Buying used solar panels is a lot like buying a used car, if you’re confident it was well cared for and the proper maintenance has been done, it can be a real bargain!
Buying used solar panels can be a tempting offer, but is it worth your investment?
Have you ever considered buying a used car? You might determine that buying used saves you enough money that you would deal with a used vehicle. Some of the things you would consider when inspecting a used car are the current condition, how well was it cared for, maintenance, seller, the current mileage, and its estimated lifespan.
If you can verify all those details, and they all come back positive, you’d call it a steal, right? You’d drive off into the sunset, pretty excited about the deal you just got.
Let’s apply that to solar panels, you would want to know the current condition of the panel, how well has it been cared for, has there been maintenance on the panel, is the seller reputable and how long will it keep working?
One of the benefits of buying a used solar panel is that the price is often lower. This gives you some wiggle room. Imagine a brand new solar panel (around 250 watts) sells new for $250 (this is actually still really cheap, the average price per watt for new panels is $2.67. This example new panel is $1 per watt).
If you can buy that same solar panel used for $90 (price per watt is $.36 here), you have some wiggle room to pay for 3 more panels before you reach the price of the new panel. Imagine this $90 panel ends up producing 215 watts instead of 250 for some reason. You can scrap that panel and buy a new one 2 times before hitting the cost of the new one. You could also just buy 4 panels and be generating between 1,100 – 1,200 watts. That makes sense but for how long will the panel produce?
Most manufacturer’s warranty their panels anywhere from 10-25 years. That means they expect the panel to be functioning as intended for that length of time. Imagine you get a panel that’s been used for 5 years. Depending on the panel quality, you could still have 20 years of power generation from that panel (most often when you buy used it doesn’t come with the manufacturer’s warranty, but that doesn’t mean the panel isn’t expected to work for the same amount of time!)
“…the solar industry is growing fast…”
All solar panels degrade over time, just like cars. You might get fewer miles per gallon as time goes on, and so it is with solar panels. The reason manufacturer’s offer their long warranty is because they expect the panel to degrade a certain rate (about 1% per year). This means that these manufacturers expect that when a panel is 20 years old, it will still be close to 80% effective! So, if you come across some used panels that aren’t very old, you can expect them to perform well!
One of the big factors that makes a panel unfit to be sold as “new” are the cosmetics. Maybe the frame has a small dent (doesn’t affect production), maybe the spacing on the panel is off (doesn’t affect production), or maybe there is a little line where the sealant is visible on the glass, aka a snail trail (doesn’t affect production). Panels with problems like these are where you make off like a bandit, because the production is unaffected, they just have minor cosmetic flaws.
Because the solar industry is growing so fast and the technology is improving so quickly, panels quickly become outdated, just like how when a new Jaguar comes out, the model from the year earlier is now “outdated”, but you’d still drive it if you could.
In conclusion, as long as you know that the used seller has done their due diligence and knows that the wiring and wattage of the panel are all in working order, it’s basically like buying new
When we talk about recharging batteries, two parts of the electric current are important: amps, and voltage.
If our battery bank runs at 12 volts, we want to recharge it with a current at or a little above that 12 volt rating.
Once we’ve reached our desired voltage in our charging current, additional amps result in a faster charge time.
EX: Our battery bank has a 400 amp/hour capacity and runs at 12 volts
A 12 volt 40 amp current will recharge our battery in around 10 hours.
A 12 volt 10 amp current will recharge our battery in around 40 hours.
You may look at this and think that maximizing your amperage is the way to go.
However, that has its own problems.
High amperage currents charge our batteries faster, but they also require much thicker wiring and specialized connectors.
As electricians will tell you, high amps means high heat.
We’d have to make sure our cables were low gauge(thick) to handle the increased heat.
High amperage current also doesn’t “play-nice.”
Expensive and bulky connections are necessary to deal with high amperage current.
Additionally, if our solar panels are far away from our battery bank, we can also run into problems of voltage loss, which occurs in low voltage current over long distances.
A 14 volt 40 amp current could drop to below usable levels if the current needs to travel over long distances.
So how do we reconcile these two facts?
If we try to charge our batteries with a high voltage current it will damage them.
High amps charge our batteries faster but make everything more complicated.
That’s where our Solar Charge Controllers can help.
Solar Charge Controllers limit the amount of voltage that reaches our battery bank.
They also limit the amount of energy lost during the night time by cutting the current off to our solar panels.
There are two kinds of solar charge controllers, MPPT and PNW.
PNW charge controllers limit the voltage coming into your battery by cutting off the voltage at the useful point.
If you have a 50 volt and 10 amp current coming from your solar panels but only want 15 volts, the PNW charge controller will send a 15 volt 10 amp current into your battery.
This is less than ideal due to the wasted energy.
On the other hand, MPPT charge controllers will convert the excess voltage into extra amperage to charge our batteries faster.
This is ideal because it allows us to wire our solar panels with the easy-to-work-with high voltage/low amperage current without wasting energy.
MPPT charge controllers are more expensive, though many find them to be worth the extra investment.
The Capacity of Your Batteries
Another way to know how many panels you need is to look at your battery bank.
Batteries come labeled with an amp/hour rating.
This rating tells you how much energy each one can store when it’s fully charged.
You’ll want to make sure your total amp/hour storage in your battery bank can handle this load each day (double the load if you’re using lead deep-cycle batteries).
Your batteries amp/hour will tell you how much energy you have available to be used in powering your appliances.
You can increase your available amp/hours by buying multiple batteries and wiring them in parallel.
As we said before, the current from your solar panels has two elements:
Voltage and Current.
These are measured by volts and amps.
If your battery bank consists of 12 volt batteries wired in parallel, your recharging current will have a voltage at or a little above 12 volts.
Once this voltage is achieved, we can calculate our recharge rate by looking at the amps of our recharging current.
If our batteries have 400 amp/hours of energy when fully charged, it would take a 40 amp current around 10 hours to recharge them fully.
It would take a 5 amp current 80 hours to recharge our battery bank.
So How Many Panels Should I Buy Then?
As we discussed in this article, there is not a one-size-fits-all answer.
To find a solution that works for you, you need to first get some idea of:
(1) What you’re hoping to power with your solar setup.
Solar energy can be used in a huge variety of applications, from home power to RV power to emergency situations.
You can use Calculatorto get some idea of how much energy you’ll need for your specific situation.
(2) The kind of battery bank you’re hoping to work with.
Lithium batteries have several advantages, but you’ll pay a premium for them.
Lead deep-cycle batteries are the cheaper solution in more ways than just price.
(Remember to keep those lead deep-cycle batteries above half charge!)
(3) Your Solar Charge Controller will help you convert current into usable voltages and amperages.
MPPT charge controllers give you more flexibility in wiring your solar panels but cost more than PNW charge controllers.
(4) Finally, we want to make sure our battery bank has the storage needed for each daily use.
Remember, we don’t want to damage lead deep-cycle batteries by allowing them to go below half charge, so be sure that your battery bank has more than double the capacity of your daily drain.
A common first step for most beginners is to look at a few key numbers when you start planning a new solar power project.
These numbers include projected wattage drain, projected number of sunlight hours, and total panel wattage.
You can calculate your potential power draw and sunlight hours through the calculator here.
After you get some idea of the power draw, it can be tempting to just go out and buy enough solar panels to cover your draw.
If you find that you typically use 2500 watt/hours each day, you may make the mistake of simply dividing that drain by the average number of sunlight hours to buy the appropriate number of panels.
If you get five hours of sunlight each day, you may think that a single 500-watt panel will be enough for your setup.
After all, 500 watts for 5 hours does equal 2500 watt/hours, your exact drain!
If you were to buy one 500 watt panel, you’d almost never generate enough power for your needs.
This is because a great number of inefficiencies nearly always accompany solar panels.
You must consider these inefficiencies when buying your solar panels, or run the risk of too little power.
We’ll be discussing some of the main inefficiencies that accompany all solar power in this article so you can be more confident in your solar design.
Panel Wattage in Lab Tests vs Real World (STC vs PTC vs Real World)
Most solar panels are rated in two ways, STC and PTC. STC stands for Standard Test Conditions, and are what you think of when you hear of unrealistic laboratory test conditions.
Every variable is perfect for solar panels to produce electricity.
They’re kept at an efficient temperature, they receive light that comes in at a perfect angle and with a uniform intensity across the entire panel.
Solar panel manufacturers measure their panels using STC.
Every panel that produces energy within a certain rating is then marked with that rating and sold.
Needless to say, STC does not represent real-world conditions.
In order to help installers get a better idea of actual solar panel performance, the PVUSA Test Conditions(PTC) rating was created.
A PTC rating is measured with a more realistic operating/ambient temperature and wind speed. On average PTC ratings reflect around 90% of the manufacturer’s STC rating.
This means that a panel rated for 500 watts will typically get a PTC rating of somewhere around 450 watts.
Every solar panel sold in California is required to receive a PTC rating, which means that almost every panel sold in the US has a PTC rating.
PTC is vastly more accurate than STC when representing real-world conditions, but it still doesn’t take into account degradation over time, off-angles, dirty panels, or other inefficiencies we’ll be covering later in this article.
Panel Degradation Over Time
We’ve covered how panels can produce less than their factory rating may imply.
Another concern to consider when buying panels is that they naturally lose efficiency as they age.
This fact can be especially important when designing solar setups with long projected lifespans.
It’s typical for solar panels to degrade by about 1% every year.
That means that in 10 years you’ll be generating about 90% of what you were when you first installed.
Let’s look at our example from earlier.
If we buy a 500-watt panel, we know it’ll likely have a PTC rating of around 450.
If we then use this panel for another ten years, it’ll drop down to around 400 watts even if we don’t take into account any other inefficiencies!
This degradation occurs for a variety of reasons.
Shortly after installation, solar panels are exposed to UV light which can rapidly lower the efficiency by a percent or two before leveling off.
Another way that degradation occurs involves thermal expansion and cooling.
As with most materials, solar panels expand slightly when warm and shrink slightly when cold.
This cycle of shrinking and growing can cause microfractures in the silicon.
You can see this degradation over time reflected in manufacturers’ performance warranties.
It’s typical to see a guarantee of 90% production for the first ten years and 80% for the first 25 years.
Voltage drop is usually considered acceptable when it’s less than 3-5% of your solar panel’s output.
Solar Charge Controllers PWM vs MPPT
Another key to increasing the efficiency of your solar panels is to buy the right kind of solar charge controller.
There are two main types of solar charge controllers.
PWM (Pulse Width Modulation) solar charge controllers limit the voltage coming into your batteries to prevent damage.
If you’ve got a 12-volt battery bank, a 60-volt current would cause serious damage to your batteries.
PWM charge controllers prevent this by limiting the amount of voltage.
All the excess current is lost with PWM charge controllers.
MPPT (Maximum Power Point Tracking) charge controllers are similar in that they limit the amount of current flowing into your batteries to prevent damage.
However, unlike PWM charge controllers, MPPT charge controllers convert the excess voltage back into the current to charge your batteries faster.
Depending on the voltage of your solar panels and battery bank, an MPPT charge controller could save you an incredible amount of efficiency.
MPPT charge controllers are more expensive than PWM charge controllers, but they could pay for themselves depending on your setup.
The last inefficiency we’ll be covering today is inefficiencies inherent in batteries.
Battery Inefficiencies
It’s not uncommon for charging and discharging your battery to be the single largest inefficiency in your solar setup.
We refer to the energy that batteries lose during the process of charging and discharging as their round trip efficiency.
There are two main types of batteries: lead deep-cycle batteries and Lithium-ion batteries.
Lead deep-cycle batteries have been used for decades and are cheap and reliable.
However, they usually have a round trip efficiency of somewhere in the 80% range.
Lithium-ion batteries have grown in popularity in recent years for their vast and varied advantages over lead deep-cycle batteries.
Lithium batteries typically have a round-trip rating in the high 90% range.
However, these batteries are far more expensive than lead deep-cycle batteries, sometimes costing up to ten times as much.
Conclusion
As you can see, the real world has a multitude of inefficiencies that can make planning your solar setup difficult.
STC ratings can be overly optimistic, panels degrade, and dirt and clouds can make everything even harder to account for.
On top of that, inefficiencies inherent to batteries and wiring can make the headache of planning even worse!
The main takeaway for your solar setup would be to buy far more power than you may think you’ll need.
You’ll never complain about having a battery bank that’s always full, or a solar panel setup that recharges them too quickly.
Remember, if it takes 10 solar panels to power an appliance, you can’t power the appliance with 9 solar panels, but you can with 10 or more.
The Four Main Elements of Most Solar Power Systems
There are four main elements in most Solar Power systems. The four elements are the following: First, your Solar Panels, which gather energy from the sun. Second, your Charge Controller, which regulates the electricity coming from your panels to avoid damaging your battery bank. Third, your Battery Bank, which stores the energy coming from the panels for use during non-sunlight hours. Fourth, your Inverter, which most setups include in order to power common AC appliances.
Step 1: Connecting Your Charge Controller to Your Battery Bank
Your battery bank could consist of a single led deep-cycle battery or an entire bank of expensive lithium-ion batteries. Regardless of your battery bank, our first step is to connect our bank to our charge controller.
Every battery bank is different. Many people wire several 12-volt batteries together in parallel to reach higher amp/hour storage, and some have a combination of batteries wired in a combination of series and parallel to reach desired voltage and amp/hour ratings. For this article, we’ll be explaining the connections with the assumption that you have either one battery or a few batteries wired in parallel.
Every charge controller is different, so be sure to read the manual for your specific design. Most solar charge controllers have a few features in common which will be going over here:
Solar charge controllers have input terminals. These input terminals are openings on the bottom of the charge controller connected to screws accessible on the front face.
To connect our battery bank to our solar charge controller, we first unscrew the metal screws marked “Battery” on the front of the charge controller. This opens space inside the input terminals, allowing us to push in our bare wire. We should have two wires: A wire that connects to a positive terminal on our battery, and a wire that connects to the negative terminals of our lead battery. Each of these wires should have one end that has bare cable. We first insert the negative bare wire into the marked opening, then the positive bare wire into its marked opening before tightening the screws back down. Be sure not to strip your screws during this step!
Step 2: Connecting Your Solar Panels to Your Charge Controller (MPPT vs PWM)
Many solar panels use MC4 connectors that look like this:
We can connect these MC4 connectors using a variety of methods, including an adaptor kit.
Most users will find it easiest to simply get additional PV wire and attach two MC4 connectors that will connect to the solar panel.
Then strip the insulation on the other end of the wire to reveal the bare wire which we can then insert into our charge controller.
The steps are similar to our battery connections. We first unscrew the screws on the front of our charge controller marked solar power, then we first insert the bare wire from our negative solar power connection followed by the bare wire from our positive solar connection.
After we’ve inserted our two wires, we screw the two screws on the front of the controller back in.
Step 3: Connecting Your Battery Bank to an Inverter
Every inverter is different. Some are heavy-duty inverters meant to support high loads, some are smaller. Before we connect our battery bank to our inverter though, we need to talk about disconnects.
It’s always a good idea to have a disconnect in-between your battery bank and your inverters. For example, if your wires are rated to handle 170 amps, it’s a good idea to have a 150 amp breaker or fuse. This ensures that the current flowing through your wires doesn’t exceed their rating, which could result in melting insulation or fires.
You can use either breakers or fuses for this purpose. Some choose to go with breakers instead of fuses. When breakers are triggered, all that’s required to reset them is to flip a switch. If a fuse breaks, it means going out and buying and installing a new one. The breaker will be installed between the battery bank and the charge controller. The inline fuse will be installed between the panel and the charge controller (usually before parallel pigtails or additional wire).
Circuit Breaker Inline Fuse
So we first attach one of our wires through our disconnect (breaker or fuse). After we’ve incorporated our disconnect, we can take a look at our inverter. Most inverters have bolts on the back that are marked for connections with battery banks. You can see pictures of SanTan inverters below:
Many inverters will have a screw-and-bolts system that looks somewhat similar to most battery terminals. We attach our wires from our battery bank to the inverter, screwing the bolts tightly to ensure a good connection.
WARNING: Many inverters will draw more than 50% of your batteries charge before disconnecting. Allowing your battery to drop below 50% charge will damage them and shorten their lifespan.
Step 4: Safety Checks
A few things to keep in mind:
Be sure that your wires are rated for the level of amps that you’re planning on passing through them. The higher your projected amp load, the lower your gauge (thicker your wires) has to be.
Be sure that your connections are insulated and kept out of the elements. This is particularly true if you’re planning on leaving your solar panels outside. Wire corrosion can occur even underneath insulation, though it can be hard to detect.
Make sure your battery bank never drops below 50% if you’re using standard deep cycle batteries. Some choose to connect their inverters directly to their charge controller to avoid this.
You can check how much power is passing through different parts of your system by incorporating a volt meter. You can ensure that dangerous levels of amps are never drawn by including disconnects like breakers or fuses.
Conclusion:
Getting a handle on all the different parts of your solar setup may seem daunting, but hopefully, now you’ve got a better grip on the different parts. We first connect our Battery Bank to our Charge Controller by loosening the screws on the face. After we’ve connected our battery, we can insert the wires from our Solar Panels and screw everything down into place. After that’s connected, we’re ready to begin powering our appliances through our Inverter. We need to make sure our wires can handle our current. We ensure this by using disconnects like breakers, fuses, or volt meters to ensure we’re not passing dangerous levels of current.
An Overview to Powering Your Appliances With Solar Energy
We can divide our solar power setup into three main sections. These are (1) the battery/power supply, (2) the appliances we want to power that drain our batteries, and (3) the solar panels that recharge our batteries. Both our batteries and our panels can be wired in series or in parallel depending on our needs. Let’s break each one down, starting with our batteries:
Part One, The Power Supply
Amp/Hours, The Energy in Your System
The amount of energy contained in a given battery is measured using something called amp/hours. The same way we might say an apple has 95 calories, we can say a standard lead battery contains 100 amp/hours. This means that, in theory, we could drain 1 amp for 100 hours or 100 amps for one hour. In real world applications, that’s not the case. A standard battery might have a label that reads 100 AH(amp/hours) @ 20 HR(hours). This means that the battery is rated to supply 5 amps for 20 hours at a useful voltage.
It’s important that we understand that amp/hours is just a measure of total energy in the battery, so here’s another example: Car batteries typically contain only around 45 amp/hours of energy. When we crank our starters to turn on our engines, 100+ amps can be drawn from the battery. However, this high drain only lasts for as long as it takes to turn your engine over. As soon as your engine turns over, the car battery is quickly recharged from the car’s alternator.
You may ask yourself then, what battery should I buy? How many amp/hours will I need? What if my daily drain is greater than any one battery can provide?
Setting up Multiple Batteries in Parallel VS Series
For most applications, a single battery with 100 AH will not supply enough energy. To get more power, we could buy multiple batteries. That leads to the question: “How do I set up my multiple batteries? There are two ways, in Series or in Parallel. When batteries are wired in Series, the voltage is added. When batteries are wired in Parallel, the amp/hours are added. If we wire four 100 amp/hour batteries together in Parallel, our system will have 400 total amp/hours of energy at 12 volts. Most common battery setup will want to wire their batteries in Parallel, but it is possible to wire in parallel and series to achieve 24/48 volts. For most applications however, 12 volts is already a useful voltage.
To wire a battery in parallel, we use wires to connect each of the positive terminals of our batteries, then, using another wire, connect all the negative terminals of our batteries. This will increase our amp/hour storage while maintaining the same voltage.
To wire a battery in series, we wire the positive of one battery onto the negative of the next one, repeating this process for each battery in our setup. This adds up the voltage of our setup while the amp/hour storage remains the same.
Part Two, The Drain from Your Appliances
Powering Your Appliances Using an Inverter
Most solar setups incorporate inverters. Inverters convert the DC(direct current) from our batteries into AC(alternating current) that can be used by most household appliances like TVs or refrigerators.
Not all inverters are created equal, and you’ll need to make sure that the inverter you buy for your system can output enough watts for your setup. This article will only be going into the power supply needed to power your appliances.
How Long Will My Batteries Be Able to Power My Setup?
We describe the power drain that household appliances pull on your batteries using Watts. To simplify, Watts is a description of the rate of amp/hours drained by a given appliance. The amount of drain is different for each application, and is influenced by whether the appliance uses DC or AC.
So if the number of Watts an appliance requires describes how many amp/hours it will pull from our batteries while in use, how can we know how long our appliance will run with a given amp/hour battery setup then?
Here’s a formula from DonRowe.com that will tell us the number of hours a given battery supply will be able to run an inverter containing our appliances.
We first add up our batteries’ total amp/hours. (Remember, this is information that you can find on the label of each battery.) In our example setup, we have four batteries wired in parallel which gives us 400 total amp/hours.
Now we need to decide what kind of appliances we want to power with this setup. Let’s say we want to power a TV which drains our amp/hours at a rate of 60 watts.
Here is the formula:
((10 X 400 Amp/Hours)/(60))/2) = around 33 hours of continuous TV usage if we start at full charge.
Another Example:
Let’s say we have the same four 100 amp/hour batteries wired in parallel, giving us 400 total amp/hours at 12 volts. What if we wanted to power our 60 watt TV, and an 80 watt refrigerator?
Ex:
(10 X 400 amp/hours)/(60 watts from TV+80 watts from Fridge)=(28/2)= around 14 hours of continuous use.
Of course, the point of our solar setup is to charge our batteries during the day while it’s sunny, so our amp/hours will be going up during the day instead of just getting drained by our appliances.
Part Three, Charging Our Batteries Using Solar Panels
Solar Panel Basics
Our solar recharging kit will have two main elements. The PANELS, which generate power from the sun, and the SOLAR CHARGE CONTROLLER, which converts this current into a voltage and amperage that can charge the battery without damaging it.
Solar Charge Controller
Most solar panels output a voltage that will damage your battery if connected directly. That’s why solar setups include Solar Charge Controllers. These devices limit the amount of voltage flowing from your panels into your batteries to prevent damage, reduce drain during the night, and to prevent overdraining from your appliances.
For example, a given solar panel may output 24 volts to your battery. With a standard 12 volt setup, no more than 14-14.5 volts would be necessary to fully charge your battery. Our solar charge controllers limit the flow of electricity to limit this damage.
There are two main kinds of charge controllers, MPPT and PWM charge controllers.
MPPT vs PWM
Both MPPT and PWM limit the voltage to your battery, but they do it in different ways.
Maximum Power Point Tracking(MPPT) charge controllers are the more expensive option. They convert the excess voltage into amperage, effectively increasing the charging rate of your battery. In our example above, an MPPT charge controller would limit our 24 volts coming from our solar panels but increase the current, so the total power flow remains about the same. Using an MPPT charge controller means you can deal with a higher overall voltage effectively, which has advantages that we’ll discuss in the Solar Panel section.
PWM controllers are cheaper, but lack this feature. In the example above, they’d also limit the voltage down to 12 volts, but they wouldn’t change the amperage. In effect, half of the generated power is lost! It’s highly recommended that you use a battery bank with a voltage matching the output of your solar panels if you use a PWM charge controller.
Wiring our Solar Panels
Just as we can wire our batteries in series or parallel, we can also wire our solar panels in series or parallel. Remember, when we wire in parallel, we add our amps. When we wire in series, we add our volts.
When charging batteries, we want our charging current to reach a certain level of volts, then increase our amperage to charge faster. A 12 volts and 3 amps charging current will charge much faster than a 12 volt and 1 amp current. So you may be tempted to always wire in parallel, especially with the knowledge that high watt solar panels already release more than 12 volts. We must be careful before we increase our amps in this way though. We have to think about our wires.
If we massively increase our amps, we’ll need extremely low gauge(thick) cables to safely carry this current. The more amps we put through our wires, the more heat they’ll generate as well. Low voltage currents will also have voltage loss if we’re carrying our energy for long distances.
This is why you may find having an MPPT charge controller may be worth the added cost. With an MPPT charge controller we can wire our panels in series, deal with an easy high voltage low amp current, and still lose almost no energy while charging our batteries.
How Many Panels Do I Need?
A common question we receive is this: “How many panels should I buy? How many watts will I need to produce to power my setup?”
Every setup is different. If you just want to charge your phone and laptop your setup will look different than someone who wants to power their fridge, TV, and several LED lights.
Here’s how you can get a rough estimate of how much energy your setup will need. Appliances use watts. You can google all the watt usage for each of your appliances, then multiply the number of hours you expect to use them per day and add them together. For example, we can use our tv and refrigerator example from earlier. We plan on using our 60 watt tv for 4 hours per day, resulting in 240 Watt/Hours drain (20 amp/hours). We want to run our 80 watt refrigerador 24 hours a day, resulting in a drain of 1920 Watt/Hours(160 amp/hours). This means that our system will have a drain of 2160 watt/hours per day.
So how many panels will we need? If we suppose that we have a single 500 watt panel and that we’ll have, on average, five hours of sunlight per day. That means we’ll be theoretically generating 2500 Watt/Hours per day. You may think that this will more than cover your uses, but it’s important to factor in power loss and inefficiencies.
It’s common to multiply your expected power generation by .85 to account for losses resulting from inefficiencies in the inverter, the charge controller, the natural battery loss of charge, wiring loss, and imperfect real-world panel performance. So our single 500 watt panel will probably produce something more like 425 watts per hour of sunlight. If we multiply this by our five hours of sunlight, that means we’re only going to be producing 2125 watt/hours of energy, less than our daily drain.
How Many Batteries Do I Need?
We never want lead deep cycle batteries to go below 50% charge to avoid damage. This means that we’ll need a battery setup that has an energy capacity of at least twice our drain. In our example above, we’d want at least 4500 watt hours(around 375 amp/hours). This means we’d want at least two lead acid batteries with 200 amp/hour ratings.
Conclusion
Solar energy setups are a complicated technology. With a little research and planning of your solar setup, with a large enough Battery/Power Supply, with a good idea of how much drain our Appliances will pull from our setup, and with an understanding of Solar Charge Controllers and Solar Panels, our setups will allow us to power our appliances anywhere in the world, on or off the grid.
Installing Solar Panels on My House: The Basics of On-Grid vs Off-Grid
From DIY’ers to door to door salesmen, home solar power has exploded in popularity in recent years. There are many reasons one might install solar energy, from environmental to economic to emergency preparation. There are two main methods with which we can install solar power into our homes. On-Grid, and Off-Grid.
You should consider what you want out of solar when selecting either approach. We’ll be giving a brief description of each before diving deeper into each method.
On-grid solar involves connecting your home solar system to the power grid of your city. On-grid solar setups will feed excess energy generated by your panels back into the electrical grid of your utility company. Depending on your local regulations, number of panels, and hours of sunlight, this could result in an electricity bill that is reduced, free, or in a bill that actually pays you for your electrical contributions to the grid. Door-to-door salesmen typically offer on-grid solar setups.
Off-grid solar, as the name implies, involves a home solar setup that is independent of any power grid. This can be appealing for cabins or other homes that are separated from the electrical grid, or as an emergency backup in case of grid failure. Off-grid solar setups typically incorporate battery banks to capture the power generated during the day for use during non-sunlight hours. It’s also common to use off-grid solar in coordination with a diesel generator for emergencies.
On-Grid Solar Energy
As summarized above, on-grid solar setups are connected to the electrical grid of your city. The main advantage of on-grid setups is that you can still use electricity during the night or on cloudy days without a battery bank. Another advantage of on-grid setups is that you can get paid back for your contributions to the electrical grid through something called Net Metering.
Net Metering VS Net Billing
Let’s say you have an array of solar panels rated at 5 kilowatts. In an idealized scenario, this will generate 5 kWh(kilowatt-hours) per hour or sunlight. If your drain is only 1 kWh(kilowatt-hour) per hour, your solar setup would be pumping 4 kWh back into the grid every hour.
How much you’re credited for this extra energy depends on your state. Each state has its own laws and regulations. The two main ways you can be credited are known as net metering and net billing.
If your state uses net metering, that means your utility company credits your bill for the exact amount that they’d have charged you if you’d consumed that energy instead of supplying it. If your utility company would have charged you 12 cents per kWh used, they’ll cut your bill by 12 cents per kWh supplied. This can get slightly complicated because some utility companies charge (and therefore pay) more for energy used during peak hours. In our example above, you’d earn 4 X .12, or 48 cents per sunlight hour credited to your account.
If your state uses net billing, that means your utility company pays you less for the energy you supply than they’d have charged you for the energy you’d have consumed. This varies by utility company, but you can probably expect to be credited at around half of what they’d have charged you. In our example above with net billing, you’d earn 4 X .06, or 24 cents per sunlight hour credited to your account.
An important note: some states have neither net billing nor net metering. In Alabama, for example, utility companies actually charge you for having a solar setup.
Other On-Grid Considerations
In order to know how much energy you’re supplying to the grid, you may need a second meter. One checks how much energy you’re taking from the grid, and the other checks how much you’re supplying. There are also certain two-way meters that can do both in the same device.
When you connect your on-grid system to the electrical grid, you’ll need to sign what’s known as an interconnection agreement with the utility company. Different companies have different agreements. The complexity between these interconnection agreements varies wildly, with some being little more than a piece of paper and others being far longer and complex. Liability can greatly vary from company to company as well. Be sure to research your state’s regulations.
Your utility company will typically send someone out to inspect your setup before connecting you back to the grid. You may find it worth your investment to pay a qualified electrician to set up the connections between your solar setup and the grid.
Another important consideration in your on-grid setup is the addition of a battery bank. Tesla, for example, uses power walls. This is basically a lithium battery bank. A battery bank will allow your house to maintain power in the case of a power outage, and allow you to drain less from the grid. If you live in a state with net billing or no net metering at all, it may make financial sense to invest in a battery bank. You can read more about the different kinds of batteries and equipment for those batteries in the off-grid section below.
On the other hand, if your state has full net metering, it may be useful to think of the power grid as a massive battery bank. You’re “charging” the power grid with excess energy during sunlight hours and only have to pay when you’ve used up all the excess energy.
Off-Grid Solar Energy
But what if you want to install solar energy into a cabin that’s far from the power lines? Perhaps you’re concerned about the stability of the power in your region, or you’re prepping for a potential collapse of society?
If any of those fit your situation, you may want to set up an off-grid solar setup. Before we talk about the different parts of an off-grid solar setup, we’ll want to know how much energy we can expect to use.
Calculating Your Energy Needs: Watts and Watt/Hours
Our first step in calculating the amount of energy that our solar panels will have to provide is to look up the watts of our appliances and then multiplying them by how long we expect them to run for. You can find the watts drained by most common appliances through google. Alternatively, you can use Solar Energy Calculator to calculate the watt drainage from multiple appliances at the same time.
Once we know our wattage drain each day, we need to buy enough solar panels to cover both our projected use plus projected cloudy days and inefficiencies in our setups. Solar panels come rated with a wattage rating. A 300-watt solar panel, for example, will provide around 300 watt/hours of energy for each hour it receives sunlight.
Each application is different. If you’re hoping to power all the appliances in a remote cabin, you’ll need far more than if you’re using just one to recharge your phone and laptop during power outages.
Solar power is by nature unreliable. You never know when you’ll have a few cloudy days in a row or when your power consumption will spike due to an unforeseen use by an appliance. Due to this unreliability, many off-grid solar users will purchase a generator to accompany their solar powers. A single gas generator will provide a great deal of peace-of-mind to your solar setup.
Now let’s talk about the different parts that make up our solar setups:
Battery Bank
However you get your energy, every off-grid solar setup will need a battery bank to store energy. Without batteries, you’d only be able to power your appliances during sunlight hours or while your generator is running. Batteries will allow you the freedom to run your appliances at any time.
There are two main kinds of batteries, lead deep-cycle batteries and lithium batteries. Lead deep-cycle batteries are cheap, but can be damaged easily if you’re unfamiliar with their limitations. For example, letting a lead deep-cycle battery get below half charge will cause permanent damage.
Lithium-ion batteries are superior in almost every way, but are far more expensive. For example, Tesla uses lithium batteries in its solar wall installations. However, lithium batteries can be up to ten times as expensive as lead deep-cycle batteries.
We want to make sure our battery bank has enough energy storage for our drain, which we can calculate using this Calculator
You can wire your batteries in series or parallel to increase their storage capacity, or to increase their voltage rating. You can read more about batteries in our other article here:
Solar charge controllers are an integral part of every solar setup, on-grid or off-grid. They perform several important jobs.
They limit the amount of voltage coming from the panels into your battery bank. Too high of a voltage can damage your batteries. They also disconnect the current when your solar panels are out of sunlight to reduce draining batteries.
There are two main types of solar charge controllers, PWM and MPPT. PWM performs all the functions that are described above, but are “dumb.” They simply cap the voltage coming into your battery so as to not damage them.
MPPT solar charge controllers are similar in the above-listed functions, but they are “smart,” in that they convert excess voltage into current which results in a faster charge time. This allows you to wire your solar panels in an easy high-voltage formation without wasting energy.
MPPT controllers are more expensive than PWM controllers, but many people find them worth the investment.
Inverter
Most appliances use electricity in the form of AC also known as alternating current. Solar panels and batteries both provide energy in the form of DC or direct current. In order to power most appliances inside your home (like a refrigerator, TV, or toaster) you’ll need to use a device to convert the DC energy coming from your panels and battery into the useable AC form. A device that performs this conversion is called an inverter.
Not all inverters are created equal! Some can provide a huge flow of power all at once, but some are cheaper and smaller. Be sure to check the rating for whichever inverter you buy and compare it to your projected load.
Some users attach their inverters directly to their Solar Charge Controllers. Running your inverter directly to your battery bank runs the risk of over-discharging your batteries. Lead deep cycle batteries will be damaged if they ever run below 50% charge. However, running your inverter off your solar charge controller typically limits the amount of current you’ll be able to draw from your batteries.
IMPORTANT NOTE: Many appliances have a larger “startup” wattage. A refrigerator for example, will draw a few hundred watts while running but can draw more than a thousand watts when it’s first activated. Be sure your inverter is designed to handle your appliance’s larger startup load!
Weatherproofing and Safety
When working with electricity, use common sense safety precautions. Be sure to check your cables periodically for corrosion, and be sure to waterproof any holes you drill while installing your wiring in the roof of your home. Electrical shorts (the visible arc of electricity) can reach temperatures of 35,000 degrees Fahrenheit and ionize the air.
Always include disconnects like circuit breakers or fuses between the different parts of your system to avoid melting wires or damage to different components.
Conclusion
Installing solar energy into your home can be a complicated yet financially attractive project. On-grid solar setups can lower your electricity bill while at the same time helping you to feel better about your carbon footprint.
Off-grid solar setups can give you a nice backup in the case of a power outage in your city, or the ability to run a hair dryer in an otherwise rural cabin.
One of the most common projects for DIY solar enthusiasts is to install solar power onto your RV. Solar power can give you the freedom to park away from ground power, or to drive almost anywhere in the world without the fear of your fridge running out of power.
While the question of “How Many Solar Panels do I need to power my RV?” may seem simple, the answer depends on what you plan to power inside your RV. Powering a single outlet for charging your phone will require far less energy than a space heater!
There are several kinds of RV setups. Most have separate electrical systems for automotive functions (like turning your engine over) and for coach functions (like vent fans or your water pump). You can typically only use coach functions if you already have a battery installed into your RV. We can use this as a base for our solar installation.
To know how many solar panels you need, we first need to get a good guess of how much energy your appliances will drain. We can think about this energy that powers your appliances using watts and watt/hours.
Calculating Your Energy Needs: Watts and Watt/Hours
You’ve probably heard the term watts before. Watts are used to describe the rate that a given appliance uses power. Watt/hours is a description of the amount of energy that’s being used per hour. We can think about the energy in batteries in terms of a given amount of watt/hours.
Example:
We turn on a 100-watt refrigerator. In one hour, our 100-watt refrigerator will have drained 100 watt/hours from our battery. (1 hour X 100 Watt drain = 100 Watt/Hours.) If we run this refrigerator all day and night, it will drain 2400 watt/hours per day.
Our first step in calculating the amount of energy that we’ll need is to look up the watts of our appliances and then multiplying them by how long we expect them to run for. You can find the watts drained by most common appliances through google. Alternatively, you can use a Solar Energy Calculator to calculate the watt drainage from multiple appliances at the same time.
Once we know our wattage drain each day, we need to buy enough solar panels to cover both our projected use plus projected cloudy days and inefficiencies in our setups. Solar panels come rated with a wattage rating. A 300-watt solar panel, for example, will provide around 300 watt/hours of energy for each hour it receives sunlight.
There are other important factors to keep in mind before buying your solar panels. If you plan on setting your solar panels up in a set-it-and-forget-it style by installing them on your roof, you must first think about how many solar panels realistically fit.
Area of Your RV
Most RV’s have around 280 square feet of area on their roof. This is the entire area of the roof, not taking into account any vents or other blockages.
If you’ve ever seen the roof of an RV, you know that the vents and windows make this unrealistic. If we optimistically plan to cover half of our roof with solar panels, we’d end up with about 140 square feet. Solar panels can produce around 15 watts per square foot, so that means we’d generate around 2100 watts per hour of sunlight if we cover every available space. If we gather sunlight for five hours per day, we’d generate just over 10,000 watt/hours per day. This is more than enough to power most applications, but still limits what we can do inside our RV.
Battery Bank
When you’re setting up your solar panels, it’s a bad idea to run them directly into your appliances for a variety of reasons. Solar power setups always include battery banks so that you can continue to use power even when the sun is down.
You can read other articles on our site to learn more about other batteries and their pros and cons, so we’ll just be doing a quick overview in this section.
There are two main types of batteries, lead deep-cycle batteries and lithium batteries. Lead deep-cycle batteries are cheap, but can be damaged easily if you’re unfamiliar with their limitations. For example, letting them get below half charge will cause permanent damage to lead deep-cycle battery.
Lithium ion batteries are superior in almost every way, and have the price point to prove it. Tesla uses Lithium batteries in their solar wall installations, for example. These batteries can be up to ten times as expensive as lead deep-cycle batteries however.
Solar Charge Controller
Solar charge controllers are an integral part of your RV’s solar setup. They perform several important jobs.
They limit the amount of voltage coming from the panels into your battery bank. Too high of a voltage can damage your batteries. They also disconnect the current when your solar panels are out of sunlight to reduce draining batteries.
There are two main types of solar charge controllers, PWM and MPPT. PWM performs all the functions that are described above, but are “dumb.” They simply cap the voltage coming into your battery so as to not damage them.
MPPT solar charge controllers are similar in the above-listed functions, but they are “smart,” in that they convert excess voltage into current which results in a faster charge time. This allows you to wire your solar panels in an easy high-voltage formation without wasting energy.
MPPT controllers are more expensive than PWM controllers, but many people find them worth the investment.
Inverter
Most appliances use electricity in the form of AC also known as alternating current. Solar panels and batteries both provide energy in the form of DC or direct current. In order to power most appliances inside your RV (like a refrigerator, TV, or toaster) you’ll need to use a device to convert the DC energy coming from your panels and battery into the useable AC form. A device that performs this conversion is called an inverter.
Not all inverters are created equal! Some can provide a huge flow of power all at once and some are cheaper and smaller. Be sure to check the rating for whichever inverter you buy and compare it to your projected load.
Some users attach their inverters directly to their Solar Charge Controllers. Running your inverter directly to your battery bank runs the risk of over-discharging your batteries. Lead deep cycle batteries will be damaged if they ever run below 50% charge. However, this does typically limit the amount of current you’ll be able to draw from your batteries.
IMPORTANT NOTE: Many appliances have a larger “startup” wattage. A refrigerator for example, will draw a few hundred watts while running but can draw more than a thousand watts when it’s first activated. Be sure your inverter is designed to handle your appliance’s larger startup load!
Weatherproofing and Safety
When working with electricity, use common sense safety precautions. Be sure to check your cables periodically for corrosion, and be sure to waterproof any holes you drill while installing your wiring in the roof of your RV. Electrical shorts (the visible arc of electricity) can reach temperatures of 35,000 degrees Fahrenheit and ionize the air.
Always include disconnects like circuit breakers or fuses between the different parts of your system to avoid melting wires or damage to different components.
Conclusion
Installing solar energy into your RV is a complicated yet rewarding project. You’ll gain the freedom to drive almost anywhere without paying for electrical ground hookups or worrying about using all your gas to power an in-built generator.
