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How many solar panels do you need? Calculate your system size.

Avatar for Ben Zientara
Published on 05/05/2020 in
Updated 05/05/2020
solar panels on a roof

One of the most important questions to ask when you’re considering solar panels for your home is “how many solar panels do I need?” It’s important because you want to make sure your solar panels can provide enough electricity to offset your power bill and pay back their cost.

Here’s the answer for the average American home:

The average home in the Unites States needs about 9,000 kilowatt-hours of electricity per year. To make that much energy, the home would need twenty-one 320-watt solar panels, for a total installation size of 6.72 kilowatts. That would be enough to produce just over 9,000 kWh in the first year, with the solar panels under warranty for at least 25 years.

That’s the easy answer for the average American house. But your house is anything but average, and many factors go into the calculation that determines what we solar folks call “system size.” To get those variables, you have to answer a few other questions before you can determine a final number of how many panels you need.

Here’s where we tell you the truth: Doing these calculations yourself can provide you with a rough estimate of what you might need, but nothing beats an expert using the most advanced tools that can scan your roof and calculate a very good estimate of your home’s solar potential.

If you’d like to get a detailed estimate of your needs, your best bet is to get a quote from one of our expert installer partners in your area.

Below, we’ll work through how we got to the numbers we used for an average home. Here’s a step-by-step list of preliminary questions you have to answer before you get to the number that will answer the final question:

  1. How much energy do you use?
  2. How much sun does your roof get?
  3. Does your utility offer net metering?
  4. Calculate your solar panel needs
  5. Will your panels fit on your roof?

Note: What we’re talking about here applies to people who live in their utility’s service area and intend to install a “grid-tied” solar panel system. If you’re interested in off-grid solar, see our guide to finding out how many solar panels you need to go off-grid.

Step 1: Determine your energy usage

Looking at usage over a year, you can get a good picture of your total energy needs. In fact, many utility companies base the total allowable size of your home solar panel system on how much energy you used in the 12-month period prior to installation, so you and your installer need to know this number.

Some utility companies make it simple to find your kWh needs by putting your year-to-date or last 12 months’ usage somewhere on your bill. Others make it more difficult by withholding that information, forcing you to call them, go on their website, or (gasp) find all 12 of your last bills and add up the monthly totals to calculate the annual usage.

Here’s an example from one utility’s online portal that shows monthly and total usage in kWh, along with the average daily temperature, which can illustrate why usage is higher:

12-month energy usage in kWh

The image above shows an annual usage of just over 9,000 kWh, which we’ll use in the next few examples as a benchmark. Like we said above, the average annual usage across the United States is about 9,000, so this number will provide a good estimate for most folks.

Usage goes up and down on a month-by-month basis, and sometimes has weird spikes. Looks like this person left town for July, but maybe hosted guests in September. Those weird spikes are why it’s important to look at your full years’ usage.

Step 2: Determine how many kWh you get per kW of solar

The next big factor in finding out how many solar panels you need is how much electricity you can squeeze outta your panels. For that, you need to know how sunny it is where you’re live and determine the number of kWh each kW of panels can make.

Thankfully, the folks over at the National Renewable Energy Laboratory have made the handy map below to make it easy. check it out:

The map shows different colored areas based on how many kWh you can generate in a year with 1 kW of solar panels in an ideal setting.

So let’s do an example: most of that map lies in the 1,500 to 1,700 range. So let’s use the average: 1,600. How do you feel about St. Louis?

If we need 9,000 kWh in Saint Louis, MO, which is in the “1,600” zone of the map, we’d take that 9,000 divide by 1,600 and get 5.625. That’s the size of our system in kW under ideal conditions. Unfortunately, solar panels don’t exist in ideal conditions. They live on your roof, where heat, wiring, and more can cause slight losses of electricity.

Expect those losses to be about 15% of that ideal number. So take your 5.625 kW and divide it by 85% (0.85). That gives you 6.62 kW and some change. So the average American home in the average American city needs 6.62-kW of solar panels installed.

If you’re following along at home, here’s the calculation so far, so you can plug your own numbers in:

12-month usage ÷ map area number ÷ 0.85 = system size

Now comes the hard part: limitations on system size.

Step 3: Check if your utility offers net metering

It’s all fine and dandy that you know exactly how much solar you need to meet all your energy needs, but solar panels can never provide all your energy at exactly the time you need it. That’s why many decades ago, some very smart people invented this thing called “net metering.”

They invented it, looked upon their creation, and decided it was good. But ever since solar panel prices have fallen so low most homeowners can afford them, utility companies have been fighting against it.

Nut metering in a net shell

Basically, net metering is the idea that your utility company will take any extra solar energy off your hands when your system produces more than you can use at any given moment, then they’ll give you energy from the grid when the sun isn’t shining. They keep track of the kWh you send to them and the kWh they send to you, and net out the difference.

For example, if you overproduce 3,000 kWh on sunny days and consume 4,000 kWh on dark nights, you get billed for 1,000 kWh. Simple, right?

Why utilities hate home solar

Most states still offer net metering, but it’s becoming increasingly threatened. You see, utility companies decided they didn’t want homeowners muscling in on their racket. After all, most electric providers are investor-owned regulated monopolies, and they have an obligation to maximize profits for their shareholders. You and a few hundred other folks in your city taking your little houses out of the equation means they don’t get to make as much profit.

Some states never got around to making net metering the law of the land. Some states have taken net metering away after years of it being policy. And many states are in the process of “studying changes” to net metering, which is done by state Public Utilities Commissions, usually staffed full of people who (you guessed it!) used to work in high levels at investor-owned utilities. That’s where the “regulated monopoly” becomes the “self-regulated monopoly.”

But there is some good news: A state has never taken net metering away from homeowners who had already installed solar panels on their houses. Well, Nevada tried once, but that did not go well for them, and net metering is back in a big way there now.

Given the above truth, it pays to know where net metering is available. Here’s a map!

map of states with net metering current as of 2018

If you live in one of the states with net metering or something like it, it’s very likely cool for you to install enough solar to meet 100% of your needs. Many places even allow systems sized up to 120% of your last 12 months’ usage, so if you plan to get an electric vehicle and charge it with solar, you’re covered.

Under net metering, you’ll get full credit for your extra kWh, and you might even be able to roll energy credits over indefinitely. You can check your state for more information about its specific policies, or you can connect with a local solar expert and get custom information for your home.

If you live in a state where some utilities offer net metering, check with your utility company.

If your state doesn’t have net metering

This is where it can get a little touchy, because policies vary widely in these states.

There are a few possible scenarios:

  • Your electric utility won’t buy back your solar energy at all, meaning if you go over your usage, you get nothing. Nada. Bupkis.
  • Your electric utility will pay only their “avoided cost rate” (read: wholesale price) for extra electricity, meaning you’ll get 3-4 cents for every extra kWh in a single month, with no rolling credits.
  • You get “net metering lite” which involves a “feed-in tariff” (price paid to you for solar kWh) less than retail, but higher than wholesale.

In all of these cases, you’ll still be able to connect to the grid and reduce your electric bills with solar. But it is not financially smart to have a full-sized solar system, because there will be months where your production exceeds your usage and you’ll get little if any credit.

Someday soon, home batteries will be financially viable and you could simply install a battery to make sure you get all the kWh your solar panels make. That day isn’t today.

Your best bet is to reduce the size of your system so that you’ll never exceed your usage. That might mean 50% of the size or 80% of the size, depending on your state’s specific policies. You know who knows those policies better than ever? Our solar expert partners. Connect with an installer near you to find out the perfect size for your system.

Step 4: Calculate your solar panel needs

Okay, you’ve wended your way through the forest of net metering policy, and you’ve come out on the other side ready to see how many panels you need! Recall from above that our average home in the average U.S. city needs a 6.62-kW system to make the 9,000 kWh it needs every year.

Assuming the average solar panel makes 320 watts under full sun (it does), how many panels is that?

To find out, take system size in kW divided by .320 kW per panel, and round.

We get 6.62 ÷ .320 = 20.69, rounded up to 21.

So the average home in the average place in the United States needs twenty-one 320-watt solar panels. That’s a 6.72-kW system capable of making about 9,100 kWh per year—a bit more than the 9,000 kWh we need, but the difference doesn’t matter that much, and the output of the panels will actually decrease slightly over the years as weather and time take their toll on the panel glass. If you live in a good net metering state, you could round up and overproduce for a few years…

If you’d like to do your own numbers, here’s the final equation:

12-month usage ÷ map area number ÷ 0.85 = system size
system size ÷ kW per panel = number of panels

Step 5: See if the panels will fit on your roof

Oh! Here’s the last wrinkle: will those 21 panels fit on your roof? Assuming you have a nice, un-shaded, south-facing roof (west-facing in a pinch), how much space do you need?

Well, the average dimensions of a home solar panel like the 320-watt panel we quoted above are 39 inches x 65 inches. That’s 17.6 square feet each. Solar panels are installed in long rows on roof racks connected to your roof by lag bolts, and they don’t need much more space than their dimensions. but there are a couple rules about providing enough space on the edges of your roof and around obstructions.

Solar panel setback

When it comes to solar panels, the term setback means a certain amount of space that needs to be clear between the panels and the sides and ridgeline of the roof.

In California, for example, the rule states that a setback of 3 feet is necessary to allow firefighters to access the parts of the roof that are essential to doing their jobs properly. As you can imagine, this eliminates a good deal of available roof space. Most other states follow California’s lead here, because they were the first to lay out these rules.

Here’s how setback looks in practice:

California solar panel roof setbacks


That 3-foot space all around the edges of the roof (except the bottom) can make a huge difference. We assume it’ll remove about 25% of total roof space from consideration, leaving 75% of your south- or west-facing roof available for solar panels.

Calculating how many solar panels will fit on your roof

The first thing to account for here is that solar panels can only go on a roof that faces south-ish. That could be southwest or southeast, but due south is best. So when calculating your roof’s area, only consider those portions that face south.

Once you know the area of your south-facing roof, follow these steps to see how many panels will fit:

  1. Take your available square footage and multiply by .75 to account for setback
    • Let’s say you have 600 square feet of sun-facing roof: 600 *.75 = 450 square feet available for solar panels.
  2. Divide the result by the average square-footage of one panel: 17.6.
    • Now take your 450 square feet and divide by 17.6: 450 ÷ 17.6 = 25 panels would fit on your roof. That’s enough for the average 21-panel, 6.72-kW system!
  3. If you don’t have enough space for your system: Simply multiply the number of panels that will fit, based on the last calculation, by 320 watts per panel, then multiply the result by the number of the area where you live in the map above. That’ll give you how many kWh your smaller system would produce. You could also choose similar-sized panels rated for higher production, but they may be more expensive.

So with 600 square feet of roof space, you’d fit our 6.6-kW system made up of twenty-one 320-watt panels. You can do the calculation yourself or use our handy solar calculator above to find out!

How much money solar panels can save you

Now this one really depends. The biggest two factors that determine how much money your solar panels save you are incentives and electricity cost. Incentives are tricky, because they vary from state to state (and sometimes from month to month). But electricity cost is easier to figure out, because you get a bill every month with the cost per kilowatt-hour (kWh) right on there!

Now that you know how much electricity your panels can produce in a year, look at your utility company’s website or check your bill for your rate. Be sure to add in the per-kWh charges for transmission and distribution, because you’ll be saving on those too!

Last of all: multiply your electric rate by the number of yearly kWh from the last step. In St. Louis, they pay about $0.12/kWh for electricity. So $0.12/kWh x 9,000 kWh/year = $1,080 saved in one year with our 6.6-kW solar panel system!

Again, if you want to get fully-customized savings estimates that take into account your home’s unique needs and characteristics, connect with a solar expert near you.

Last modified: May 5, 2020

7 thoughts on “How many solar panels do you need? Calculate your system size.

  1. Avatar for Greg Chimes Greg Chimes says:

    A fantastically in-depth guide on calculating your solar requirements! Something that most consumers are very much unaware of and grossly under or overestimate at the start. At Southampton Solar we use Google Earth technology to assist the customer with cost estimations, as well as production estimations based on shading, roof direction etc. Great post, cheers

  2. Avatar for Jose Patriarca Jose Patriarca says:

    Sir Ben Zientera, can i ask map area number of the Phillipines.

    1. Avatar for Ben Zientara Ben Zientara says:

      Looks like it’s about 1900 kWh/m²
      Philippines insolation map

  3. Avatar for Raju Paul Raju Paul says:

    I want to install solar, I have 1.1ton inverter ac, 2nos fan,5 led bulb (15 at each),1nos inverter fridge. How many kwt would be required?

  4. Avatar for Raju paul Raju paul says:

    Where I get map area number of India ?

    1. Avatar for Ben Zientara Ben Zientara says:

      That is an excellent question. I searched a bit and found this one with average daily and annual totals:

      Solar PV resource in India

  5. Avatar for Bob Patrick Bob Patrick says:

    Really very interesting. COnogratulations for your post.

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