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How many solar panels do you need?

One of the most-asked questions in the solar installation business is “how many panels do I need?” And the answer, as always, is “it depends.”

There are many variables that go into deciding the size of system you need; cost, the amount of sun you get, the amount of electricity you want to generate, roof size and shape, and more goes into that decision.

Your best bet is to get a quote from one of our expert installer partners in your area.

Determining your ideal solar system size

The main variables that go into determining how many solar panels you need for your home are:

  • The state where you live
  • How much electricity you use
  • How much roof space you have

Here’s why these variables matter: Many states and localities have rules that base the maximum allowed system size on 100% or 120% of your average annual usage. On top of that, some states don’t have net metering; that is, they don’t allow you to sell excess power back to the grid.

The final variable is roof space, and it’s the easiest to calculate. If you have a nice, unshaded, south- or southwest-facing roof to work with, the only considerations are the size and efficiency of the solar panels size and the available square footage.

Read on if you’d like to find out how many of those beautiful panels you can cram on your roof, or try our handy solar panel calculator!

Average size and output of a solar panel

There are many many different kinds of solar panels out there in the world, and they vary in size, efficiency, and energy produced.

Luckily, the sunny state of California keeps excellent records on all the solar installations it approves, so with some quick pivot table work, we discovered the top 5 solar panels on the market (representing 30% of all installations). Then we figured out the average size of those panels and how much energy they are rated to produce.

Here’s a look at what we found, with the averages at the bottom:

Kyocera Solar
39 65.4 265 15
REC Solar
39.02 65.55 260 14.6
SW280 Mono Black
39.4 65.95 280 15.5
Canadian Solar
38.7 64.5 265 15.3
Yingli Energy
39 65 260 14.8
AVERAGE: 39 65 265 15

So how many solar panels will fit on my roof?

As you can see above, the average solar panel these days is about 3.25 feet by 5.4 feet (about 17.5 square feet) and puts out about 265 watts of electricity. That makes it simpler to see how many panels you can fit and how much electricity the system would be rated for, except for one thing: solar panel setback.

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, 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.

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 420 square feet of sun-facing roof: 420 *.75 = 315 square feet available for solar panels.
  2. Divide the result by the average square-footage of one panel: 17.5.
    • Now take your 315 square feet and divide by 17.5: 315 / 17.5 = 18 panels would fit on your roof.
  3. Each panel produces 265 watts, but systems are measured in kilowatts (kW), so you have to divide by 1,000: 265 / 1,000 = .265 kW per panel.
  4. Multiply those 22 panels by .265 kilowatts each: 18 x .265 = 4.77 kW.

So with 420 square feet of roof space, you’d get a 4.77-kW system made up of 22 panels. You can do the calculation yourself or use our handy little solar calculator below to find out!

How much energy your solar panels will produce

The big factor in finding out how much juice you can squeeze outta your panels is where you’re live, and thankfully, the folks over at the National Renewable Energy Laboratory have made the handy map below to make it easy. Check it out:


  1. Find your city in the map and match it to the numbered area.
    Let’s start with our home, Portland, Oregon. Our city lies within the 1,400 band, so 1,400 is our number.
  2. Multiply the number of kW your system is rated for (we get 4.77 from above) by the number of the map color in your area. For us it’s: 4.77 x 1,400 = 6,678.
  3. Next, multiply that number by 0.78. Why 0.78? It represents the percentage of electricity you can expect to capture, based on inverter efficiency, panel performance, and losses from wiring. We calculate: 6,678 x 0.78 = 5,209 kWh generated per year.

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 Portland, we pay about $0.11/kWh for electricity. Here’s how that looks for us: $0.11/kWh x 5,209 kWh/year = $573 saved in one year with our 4.77-kW solar panel system!

Now that you’ve read about all that math, let us help you do it:

How much can you save with a solar roof in ?

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19 thoughts on “How many solar panels do you need?

  1. Lou says:

    There is no natural gas in my area. That leaves electricity for hot water, and and heating. In the winter, I use 4500 KWH per month. Summer I use about 1/3 of that to manage a well pump and pool filter etc. What type of system would be best ? Solar panels, solar evacuated tubes, or ground source heat pump ?

  2. Matt meiresonne says:

    Hey Ben, awesome article. You really broke it down into a science which is super useful. If I wanted to install extra solar panels and tie it to the utility grid, how long do you think it would take for those panels to pay for themselves?

  3. Bob says:

    Could you share a table showing approximate current cost per panel, panel manufacturer, respective max watt rating and whether they use micro inverters or a collective inverter, not including installation cost?

  4. Krystal says:

    Also, be aware that vents, sat. dishes, chimneys and other roof obstructions can impact how many panels can fit.

  5. Krystal says:

    Many of the big installers are rapidly switching supply chains to 280 or 295 watt panels as the “standard” offering. Also, in the Southwestern US, a west facing roof can get almost as much production as a South face. In sunny AZ, NM and southern CA you can even use due North face. Not the most optimal but it can pencil out. Do your own calcs but please have a professional installer do a survey. They have access to very accurate sun reading tools and mountains of data regarding sun hours and real world performance that you do not.

  6. Jenn says:

    My roof is east and west and I get full sun to my south is a hill

  7. Scott says:

    Are these solar panel outputs that California has calculated per day, month or year?

    1. Ben Zientara says:

      Hi, Scott-

      The calculations above are all per year. For example, when I input 400 sq-ft for Southern California (map area 2000), I get the following information:

      Your roof can hold 23 solar panels… which is 6.10 kW of generation power! Your panels will produce 9,516 kWh/year.
      Here’s the big one: you’ll save $ 1,712.88 on electricity in year 1, and the savings keep growing from there.

  8. Chad says:

    The basis for the calculations in this article are very misleading. Big rectangular panels seldom pack in a space the way you would like. For example, my roof is all hips, so from the get-go I am fitting rectangular panels into a triangular space. One face of my roof for example is 200 sq. ft., and while this article claims I could fit 11 panels, I can actually only fit 7. That’s not much more than 1/2 what this article says! Don’t overestimate rectangular roofs either. You might lose and entire row, or column, or both if things don’t align right.

    The only accurate way to determine the generation potential of your roof is to actually mock-up the panel layout on your roof using the dimensions of the panels you choose, and only then will you really know how many panels will fit.

    1. Ben Zientara says:

      Thanks for the reply, Chad. And it’s an excellent point you’re making. This article does assume a rectangular roof, or at least that you can carve rectangular sections out to fit your panels on. Maybe we’ll tackle hips in a future post!

  9. saidas m.g says:

    i want to know for a 1kw power how much square feet will be needed how it can be more thing for industries how they can use this solar panel for their working process eg:palywood industry

  10. Kukati says:

    How much many I can save from 120 panals I need information in Rupee’s could please give that information

  11. Douglas Eskridge says:

    My roof allowing a three foot setback on all sides is 1198 sq. ft. I have a five ton heat pump, 50 gallon hot water heater, fridge,washer/dryer,stove top 4 burner is my roof adequate for my needs ? Thanks !

  12. prince says:

    what is the average cost of 1 solar panel plus and minus installation THANK you

  13. Marty says:

    How much

  14. Charles says:

    Since you can purchase an all-electric car and plug it into your solar panels, you don’t pay road use taxes. Correct?

  15. Ethan says:

    Im planning to live in a bus, running a space heater in a 35 cubic ft area of it and a 3.5 cubic ft mini fridge. Is 3 panels enough.

  16. Buchi says:

    Thanks for the explanation. I ll like to read more of your article about solar configuration and installtion.

  17. Ramachandra v says:

    my hose roof top is 1100 sqft for install solar pannels full roof required for budgetery quote for polycrystiline pannels & manocrystilline pannals how much powe will it n=be generate per day

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