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How many square feet do I need for solar panels on my roof?

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 more goes into that decision. Your best bet is to get a quote from one of our expert installer partners in your area.

People also ask a related question: “how many solar panels can I fit on my roof?” The answer is, again, “it depends.” But it depends on a whole lot less! If you have a nice, unshaded, south- or southwest-facing roof to work with, the only variables are panel size and available square footage.

Read on if you’d like to find out how many of those beautiful panels you can cram up there, or skip ahead to our handy calculator!

Panel Size

There are many many different kinds of 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:

MAKE/MODEL WIDTH (IN.) LENGTH (IN.) WATTS WATTS/SQ. FT.
Kyocera Solar
KU265-6ZPA
39 65.4 265 15
REC Solar
REC260PE
39.02 65.55 260 14.6
SolarWorld
SW280 Mono Black
39.4 65.95 280 15.5
Canadian Solar
CS6P-265P
38.7 64.5 265 15.3
Yingli Energy
YL260P-29b
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 simple to see how many panels you can fit and how much electricity they would be rated for:

  1. Take your available square footage and divide by 17.5.
    Let’s say you have 385 square feet of sun-facing roof: 385 / 17.5 = 22 panels would fit on your roof.
  2. 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.
  3. Multiply those 22 panels by .265 kilowatts each: 22 x .265 = 5.83 kW.

So with 385 square feet of roof space, you’d get a 5.83-kW system made up of 22 panels. You can do the calculation yourself or use our handy little solar calculator below to find out!

And how much energy will they 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:

Next:

  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 5.83 from above) by the number of the map color in your area. For us it’s: 5.83 x 1,400 = 8,162.
  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: 8,162 x 0.78 = 6,366 kWh generated per year.

OK, but what I really want to know is: how much money can I save on electricity?

Now that 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 6,366 kWh/year = $700.29 saved in one year with our 5.83-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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13 thoughts on “How many square feet do I need for solar panels on my roof?

  1. 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.

  2. 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!

  3. saidas m.g says:

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

  4. Kukati says:

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

  5. 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 !

  6. prince says:

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

  7. Marty says:

    How much

  8. 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?

  9. 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.

  10. Buchi says:

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

  11. 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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