How many panels do you need?
Sample dataWork out roughly how many panels your own electricity use needs, before anyone tells you. It is the fastest way to spot a quote sized to a commission rather than to your house — in either direction.
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How this works
- SAMPLE PRODUCTION FACTORS. The kW-per-annual-kWh figures here are broad regional rules of thumb and are marked sample. The accurate answer for your specific roof comes from NREL's PVWatts model, which is free, public, and what our payback tool uses — it is linked below and you should run it.
- The arithmetic in one line: your annual kilowatt-hours, divided by how many kilowatt-hours a kilowatt of array produces where you live in a year, gives the kilowatts of array you need. Divide that by the wattage of the panels being quoted and you have a panel count.
- Find your annual usage on your own bills — most utilities show twelve months of history on the bill or in your online account. Use a real year rather than a guess; it is the input everything else scales from.
- Panel wattage has moved a lot. Residential panels quoted today are commonly in the 400 to 450 watt range, so a 7 kW array is roughly 16 to 18 panels. Ask what wattage is being quoted, because the panel count on its own tells you nothing.
- Orientation and shade change the answer more than most people expect. An east or west facing array is not a problem, it simply needs more panels for the same annual output. Shade is different: it is the one constraint no design fully solves, and it should be measured rather than eyeballed.
- Sizing to 100% of your usage is a convention, not a law. What is actually right depends on what your utility pays for exported power, which is set by your state and your utility — see the net-metering tool. Where exports are credited poorly, a smaller array that covers less of your usage can be the better investment.
- Be deliberate about sizing for future load. An electric car or a heat pump genuinely raises your usage, and it is also the most common reason a quote is bigger than your bills justify. Both can be true; decide it yourself rather than accepting it.
- Nothing you enter leaves your device.
Sources
- NREL Developer Network: PVWatts v8 API documentation
- NREL: PVWatts Calculator
- NREL: PVWatts solar resource data and NSRDB background
- U.S. Energy Information Administration: Electric Power Monthly, average retail price of electricity by state and sector
- U.S. Department of Energy: Homeowner's guide to going solar
- U.S. Energy Information Administration: Residential Energy Consumption Survey (average household electricity use)
- Expert review
- Not yet reviewed
- Last verified
- September 18, 2026
A rough sizing guide, not a system design. The production factors are SAMPLE regional rules of thumb; NREL's PVWatts gives a far better answer for your actual roof and is free to use. What size is right for you also depends on what your utility pays for exported power.
Common questions
How do I find my annual kWh?
Your utility bill, or your online account, will show twelve months of usage history. Add the twelve months up, or take the figure the bill gives you. Use a real year rather than estimating from one month, because summer and winter differ enormously and the annual total is what an array is sized against.
How many panels is a kilowatt?
At today's common residential panel wattages of roughly 400 to 450 watts, a little over two panels per kilowatt. So a 7 kW array is about 16 to 18 panels. Always ask what wattage is on your quote — a panel count without a wattage is not a specification.
Should I size the system to cover all my usage?
Not automatically. It depends on what your utility pays for power you export. Where exports are credited at the retail rate, covering all of your usage makes sense. Where they are credited at a lower wholesale rate, a smaller array that you mostly consume yourself can pay back better. Our net-metering tool explains how to check which applies to you.
My quote is much bigger than this suggests. Is that wrong?
Not necessarily, but it deserves a specific answer. Legitimate reasons include an electric car, a planned heat pump, or a roof where shade or orientation cuts production. The reason you should be alert to is that a bigger array is a bigger sale. Ask which one applies and ask to see the production model.
Why should I run PVWatts myself?
Because it is the national laboratory's own model, it is free, and it takes about two minutes. It gives you an independent production figure for your address, roof pitch, orientation and shading — which is exactly the number an installer's payback model rests on. Having your own copy of it changes the conversation.
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