How Many Solar Panels Do I Need?

What the calculator counts

This tool answers one question: how many solar panels cover your electricity use. It does not quote an installed price, and it does not guess your roof shape. It takes the power you already use, the panels you might buy, and the sun where you live, then returns a panel count, a system size in kilowatts, and the roof area those panels need.

Start with your yearly use in kilowatt hours. That number sits on your electric bill as a 12 month total, or you can add up the last twelve monthly readings. The U.S. Energy Information Administration's residential data shows the average U.S. home used about 10,791 kWh in 2022 and about 10,359 kWh in 2024, so most households land near 10,000 to 12,000 a year.

The second input is panel wattage. New home panels are commonly rated at 400 watts, but you will see 350 watt and 450 watt models. A higher wattage means fewer panels for the same power, which matters when roof space is tight. The third input is sun hours, the average hours per day your roof gets full strength sun. Put those three together and the math is direct.

The four numbers that set your panel count

Four inputs drive every result. Yearly kilowatt hours is your demand. Panel wattage is the size of each unit. Sun hours is your local resource. System efficiency is the losses from wiring, inverter, heat and dirt, usually about 80 percent for a well built home array.

The chain runs like this. Annual production per panel equals wattage divided by 1,000, times sun hours, times 365 days, times efficiency. A 400 watt panel at 4.5 sun hours and 80 percent efficiency makes 0.4 x 4.5 x 365 x 0.80 = 526 kWh a year. Divide your yearly use by that and you get the number of panels, rounded up because you cannot buy a fraction of a panel.

System size in kilowatts is the panel count times the wattage, divided by 1,000. Twenty 400 watt panels are 8,000 watts, or 8 kilowatts. That is the number installers quote when they size a system, and it is the number your permit and utility application will ask for.

Worked example: a typical U.S. home

Take the average home: 10,359 kWh a year, 400 watt panels, 4.5 sun hours, 80 percent efficiency, and a goal of covering 100 percent of use. Step one is system size. 10,359 / (4.5 x 365 x 0.80) = 10,359 / 1,314 = 7.88 kW.

Step two is the panel count. 7.88 kW is 7,883 watts. Divide by 400 and you get 19.7, which rounds up to 20 panels. Step three is the proof. Twenty panels at 0.4 kW each make 8.0 kW, and 8.0 x 4.5 x 365 x 0.80 = 10,512 kWh a year, a hair above the 10,359 you use, so the array covers the home with a small buffer.

Step four is the footprint. A 400 watt panel is about 18 square feet, so 20 panels need about 360 square feet of clear, south facing roof. At the U.S. average residential price of 16.48 cents per kWh in 2024, per the Energy Information Administration's 2024 price data, those 10,512 kWh are worth about 10,512 x 0.1648 = $1,732 a year on the retail bill, which is the value the system offsets.

Worked example: the same home in Seattle

Keep the same 10,359 kWh home and the same 400 watt panels, but drop sun hours to 3.5, the rough NREL average for Seattle. System size becomes 10,359 / (3.5 x 365 x 0.80) = 10,359 / 1,022 = 10.13 kW.

In watts that is 10,134, divided by 400 gives 25.3, which rounds up to 26 panels. The proof: 26 x 0.4 x 3.5 x 365 x 0.80 = 10,629 kWh a year, again just over the home's use. Roof area is 26 x 18 = 468 square feet, about 30 percent more roof than the 4.5 sun hour case for the exact same house.

This is the single biggest reason two neighbors in different states need different arrays. The home did not change. Only the sun changed, and that alone moved the count from 20 panels to 26. If you live in the cloudy Pacific Northwest, plan for more roof than the national average suggests.

Worked example: a smaller home in Phoenix

Now a smaller home that uses 7,000 kWh a year, with strong sun. Phoenix averages about 6.5 sun hours a day per NREL data. System size is 7,000 / (6.5 x 365 x 0.80) = 7,000 / 1,898 = 3.69 kW.

In watts that is 3,687, divided by 400 gives 9.2, which rounds up to 10 panels. The proof: 10 x 0.4 x 6.5 x 365 x 0.80 = 7,592 kWh a year, covering the 7,000 kWh home with room to spare. Roof area is only 10 x 18 = 180 square feet.

Put the three homes side by side. The small Phoenix house needs 10 panels on 180 square feet. The average home at 4.5 sun hours needs 20 panels on 360 square feet. The Seattle home needs 26 panels on 468 square feet. Same country, same panels, different roofs, because sun hours do the heavy lifting.

Sun hours by city, and why they matter

Sun hours are not the hours of daylight. They are the hours per day when sunlight hits your roof at full panel strength, about 1,000 watts per square meter. The National Renewable Energy Laboratory's PVWatts tool publishes these averages by location, and they range from the low 3s in the Pacific Northwest to the high 6s in the desert Southwest.

These counts use 400 watt panels and 80 percent efficiency for a 10,359 kWh home, so you can see the spread at a glance. A roof that faces east or west, or that leans steeply, will land between these numbers. Shade from trees or a chimney cuts production further, which the calculator cannot see, so add a panel or two if your roof is not ideal.

From panels to roof space and system size

Two numbers turn a panel count into something you can plan a roof around. System size in kilowatts tells the installer and the utility how big the array is. Roof area tells you whether it fits.

Each panel is about 18 square feet, so roof area is just panels times 18. Before you trust a count, measure the clear, unshaded, south facing space on your roof and compare it to the area column. A typical U.S. home has enough roof for a 20 panel array, but a cut up roof with vents and skylights may not fit a 26 panel one without using the east or west face.

The Department of Energy's homeowner guide to going solar notes that roof condition, age and orientation all shape what you can build, so treat the panel count as a target the site survey then confirms.

What the panels are worth each year

The panel count is only useful once you know what it saves. Multiply yearly output by your retail rate to see the bill value the array offsets. At the 2024 U.S. average of 16.48 cents per kWh, the 20 panel average home makes 10,512 kWh worth about $1,732 a year. In a high price state the same panels save far more, and in a low price state they save less.

This is the value the system replaces on your bill, not the payback or the price you pay to install it. Installation cost, incentives and financing sit on top of this number, and they vary by state and by year. The calculator here stays focused on the panel count and the power, because those are the parts the sun and your roof decide, and they are the parts you can check yourself before any sales call.