How Many Solar Panels Do I Need?

Quick answer: panels by monthly bill

Panel counts for a 100% offset at the US average rate of $0.17 per kWh, 4.5 peak sun hours a day, 400 W panels, and a 78% performance ratio. Usage is bill ÷ rate; the system size is usage × 12 ÷ (4.5 × 365 × 0.78).

Rates vary from about $0.11 in Idaho to over $0.30 in California and Hawaii. A $200 bill at $0.30 is only 667 kWh and needs 16 panels, not 28.

Panels by monthly kWh usage

If your bill prints kWh, use this table instead; it skips the rate and is more accurate. Same assumptions: 100% offset, 4.5 sun hours, 400 W panels, 0.78 performance ratio.

The 1,000 kWh row gets its own walkthrough in how many solar panels for 1,000 kWh per month.

How sun hours change the count

The same 1,000 kWh a month needs very different arrays in Seattle, Kansas City, and Phoenix. Peak sun hours come from NREL's solar resource maps or the PVWatts tool for your address; 3.5 is typical of the Northeast and Pacific Northwest, 4.5 of the Midwest and Southeast, and 5.5 of the Southwest.

1,000 kWh per month, 100% offset, 0.78 performance ratio. Moving from 3.5 to 5.5 sun hours cuts the array by more than a third.

Roof area, orientation, and shade

A modern residential panel measures roughly 3.3 to 3.7 feet by 5.4 to 6 feet, or 18 to 22 sq ft, so the calculator allows 20 sq ft each. That covers the panel and a little spacing but not the fire-code setbacks most jurisdictions require, usually 18 to 36 inches along the ridge and edges. As a rule, count only the roof faces that are flat rectangles of usable area, subtract vents and skylights, and expect the installer to fit 80 to 90% of what the raw square footage suggests.

Direction matters as much as area. In the US a south-facing roof at a 20 to 35 degree pitch gives the sun-hours figure on the map. Panels split between east and west faces produce about 10 to 20% less over the year, and a north face 30% or more less. To keep the same yearly output on an east-west roof, divide the system size by roughly 0.85: a 9.4 kW south array becomes 11.0 kW, or 28 panels instead of 24. Shade is worse than orientation because it hits at the panel level; a chimney or tree shadow crossing three panels every afternoon can cost those panels a third of their output. Trim what can be trimmed, and ask for microinverters or optimizers so one shaded panel does not drag down the string.

Panel wattage trade-offs

Wattage changes the count, not the energy. A 9.4 kW system produces the same 12,000 kWh a year whether it is 27 panels at 350 W or 21 at 450 W, and installers price by the watt, so the system cost is similar. Higher wattage helps when roof space is the constraint, since 450 W panels cut the count by about 22% compared with 350 W ones. The trade-off is that the highest-wattage panels are sometimes physically larger commercial-format modules that fit awkwardly on a cut-up roof, and they can carry a small price premium. For most homes with a clear south face, 400 to 430 W panels are the sensible default.

Do it by hand: a worked example

Take a $150 monthly bill at $0.17 per kWh in a 4.5 sun-hour location, 400 W panels, and a 100% offset:

1. Monthly usage: $150 ÷ $0.17 = 882 kWh.
2. Yearly usage: 882 × 12 = 10,588 kWh.
3. Yearly output per kW installed: 4.5 × 365 × 0.78 = 1,281 kWh.
4. System size: 10,588 ÷ 1,281 = 8.3 kW.
5. Panels: 8.3 ÷ 0.4 = 20.7, rounded up to 21 panels.
6. Roof area: 21 × 20 = 420 sq ft.

If you know your kWh, skip step 1. If you only want to cover 80% of the bill, multiply the yearly usage by 0.8 before step 4. For the cost side, the solar panel cost calculator takes the 8.3 kW figure and applies your local price per watt and the 30% federal credit; the system size calculator explains where the sun-hours and performance-ratio inputs come from in more depth.

Homes with an EV or heat pump

Last year's bill understates next year's usage if you are about to plug in a car or swap a gas furnace for a heat pump. An EV driven 12,000 miles a year at 3.3 miles per kWh uses about 3,600 kWh, or 300 kWh a month. Adding that to a 900 kWh home takes the count at 4.5 sun hours from 22 panels to 29. A heat pump depends heavily on climate and home size: a well-insulated house in the Carolinas might add 2,500 kWh a year, while a large house in Minnesota can add 6,000 or more, and most of it lands in winter when solar output is lowest. Adding 4,800 kWh a year to the same 900 kWh home pushes it to 1,300 kWh a month and 31 panels.

The practical approach is to estimate the new load, add it to your monthly kWh, and enter the total in the calculator, then check that the roof can hold the result. If it cannot, size for today's usage plus the EV and accept a partial offset on the heating, since winter production would fall short of a heat pump's demand anyway. Where your utility credits exports at a low rate, the extra panels for a heat pump pay off less than a battery or a time-of-use plan that charges the car at midday.