Real Time Solar

Solar Panel Calculator

How many panels your home needs, worked from your actual usage and the peak sun hours where you live — not a national average that is wrong almost everywhere.

In short: Panel count is annual kWh divided by (peak sun hours x 365 x system efficiency), then divided by panel wattage. A typical US home using 10,500 kWh a year needs about 16 panels of 400W at average sun.

Panels needed

20

System size

8.0 kW

Roof space

360 sq ft

Production per kW

1,314 kWh

Roughly 18 square feet per panel including spacing, and 80% system efficiency. The roof figure is usable south-facing area, not total roof area.

How this is worked out

kWh per kW   = peak_sun_hours × 365 × 0.80
system kW    = annual_kWh ÷ kWh_per_kW
panels       = ceil(system_kW × 1000 ÷ panel_watts)

The only location-dependent term is peak sun hours, and it dominates. A 400W panel in Arizona and the same panel in Washington are not the same panel in any way that matters to this calculation, which is why sizing from wattage alone is close to meaningless.

Questions

How many solar panels does an average house need?

For a typical US household using about 10,500 kWh a year with 400W panels, somewhere between 14 and 25 depending on where you live. Arizona needs around 14 because it averages 6.5 peak sun hours a day; Washington needs around 25 at 3.6 hours. Anyone quoting a single national number is ignoring the only variable that matters.

Where do I find my annual kWh usage?

It is on your electricity bill, usually as a 12-month total or a monthly average you can multiply by twelve. Use your real figure rather than the national average — household consumption varies by a factor of three or more, and this is the input everything else is derived from.

Why is the system size bigger than my usage suggests?

Panels never deliver their rated output in practice. Inverter conversion, wiring, heat, dust and imperfect orientation all take a share. We assume 80% of nameplate, which is close to what installers quote; some calculators use 86%, which produces a smaller system than you will actually need.

Will my roof fit them?

The square footage shown allows roughly 18 square feet per panel including spacing. That is usable south-facing roof area, not total roof area — chimneys, vents, dormers and north-facing slopes do not count. If the number is close to what you have, assume it will not fit and get it surveyed.

Panels needed by state

Fewest first, for the same typical household usage.

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