The simple version
A 400-watt panel makes 400 watts only in strong, direct sun. Peak sun hours add up a day's sunlight into "hours of that full-strength sun". A winter day with 9 hours of daylight might give you just 2.5 peak sun hours.
What a panel makes in a day ≈ panel watts × peak sun hours × 0.77 (after wiring and heat losses).
| State (darkest month) | Tilted panels | Flat roof | A tilted 400 W panel makes |
|---|---|---|---|
| Arizona (December) | 4.9 | 3.1 | ~1.5 kWh/day |
| Missouri (December) | 3.2 | 1.9 | ~1.0 kWh/day |
| Washington (December) | 1.8 | 0.9 | ~0.6 kWh/day |
Source: NASA POWER long-term averages at each state's center of population. Every state, every month: winter sun by state.
Why size for the darkest month
Size for the yearly average and you'll run short all winter. Size for the darkest month and every other month is a bonus. That's how every guide on this site does it.
Find yours: winter sun hours by ZIP · Size a system: sizing calculator
Questions people ask
How many peak sun hours do I get?
Look it up for your ZIP with the winter sun hours tool. It shows your darkest month, which is what to size for.
Is peak sun hours the same as daylight hours?
No. A long day with low or hazy sun might only add up to 2–3 hours of full-strength sun.
Get the exact setup
A few taps and you'll know exactly what to buy for your situation.