Guide
How Many Solar Panels Do I Need?
From your power bill to a system size to a panel count — the back-of-envelope version.
Updated September 2026
Start from your kWh, not your roof
Size solar to your electricity use. Find your average monthly kWh on your utility bill (the US average is around 900 kWh/month). The system size in kilowatts is roughly your daily kWh divided by your area's peak sun hours, with a little headroom for real-world losses.
Try it Solar Panel Calculator → Estimate the solar system size and number of panels to cover your electricity use, from your monthly kWh.A worked example
900 kWh/month is about 30 kWh/day. In a region with 5 peak sun hours, that's 30 ÷ 5 = 6 kW, and dividing by ~0.8 for losses gives roughly a 7.5 kW system. At 400-watt panels, that's about 19 panels.
- Peak sun hours vary a lot by region — the Southwest sees far more than the Northeast.
- Higher-wattage panels mean fewer panels for the same system size.
- Roof space, shading and orientation set the practical ceiling.
What this estimate doesn't include
This is a planning number, not a quote. Actual production depends on roof tilt and azimuth, shading, panel efficiency and inverter losses, and net-metering rules differ by utility. Use it to sanity-check installer proposals — not to replace a site assessment.
FAQ
How many solar panels for a 900 kWh/month home?
Roughly a 7–8 kW system — about 18–20 panels at 400 watts — in a region with ~5 peak sun hours. Sunnier areas need fewer.
How do I find my kWh usage?
It's on your monthly utility bill. Average the last 12 months for a better estimate, since usage swings with the seasons.
Do more efficient panels mean fewer panels?
Yes — higher-wattage panels produce the same system size with fewer units, which helps on a small roof.