How many solar panels do I need?
Divide your yearly electricity use by what one kilowatt of panels makes where you live. That second number is 60% higher in El Paso than in Seattle, which is why a national average gives the wrong answer. Use yours.
Your numbers
10,800 kWh/yr × 100% ÷ 1,366 kWh per kW (U.S. median city — pick a location for yours) = 7.9 kW → 19 panels.
Roof space is usually the real limit. See exactly how many panels fit — and which side of the roof they go on — in the planner.
Fit panels on my roof →Panels needed by monthly usage
420 W panels, 100% of annual use, south-facing 25° roof, PVWatts-style losses. Find your kWh on your bill.
| Monthly use | Cloudy (e.g. Seattle) 1,138 kWh/kW | U.S. median city 1,366 kWh/kW | Sunny (e.g. Phoenix) 1,701 kWh/kW |
|---|---|---|---|
| 500 kWh/month | 13 panels · 5.3 kW | 11 panels · 4.4 kW | 9 panels · 3.5 kW |
| 750 kWh/month | 19 panels · 7.9 kW | 16 panels · 6.6 kW | 13 panels · 5.3 kW |
| 900 kWh/month | 23 panels · 9.5 kW | 19 panels · 7.9 kW | 16 panels · 6.4 kW |
| 1,000 kWh/month | 26 panels · 10.5 kW | 21 panels · 8.8 kW | 17 panels · 7.1 kW |
| 1,250 kWh/month | 32 panels · 13.2 kW | 27 panels · 11.0 kW | 22 panels · 8.8 kW |
| 1,500 kWh/month | 38 panels · 15.8 kW | 32 panels · 13.2 kW | 26 panels · 10.6 kW |
| 2,000 kWh/month | 51 panels · 21.1 kW | 42 panels · 17.6 kW | 34 panels · 14.1 kW |
| 2,500 kWh/month | 63 panels · 26.4 kW | 53 panels · 22.0 kW | 43 panels · 17.6 kW |
| 3,000 kWh/month | 76 panels · 31.6 kW | 63 panels · 26.4 kW | 51 panels · 21.2 kW |
Why installers quote different sizes
Different offset targets (80% vs 110%), different production models, and planned loads like an EV. Under net billing, sizing to 100% is often not the best value. Offset explained.
Will they fit?
A 420 W panel is about 1.72 × 1.13 m (5.6 × 3.7 ft). Fire-code setbacks and vents eat roof space. Lay panels out on your actual roof.