Solar panel calculator: how many panels do I need?
System size (kW) = yearly kWh ÷ (peak sun hours × 365 × 0.86). Divide by your panel's watts for the panel count. 0.86 allows for NREL PVWatts' default 14% system losses.
Panels for an average US home
An average home used 863 kWh a month in 2024 (EIA), or 10,356 kWh a year. With 430 W panels:
| Peak sun hours | System size | Panels | Panel area |
|---|---|---|---|
| 3.75 hours | 8.8 kW | 21 panels | 376 sq ft |
| 4.5 hours | 7.3 kW | 18 panels | 322 sq ft |
| 5.0 hours | 6.6 kW | 16 panels | 286 sq ft |
| 5.75 hours | 5.7 kW | 14 panels | 251 sq ft |
NREL's resource classes run from under 3.75 to over 5.75 kWh/m² per day across the US. An installer's PVWatts estimate for your roof's tilt, direction and shading is the number to trust.
What the estimate leaves out
Shading, roof direction and tilt, snow, and your utility's net-metering rules all change the right size. Most panels quoted to homeowners are 400–460 W (EnergySage). See how many solar panels do I need for worked examples.