Most US homes need somewhere between 15 and 25 solar panels to cover their electricity use. The exact number comes down to three things: how much electricity you use, how much sun your roof gets, and how powerful each panel is. Here’s the simple way to estimate it.
The quick formula
You can ballpark your panel count in four steps:
- Find your annual usage. Check a year of utility bills, or multiply your monthly kWh by 12. The US average is about 10,800 kWh/year (~900 kWh/month).
- Find your sun hours. Look up your state’s average daily peak sun hours — roughly 3.5 in the Pacific Northwest to 6.5 in the Southwest.
- Calculate system size:
System kW = Annual kWh ÷ (Sun hours × 365 × 0.8). The 0.8 accounts for real-world losses. - Convert to panels: divide system watts by panel wattage (modern panels are ~400 W).
A worked example
A home using 10,800 kWh/year in a region with 4.5 sun hours:
- System size = 10,800 ÷ (4.5 × 365 × 0.8) ≈ 8.2 kW
- Panels = 8,200 W ÷ 400 W ≈ 21 panels
The same home in sunny Arizona (6.5 sun hours) would need only about 14 panels for the same output — sunshine does real work.
Rather skip the math? Our solar calculator does all of this from just your monthly bill and state.
Panel count by usage and sun
| Monthly usage | Low sun (4 hrs) | High sun (6 hrs) |
|---|---|---|
| 500 kWh | ~13 panels | ~9 panels |
| 900 kWh | ~24 panels | ~16 panels |
| 1,400 kWh | ~37 panels | ~25 panels |
(Assumes 400 W panels and an 80% performance ratio.)
What changes the number
Panel wattage. Higher-efficiency panels (420–450 W) mean fewer panels for the same output — useful when roof space is tight.
Roof orientation and tilt. South-facing roofs produce the most in the US. East/west roofs lose roughly 10–20%, which can mean a few extra panels.
Shading. Even partial shade from a chimney or tree can cut output. Installers use this to decide panel placement and whether to add microinverters or optimizers.
How much you want to offset. You don’t have to cover 100% of your usage. Some homeowners size for 80–90% to keep the system smaller and cheaper, especially where net metering is limited.
Will they fit on my roof?
A 400 W panel is about 21 square feet. So 20 panels need roughly 420 sq ft of usable, unshaded roof — well within reach for most single-family homes. Complex or small roofs may push you toward higher-wattage panels to fit the needed capacity.
Bottom line
Take your annual kWh, divide by your sun hours times 292 (that’s 365 × 0.8), and divide the result by 0.4 to get a panel count. For most homes that lands in the high teens to mid-twenties. For a precise, no-math estimate tied to your state’s real sun and rates, run the savings calculator — and see how much it all costs once you have a size.
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