The number of solar panels you need depends on two things: how much electricity you use and how much sun your location gets. Get those two numbers and the sizing is straightforward arithmetic.
This guide walks through the calculation, gives a worked example, and shows panel counts by usage. To turn a system size into a payback estimate, use the solar payback calculator.
The three-step sizing method
Step 1 — Find your annual usage. Add up the kWh from 12 months of electricity bills. The US average is about 10,800 kWh/year (roughly 900 kWh/month), per EIA, but yours may differ a lot.
Step 2 — Divide by your production factor. The production factor is how many kWh each kW of panels makes per year in your area — from local sun hours. It ranges from about 1,800 kWh/kW in sunny Arizona down to about 1,090 in cloudy Washington.
System size (kW) = Annual usage (kWh) ÷ Production factor (kWh/kW/yr)
Step 3 — Convert kW to panels. Divide the system watts by your panel wattage (modern residential panels are about 400 W).
Panel count = System size (kW) × 1,000 ÷ Panel wattage (W)
Worked example
A home using 10,800 kWh/yr in a state with a 1,500 kWh/kW production factor:
| Step | Calculation | Result |
|---|---|---|
| System size | 10,800 ÷ 1,500 | 7.2 kW |
| Watts | 7.2 × 1,000 | 7,200 W |
| Panels (at 400 W) | 7,200 ÷ 400 | 18 panels |
| Roof space (~19 sq ft each) | 18 × 19 | ~340 sq ft |
So about 18 panels to cover this home’s usage. In sunny Arizona the same home needs only ~6 kW (about 15 panels); in Washington it needs ~10 kW (about 25 panels).
Panel count by annual usage
Assuming 400 W panels and a mid-range 1,500 kWh/kW production factor:
| Annual usage | System size | Approx. panels (400 W) |
|---|---|---|
| 6,000 kWh | ~4.0 kW | ~10 |
| 9,000 kWh | ~6.0 kW | ~15 |
| 10,800 kWh (US avg) | ~7.2 kW | ~18 |
| 13,000 kWh | ~8.7 kW | ~22 |
| 16,000 kWh | ~10.7 kW | ~27 |
In a sunnier state, shift down a row or two; in a cloudier one, shift up. Your exact production factor is on each state page.
Should you size for 100% of your usage?
Not always. The right target depends on your net-metering rules:
- Full retail net metering: sizing to cover ~100% of annual usage usually maximizes savings, because exports earn the same rate you’d pay.
- Net billing / low export value (e.g. California’s NEM 3.0, much of the Southwest): exported power is worth less, so sizing closer to your daytime self-consumption — or adding a battery — often improves payback. See net metering basics.
Other practical limits:
- Roof area and orientation — south-facing, unshaded roof produces most; east/west lose ~10–20%.
- Budget — bigger systems cost more up front, which matters more in 2026 now that the federal tax credit has expired.
- Utility interconnection caps on system size.
From panel count to payback
Once you have a system size, the financial question is how fast it pays back. Multiply size by cost-per-watt (typically $2.50–$3.50/W in 2026) for the price, then run it through the payback calculator. For the formulas behind the result, see solar payback period explained.
General information only. Always confirm sizing with an installer site assessment, which accounts for shading, roof pitch and local code.