How many solar panels do I need?
Four numbers size a home system: your yearly use, your local sunshine, the panel wattage and how much you want to cover.
Panels = yearly kWh use ÷ yearly kWh one kW of panels makes where you live ÷ panel size in kW. The average US home (10,791 kWh a year, EIA) in Phoenix needs about 6.1 kW, or 16 panels of 400 W.
1. Find your yearly electricity use
Add up the kWh on your last 12 electric bills; most utilities show a 12-month history on the bill or in your account. With only a dollar amount, divide the yearly total by your energy charge per kWh. A US home averaged about 899 kWh a month in 2022.
2. Find what one kW makes where you live
NREL's PVWatts model estimates yearly output for one kW of panels on a south-facing roof at 20°. The same system makes more in Arizona than in Texas: Phoenix 1,755 kWh, Tucson 1,788 kWh, San Antonio 1,503 kWh, Austin 1,483 kWh, San Diego 1,716 kWh, Fresno 1,654 kWh.
3. Divide, then pick a panel size
System kW = yearly use ÷ kWh per kW. Panels = system kW ÷ panel kW. I use 400 W panels as an example; with larger panels you need fewer of them for the same kW.
| City | kWh a year per kW | 600 kWh a month | 900 kWh a month | 1,200 kWh a month |
|---|---|---|---|---|
| Phoenix, AZ | 1,755 | 4.1 kW · 11 panels | 6.2 kW · 16 panels | 8.2 kW · 21 panels |
| Tucson, AZ | 1,788 | 4.0 kW · 11 panels | 6.0 kW · 16 panels | 8.1 kW · 21 panels |
| San Antonio, TX | 1,503 | 4.8 kW · 12 panels | 7.2 kW · 18 panels | 9.6 kW · 24 panels |
| Austin, TX | 1,483 | 4.9 kW · 13 panels | 7.3 kW · 19 panels | 9.7 kW · 25 panels |
| San Diego, CA | 1,716 | 4.2 kW · 11 panels | 6.3 kW · 16 panels | 8.4 kW · 21 panels |
| Fresno, CA | 1,654 | 4.4 kW · 11 panels | 6.5 kW · 17 panels | 8.7 kW · 22 panels |
Production: NREL PVWatts v8 for the city center, south-facing roof at 20°, 14% losses. Shade and an east- or west-facing roof lower real output.
4. Decide how much to cover
Covering 100% of your use is not always the best deal. Where the utility pays less for power you send to the grid than it charges for power you buy, panels that make more than you use during the day earn little:
- Arizona Public Service (Phoenix): exports 5.55¢ vs about 14.1¢ for power you buy
- Salt River Project (Phoenix): exports 3.45¢ vs about 15.0¢ for power you buy
- Tucson Electric Power (Tucson): exports 5.13¢ vs about 14.8¢ for power you buy
In California, new customers are on net billing, where exports earn hourly avoided-cost values. There a smaller system, or one paired with a battery, can pay back faster than one sized for 100%.
Roof space
Panel sizes differ by model: take the dimensions from the spec sheet in your quote and multiply by the number of panels, then leave room for the edge setbacks your local fire code requires. Your installer measures the real layout.
Run your own numbers
Enter your ZIP and monthly bill: the calculator sizes a system from your bill and local NREL production.
Keep reading
- How much solar panels cost in 2026 — Average price per watt by state and city, and what a 5, 7 or 10 kW system costs before incentives.
- Is solar worth it in 2026? — Payback without the federal credit in the cities I cover, and when solar is a weak deal.
- How much solar panels save — Yearly and monthly savings by utility, and why they change from month to month.
Updated 2026-09-15.