Solar Panel Size Calculator
Enter your monthly electricity use and sunshine to see how many solar panels you need.
Method: Daily kWh ÷ (peak sun hours × 0.8 system derate), using NREL-style peak sun hours and 400 W, 20 sq ft panels.
Find it on your bill. The average US home uses about 900 kWh a month.
Solar panels needed
21 panels
8.4 kW system · about 420 sq ft of roof
| Daily use | 29.6 kWh |
| System size needed | 8.22 kW |
| Panels (400 W each) | 21 |
| Installed size | 8.4 kW |
| Roof area | 420 sq ft |
| Yearly production | 11,038 kWh |
- • Sun hours mean full-strength sunlight hours per day, averaged over the year. Your local figure is on NREL's PVWatts tool.
- • Panels are assumed 400 W (about 20 sq ft each), and the system loses about 20% to wiring, heat, dust and the inverter.
- • Roof direction, shade and tilt change the result. South-facing, unshaded roofs do best in the northern hemisphere.
Solar panels by monthly use
With about 4.5 peak sun hours and 400 W panels.
| Use per month | System size | Panels | Roof area |
|---|---|---|---|
| 400 kWh | 3.65 kW | 10 | 200 sq ft |
| 600 kWh | 5.48 kW | 14 | 280 sq ft |
| 900 kWh | 8.22 kW | 21 | 420 sq ft |
| 1,200 kWh | 10.96 kW | 28 | 560 sq ft |
| 1,500 kWh | 13.7 kW | 35 | 700 sq ft |
| 2,000 kWh | 18.26 kW | 46 | 920 sq ft |
Sunshine matters
| Region type | Sun hours | Panels for 900 kWh/month |
|---|---|---|
| Cloudy north | 3.5 | 27 |
| Moderate | 4.5 | 21 |
| Sunny | 5.5 | 17 |
| Desert / very sunny | 6.5 | 15 |
How it works
Daily use divided by peak sun hours gives the average power the panels must make. About 20% is lost to heat, wiring and the inverter, so the system is sized up to cover it. Dividing by the wattage of one panel gives the panel count.
More solar guides
Frequently asked questions
How many solar panels to power a house?
For the average US home using about 900 kWh a month, around 21 panels of 400 watts in a moderately sunny place, or about 15 in a very sunny one.
How much roof do solar panels need?
About 20 square feet for each 400 watt panel, so a 21-panel system takes around 420 square feet.
Do I need a battery with solar?
No. Grid-tied systems work without one, but panels alone shut off during a grid outage. A battery keeps your essentials running.