How to use this solar panel calculator
- Find your average monthly kWh on your electric bill (average 12 months if possible).
- Look up peak sun hours for your location (3–6 in most of the US).
- Enter the panel wattage you are considering.
- Keep 80% efficiency to account for inverter, wiring, heat and shading losses.
Example
A home using 1,200 kWh per month in a sunny area with 5 peak sun hours needs a system of about 9.87 kW, which is 25 × 400 W.
What to know before you buy
How much roof space do you need?
A typical 400 W residential panel measures about 5.6 × 3.7 ft (around 21 sq ft). Allow extra space for setbacks required by fire codes around roof edges and ridges.
Batteries and net metering
With net metering, extra daytime production is credited by your utility, so batteries are optional. Without it, or for backup power during outages, a battery lets you store solar energy for the evening and night.
Panels (400 W) by monthly use, 4.5 peak sun hours, 80% efficiency
| Monthly use | Panels needed | System size |
|---|---|---|
| 500 kWh | 12 × 400 W | 4.57 kW |
| 750 kWh | 18 × 400 W | 6.85 kW |
| 900 kWh | 21 × 400 W | 8.22 kW |
| 1,200 kWh | 28 × 400 W | 10.96 kW |
| 1,500 kWh | 35 × 400 W | 13.71 kW |
| 2,000 kWh | 46 × 400 W | 18.27 kW |
Formula
System kW = monthly kWh ÷ 30.4 ÷ (peak sun hours × efficiency). Panels = system W ÷ panel W. A 400 W panel is about 21 sq ft.
Tips from the pros
- South-facing roofs (in the northern hemisphere) at a 20–35° tilt produce the most energy.
- Look up your local peak sun hours with the free NREL PVWatts tool.
- Check net metering rules with your utility before sizing the system.
Common mistakes to avoid
- Using one summer month's bill instead of the yearly average.
- Ignoring shading from trees or chimneys, which can cut output significantly.
Frequently asked questions
How many solar panels to power a house?
An average US home using about 900 kWh per month needs roughly 8 kW, or about 21 panels of 400 W, with 4.5 peak sun hours and typical system losses.
What are peak sun hours?
The number of hours per day when sunlight averages 1,000 W per square meter. It depends on your latitude and climate.
How many kWh does a solar panel produce?
A 400 W panel with 4.5 peak sun hours produces about 1.4–1.8 kWh per day, or 40–55 kWh per month.
How many solar panels for 1,000 kWh per month?
About 23 panels of 400 W with 4.5 peak sun hours and 80% efficiency.