Solar Panel Calculator
Want to offset your electric bill with solar? The system size depends on how much power you use and how much sun you get. Enter your monthly kWh and the peak sun hours for your area to estimate the system size, the number of panels, and the roof space they need.
Your numbers
Result
- Number of panels
- 21
- Roof area neededsq ft
- 378
- Est. monthly generationkWh
- 900
| Item | Value |
|---|---|
| System size | 8.33 kW |
| Panels | 21 |
| Roof area | 378 sq ft |
| Est. monthly generation | 900 kWh |
How the system is sized
Divide your daily use (monthly kWh ÷ 30) by the peak sun hours times a loss factor (~80%, for the inverter, wiring and heat). For 900 kWh/month and 4.5 sun hours: 30 ÷ (4.5 × 0.8) ≈ 8.3 kW. With 400 W panels that is about 21 panels, needing roughly 375 sq ft of roof.
Peak sun hours vary by location
Peak sun hours measure how much usable sunlight your area gets per day — roughly 3 in the cloudy Northwest to 6 in the desert Southwest. Use a local figure (solar maps and installer tools have them). Less sun means more panels for the same offset.
An estimate, not a design
This gives the ballpark size. A real design factors in the exact irradiance, your roof's orientation and tilt, shading, the inverter, and local net metering rules that credit power you send back to the grid. Get a site-specific quote from a licensed installer before buying.
Frequently asked questions
Sources
- Residential solar sizing (daily kWh, peak sun hours, panel wattage)
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