Enter your monthly kWh usage and local sun hours. The calculator tells you the system size in kW and approximate panel count your home needs to offset that usage.
Starting estimate. Roof shading and panel choice will refine the final panel count.
The right system is sized to your actual electricity use and adjusted for local sun, not to how much roof you happen to have. Your monthly kWh converts to a daily output target. Divide that by peak sun hours, then apply an 80% efficiency factor to account for inverter losses, wiring resistance, and heat effects on panel output. The result is the system size in kW. A home in Phoenix and an identical home in Seattle with the same electric bill need different sized arrays, sometimes by several kilowatts, because peak sun hours differ that much.
The calculator converts your monthly kWh to a daily target, divides by your local peak sun hours and an 80% efficiency factor to get system size in kW, then divides by 400W per panel to estimate your panel count. Each step accounts for a real constraint on your roof rather than a rule of thumb.
| Monthly kWh / sun hours | System size | Panels (400W) |
|---|---|---|
| 600 kWh / 5.5 hrs (high sun) | 4.5 kW | 12 panels |
| 900 kWh / 4.5 hrs (average sun) | 8.3 kW | 21 panels |
| 1,400 kWh / 3.8 hrs (low sun) | 15.4 kW | 39 panels |
The third row shows why sun hours matter as much as usage: that home uses only 56% more electricity than the second row but needs almost double the panel count, because it also has the least peak sun. Enter any of those three kWh and sun-hour pairs above to confirm the same result.
Once you have a system size, the cost calculator turns it into a price, and the tax credit calculator shows what the federal credit is worth on that price.
Enough kilowatts to cover your daily energy target given local sun hours. For most US single-family homes that works out to 6 to 10 kW, though high-usage households or cloudy locations may need more.
Peak sun hours measure the equivalent number of hours per day at full solar intensity for your location. A site with 5 peak sun hours receives the same total solar energy as 5 hours of noon-strength sunlight, even if daylight lasts much longer.
Real-world systems lose roughly 20% of theoretical output to heat, wiring resistance, and inverter conversion losses. The 0.80 efficiency factor is the standard industry assumption and typically produces accurate results.
No. This is a starting estimate. An installer will refine the count based on your specific roof layout, shading from trees or chimneys, and the exact panel model being proposed.