Solar Panel Calculator
Estimate how many solar panels and how large a system you need from your monthly electricity use and local sun hours. Example: 900 kWh a month with 4.5 peak sun hours needs about a 7.65 kW system, or 20 panels of 400 watts.
Show steps
How to use the Solar Panel Calculator
- Enter your average monthly kWh from your electric bill.
- Enter the peak sun hours for your area (4 to 6 in most of the US).
- Enter the panel wattage, system losses and price per watt.
- Press Calculate for panel count, system size, cost and payback.
Formula and method
Daily use = monthly kWh x 12 / 365. System kW = daily use / (peak sun hours x (1 – losses)). Panels = system watts / panel watts, rounded up. Yearly output = array kW x sun hours x (1 – losses) x 365.
Worked example: 900 kWh a month
| Step | Result |
|---|---|
| Daily use (900 x 12 / 365) | 29.59 kWh |
| System size (divide by 4.5 x 0.86) | 7.65 kW |
| Panels (7,646 W / 400 W, rounded up) | 20 |
| Cost at $3 per watt | $22,937 |
Panels needed by monthly use
| Monthly kWh | System size (4.5 sun hours) | 400 W panels |
|---|---|---|
| 400 | 3.40 kW | 9 |
| 600 | 5.10 kW | 13 |
| 900 | 7.65 kW | 20 |
| 1200 | 10.19 kW | 26 |
| 1500 | 12.74 kW | 32 |
What changes the answer
Peak sun hours, roof direction and tilt, shade, panel efficiency and the inverter all change output. Typical system losses are 14 percent. Use the free NREL PVWatts tool for a site-specific estimate before you buy.
Frequently asked questions
How many solar panels do I need?
Divide your daily kWh use by sun hours and system efficiency to get kW, then divide by the panel wattage. 900 kWh a month needs about 20 panels of 400 W.
How many peak sun hours do I have?
Most US locations get 4 to 6 hours. NREL publishes maps and the PVWatts tool.
What are system losses?
Heat, wiring, inverter and dirt losses. 14 percent is a common default.
How much does solar cost?
Roughly $2.50 to $3.50 per watt installed before incentives, though prices vary.
How big a system covers my whole bill?
One that produces about as many kWh a year as you use.
Do batteries change the panel count?
Not the panel count, but they add cost and let you store extra production.
Sources
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