Solar Panels Needed Calculator
Find how many solar panels cover a target daily energy use, using peak sun hours and a system derate factor.
The formula, every assumption, and the primary sources behind this tool are published on this page.
Results
Panels needed: 8 panels
Exact (unrounded): 7.41
Array size: 3.2 kW
Estimated production: 10.8 kWh/day
Solar panels needed for 5, 10, or 20 kWh per day
| Daily use | 3 h sun | 4.5 h sun | 6 h sun |
|---|---|---|---|
| 5 kWh/day | 6 panels | 4 panels | 3 panels |
| 10 kWh/day | 12 panels | 8 panels | 6 panels |
| 20 kWh/day | 23 panels | 15 panels | 12 panels |
Select a result to load its inputs into the calculator.
Storing energy for nights or cloudy days? Size an off-grid battery bank.
How it works
Each panel produces its wattage times the day's peak sun hours, cut by real-world losses. Divide your daily need by that and round up.
panels = ceil(daily_kwh × 1000 / (panel_w × peak_sun_hours × derate))
daily_kwh- Energy you want the array to cover per day, in kilowatt-hours.
panel_w- Nameplate rating of one panel, in watts.
peak_sun_hours- Equivalent hours of full-strength sun per day for your site.
derate- Fraction of ideal output after losses (soiling, wiring, inverter, heat).
panels- Whole panels required, rounded up.
Assumptions
- Peak sun hours default to 4.5 (a mid-range U.S. annual average). Your location may be higher or lower — check the NREL map.
- The 0.75 derate follows NREL PVWatts' default system losses plus typical inverter loss. Cleaner installs may reach 0.80.
- This sizes panels to daily energy. It does not add battery-charging overhead — pair it with the battery bank calculator for off-grid storage.
- Panel count is rounded up: a fractional panel can't be installed, so the array slightly over-produces.
FAQ
How many solar panels do I need for 10 kWh a day?
With 400 W panels, 4.5 peak sun hours and a 0.75 derate, each panel makes about 1.35 kWh/day, so 10 kWh needs 8 panels (a 3.2 kW array).
What are peak sun hours?
Peak sun hours are the equivalent hours per day of 1000 W/m² sunlight. A location averaging 4.5 gets the same daily energy as 4.5 hours of full-strength sun, spread across the day.
Why round up the number of panels?
You can't install a fraction of a panel, and undersizing leaves you short on cloudy days. Rounding up gives a small production margin.
Sources
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