Solar Panel Output Calculator
A Panel's Nameplate Rating Is Not What It Actually Produces
A "400-watt" solar panel only produces 400 watts under laboratory test conditions that rarely occur on a real roof — full sun, a specific panel temperature, and zero shading or soiling. Real-world output depends on how many hours of strong, direct sunlight the site actually gets and how efficiently the whole system converts that sunlight into usable electricity. This calculator combines those factors into a daily and annual energy figure.
The Formula
"Sun hours" refers to peak sun hours — the number of hours per day the sunlight would need to shine at its peak intensity to deliver the same total energy the site actually receives, not the number of daylight hours. System efficiency accounts for inverter losses, wiring losses, temperature effects, and other real-world derating.
Where This Is Useful
- Sizing a solar array — check whether a proposed panel count and wattage will actually meet a household's energy needs.
- Evaluating installer quotes — verify the annual production figure a solar installer quotes against an independent calculation.
- Comparing locations — the same panel setup produces very different output depending on regional peak sun hours.
Worked Example
Ten 400-watt panels, 5 peak sun hours per day, at 85% system efficiency:
| Panel wattage | Number of panels | Sun hours/day | Efficiency | Daily output | Annual output |
|---|---|---|---|---|---|
| 400 W | 10 | 5 h | 85% | 17.00 kWh/day | 6,205 kWh/year |
How to Use This Calculator
- Enter the Panel Wattage in watts.
- Optionally enter the Number of Panels — defaults to 1 if left blank.
- Enter the Peak Sun Hours per Day for the site.
- Optionally enter System Efficiency % to account for real-world losses — defaults to 100% if left blank.
- Select Calculate to get daily and annual energy output.
Related Calculations
Work backward from a target energy need with the Solar Panel Size Calculator, or see how this output translates to savings with the Solar Payback Period Calculator.