This estimator calculates the capacity factor of a rooftop solar system from actual or forecast annual generation and rated DC capacity. Capacity factor expresses annual energy as a percentage of the output the array would produce if it operated at rated power every hour of the year.
Use it to compare systems of different sizes, check production assumptions, or translate annual generation into a normalized performance measure. It is not the same as module efficiency: efficiency describes conversion of sunlight at an instant, while capacity factor reflects the full year of solar availability and system operation.
1. Enter annual generation
Use metered energy or a production forecast for the same period.
2. Enter rated capacity
Use the array’s total nameplate DC capacity in kilowatts.
3. Confirm period hours
Use 8,760 for a normal year or 8,784 for a leap year when the energy covers the full year.
4. Interpret the normalized result
Review capacity factor together with average power and equivalent full-power hours.
Capacity factor = Annual energy ÷ (Rated capacity × Hours in period) × 100
Equivalent full-power hours = Annual energy ÷ Rated capacity
The energy period and hour count must match. The calculation can use AC or DC capacity, but comparisons are meaningful only when the same convention is used.
The percentage indicates how much of the array’s theoretical full-power annual output was delivered on average.
Capacity factor combines solar resource and system performance; it does not isolate equipment efficiency or weather effects.
Given: 11,300 kWh annual output, 8 kW rated capacity, and 8,760 hours.
Calculation: Maximum theoretical energy = 8 × 8,760 = 70,080 kWh. Capacity factor = 11,300 ÷ 70,080 × 100 = 16.12%.
Result: The rooftop array has an estimated 16.12% capacity factor.
Usually it indicates more annual energy per unit of rated capacity, but climate, orientation, inverter sizing, and curtailment should be considered before judging design quality.
Either can be used if you label it clearly and compare like with like. DC-based and AC-based capacity factors will differ.
Not when energy and capacity are measured consistently over the same period. A result above 100% usually signals a unit, period, or capacity mismatch.
Panel efficiency measures conversion of incident sunlight into electricity under stated conditions. Capacity factor measures delivered energy over time relative to continuous rated output.
Yes. Use monthly energy and the exact number of hours in that month instead of annual values.