Solar Power Generation Estimator
Estimate the energy a solar array produces from its wattage, peak sun hours, a system derate, and the number of days.
Result
- Daily generation20.00 kWh
- Monthly (~30.4 days)609 kWh
- Annual (365 days)7,300 kWh
- Total over 365 days7,300.0 kWh
- Effective output per kW1,460 kWh/kW/year
Step-by-step
- Daily = (W ÷ 1000) × sun hours × derate = (5,000÷1000) × 5 × 80% = 20.00 kWh.
- Over 365 days: 20.00 × 365 = 7,300.0 kWh.
- Annualized that is 7,300 kWh, or 1,460 kWh per kW installed.
How to use this calculator
- Enter your array size in watts (total rated panel wattage).
- Enter peak sun hours per day for your location (US average ~4–5).
- Set the system derate (80% conservative, 86% PVWatts default).
- Choose the number of days (365 for annual) and read the energy generated.
About this calculator
A solar array’s energy output over time is governed by its size, how much usable sun your location gets, and real-world system losses. This estimator multiplies the array’s rated kilowatts by the peak sun hours per day — the equivalent hours of full-strength (1000 W/m²) sunlight — and by a derate factor that accounts for inverter inefficiency, wiring and soiling losses, and heat. The result is daily energy in kilowatt-hours, which it scales to monthly, annual, or any number of days you choose. The derate matters: NREL’s PVWatts model uses roughly 14% total losses (an 86% factor), while 80% is a more conservative planning number. The tool also reports the effective annual yield per kilowatt installed, a handy figure (often 1,200–1,600 kWh/kW/year in sunny regions) for comparing locations or sizing a system against your electricity usage.
How it works — the formula
Daily kWh = (Watts ÷ 1000) × Peak sun hours × Derate
Total = Daily × Days
Yield = Annual kWh ÷ kW installedRated power times usable sun gives ideal output; the derate trims it to realistic generation, scaled across the chosen period.
Worked examples
- Inputs:
- watts=5000, sunHours=5, derate=80, days=365
- Output:
- 20 kWh/day → 7,300 kWh/year
- Inputs:
- watts=400, sunHours=4.5, derate=80, days=30
- Output:
- 1.44 kWh/day → 43.2 kWh/month
- Inputs:
- watts=10000, sunHours=6, derate=86, days=365
- Output:
- 51.6 kWh/day → ~18,834 kWh/year
Limitations
- Uses average peak sun hours and a flat derate — no seasonal modeling.
- Ignores tilt, orientation, and temperature specifics.
- For precise output use NREL PVWatts for your coordinates.
Planning estimate; validate with PVWatts and your utility data.