SunReckon → Solar Output Calculator
Solar Output Calculator
Estimate how much energy your solar array will make — daily, monthly, and yearly kWh from your system size, peak sun hours, and a real-world derate factor.
Solar & off-grid explainer
Run the numbers first, then read the why. Start with the calculator below — the example values are pre-filled so you can see how it behaves — then keep scrolling for the method, a worked example, and the questions that trip people up. Everything runs in your browser; nothing you type is stored.
Calculator
Key takeaways
- Daily kWh = system kW × peak sun hours × derate factor.
- Peak sun hours bundle a whole day of sunlight into hours of full-strength (1,000 W/m²) sun.
- A ~80% derate covers inverter, wiring, heat, and dust losses.
- A 5 kW system at 4.5 sun hours and 80% derate makes 18 kWh/day (≈6,570 kWh/year).
How to estimate solar output
Solar production comes down to three things: how big your array is, how much usable sun your site gets, and how much energy is lost on the way to the meter. Start with the system's DC rating in kilowatts, multiply by your location's average peak sun hours, then apply a derate factor for real-world losses. The result is the energy your panels actually deliver each day.
The derate factor is the lever most beginners overlook. Panels are rated under lab conditions, but inverter conversion, hot cells, dusty glass, and wiring all shave output — so multiplying by roughly 0.80 turns the nameplate rating into honest, usable AC energy.
Worked example: 5 kW, 4.5 peak sun hours
Daily output = 5 × 4.5 × 0.80 = 18.0 kWh/day. Over a month that's 18 × 30.4 = 547 kWh, and across a year 18 × 365 = 6,570 kWh. Move to a sunnier site with 5.5 peak sun hours and the same array climbs to about 22 kWh/day — sun hours scale output directly.
Daily output by system size
| System size | Daily output | Annual output |
|---|---|---|
| 3 kW | 10.8 kWh | ≈3,942 kWh |
| 5 kW | 18.0 kWh | ≈6,570 kWh |
| 8 kW | 28.8 kWh | ≈10,512 kWh |
| 10 kW | 36.0 kWh | ≈13,140 kWh |
All four assume 4.5 peak sun hours and an 80% derate. Scale them up or down for your own sun hours.
Pair output with the rest of your system
An output estimate is only the first step. Make sure your panel count actually matches the load with the solar array sizing calculator, then see how fast the system pays for itself using the solar payback calculator. Choosing a realistic derate and your worst-month sun hours keeps the numbers honest, especially for off-grid builds that can't lean on the utility.
Frequently asked questions
How much energy will my solar array produce?
Multiply system kW × peak sun hours × derate. A 5 kW system at 4.5 sun hours and 80% derate makes about 18 kWh/day, or ~6,570 kWh/year.
What are peak sun hours?
Hours per day that sunlight averages 1,000 W/m² — full-strength sun. 4.5 peak sun hours equals 4.5 hours of noon-strength sun spread across the day.
What is the derate or loss factor?
It accounts for inverter, wiring, dust, and heat losses. About 80% (0.80) is a common default, so you keep ~80% of the rated DC output as usable AC energy.
How much does a 5kW system make per day and year?
At 4.5 sun hours and 80% derate: ~18 kWh/day, ~547 kWh/month, and ~6,570 kWh/year. Sunnier sites produce proportionally more.
Does output vary by season and location?
Yes — peak sun hours change with latitude, weather, and season. Use a yearly average for annual estimates, or your worst month for off-grid reliability.
Should I use the AC or DC rating?
Enter the DC array rating in kW; the derate converts it to AC energy. The 80% default already includes the inverter's DC-to-AC loss.
The production method here — system size × peak sun hours × derate — follows the model used by NREL PVWatts. Monthly and annual figures are exact arithmetic from the daily estimate.
Last reviewed June 2026