SolarMathKit

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Solar production

How Many kWh Will My Solar Panels Produce?

Enter one panel or your installed panel count to estimate solar energy in kWh per day, month, and year. Use the quick formula for a planning average or enable historical PVWatts/NSRDB weather for a location model. Need panel count from a home or off-grid energy target? Use the sizing calculator or the integrated system planner.

Describe the installed array

Output assumptionsEditable planning values
Seasonal comparisonOptional

Enter sourced scenario values. These do not come from location data.

Location weather estimateOptional · PVWatts / NSRDB

Your exact coordinates are sent to NREL only when you calculate with this option. No address is stored by SolarMathKit.

Your planning estimate

Local calculation

Enter your values, then calculate. Results will show the formula inputs and rounding used.

Formula used

daily kWh = panel count × panel W × peak sun hours × system efficiency × seasonal multiplier ÷ 1,000

The quick estimate uses 30.4375 days for monthly output and 365 days for yearly output, extending the same average-day assumption rather than simulating hourly weather. The optional location-weather section adds a separate PVWatts/NSRDB historical-weather estimate for the coordinates you enter.

See the full assumptions and rounding policy.

Maintenance and verification

Maintained by: SolarMathKit Formula version: 1.3.0 Last reviewed:

The calculator calls the shared, tested TypeScript calculation modules documented in the methodology. Automated tests cover published examples, invalid inputs, rounding, and result invariants.

SolarMathKit maintains this implementation. The review is an internal technical check, not an independent engineering certification.

Worked example: ten 400W panels

Assume 5 peak sun hours, 80% system efficiency, and a seasonal multiplier of 1.00.

  1. Array size: 10 × 400W = 4,000W, or 4kW.
  2. Daily energy: 4,000 × 5 × 0.80 = 16,000 Wh.
  3. Monthly average: 16 kWh × 30.4375 = about 487 kWh.
  4. Yearly estimate: 16 kWh × 365 = 5,840 kWh.
Result: The array estimate is 16 kWh per average day, about 487 kWh per average month, and 5,840 kWh per year.

Common mistakes to avoid

  • Calling a 4kW array a 4kWh array.
  • Using panel STC watts as guaranteed field output.
  • Using the same peak sun hours for every season without checking the planning goal.
  • Treating the yearly extension as a weather-specific production model.

Questions people ask

How much energy does one 400W solar panel produce per day?

At 3 to 6 peak sun hours and 80% system efficiency, one 400W panel produces about 0.96 to 1.92 kWh per average day. Use site-specific sun hours and losses instead of treating that range as a guarantee.

Why is output lower than array watts times daylight hours?

Peak sun hours already compress changing irradiance into an equivalent full-sun period, and the efficiency factor accounts for additional losses.

What is the seasonal multiplier for?

It lets you test a sourced seasonal scenario without changing the base formula. Keep it at 1.00 if you do not have a defensible adjustment.

Does this solar panel output calculator use location weather?

The default quick estimate uses the peak sun hours and seasonal multiplier you enter. If you enable Location weather estimate, the page also models historical PVWatts/NSRDB data for the coordinates you enter. That optional result is not a forecast or a site-specific shade survey.

Can this predict a specific day's output?

No. The quick estimate is an average planning estimate, and the optional location result uses historical typical weather rather than a forecast or hourly site survey.

Before you build: Check voltage, current, temperature, protection, cable sizing and installation rules with the equipment manuals and a qualified professional.