SolarMathKit

Solar production

Solar panel output calculator

Start with an array you already have or plan to install. The calculator keeps array power in kW separate from the energy it may produce over a day, month, or year.

Runs in your browserEditable assumptionsPlanning estimate

Inputs

Describe the installed array

Output assumptionsEditable planning values

Planning estimate

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

Monthly output uses 30.4375 days and yearly output uses 365 days. Those periods extend the same average-day assumption; they are not hourly weather simulations.

See the full assumptions and rounding policy.

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

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.

Can this predict a specific day's output?

No. It is an average planning estimate and does not read a forecast or model hourly shade and temperature.

Before you build: This calculator is for planning. Check voltage limits, current limits, temperature corrections, protection devices, cable sizing, and installation rules against equipment manuals and a qualified professional.