SolarMathKit

Solar array sizing

Solar panel calculator

Convert daily or monthly energy use into a solar array and whole-panel count. Peak sun hours and system efficiency stay manual so the calculator never hides a location assumption.

Runs in your browserEditable assumptionsPlanning estimate

Inputs

Size the solar array

Panel dimensions for roof areaOptional
Advanced 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

required array W = daily Wh × coverage ÷ (peak sun hours × system efficiency)

The theoretical array is divided by the selected panel rating and rounded up. Production is then recalculated from the installed whole-panel array rather than the smaller theoretical value.

See the full assumptions and rounding policy.

Worked example: 10 kWh per day

Assume 5 peak sun hours, 80% system efficiency, 100% coverage, and 400W panels.

  1. Required array: 10,000 Wh ÷ (5 × 0.80) = 2,500 W.
  2. Theoretical panel count: 2,500 ÷ 400 = 6.25.
  3. Round upward to 7 panels, or 2,800 W installed.
  4. Installed daily output: 2,800 × 5 × 0.80 = 11,200 Wh.
Result: The planning result is seven 400W panels, a 2.8 kW installed array, and about 11.2 kWh on the entered average day.

Common mistakes to avoid

  • Using daylight hours instead of peak sun hours.
  • Entering a loss percentage where an efficiency percentage is expected.
  • Rounding panel count down from a fractional result.
  • Showing production from the theoretical array after a larger whole-panel array is installed.

Questions people ask

Where do I get peak sun hours?

Use a reputable solar-resource source for the site and season you want to plan around. NREL PVWatts is one useful check for US locations.

What does system efficiency include?

It is a combined planning factor for temperature, wiring, controller, inverter, mismatch, and other losses. Replace the preset with a value supported by your design inputs.

Does this calculate cost or tax credits?

No. Prices, incentives, and policy change by location and date, so they are outside the first release.

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.