Solar System Size Calculator

How to use this calculator

Enter Annual use (kWh), Peak sun hours/day, Performance (%). The result updates in your browser as you edit an input. The prefilled values are examples only.

How the estimate works

Divide annual kWh by 365, peak sun hours, and the performance factor. A site-specific model is needed for a reliable design.

What can change the answer?

Peak sun hours is not the same as daylight hours. Shade, orientation, snow, temperature, and inverter losses require a location-specific production model.

Before relying on the result

Check that the figures use the same units and period. Compare the result against a current label, quote, contract, employer policy, or official dataset when appropriate. This estimate does not include factors you have not entered.

Further reference

For source data or a more detailed method, see National Laboratory of the Rockies: PVWatts production model. The calculator does not automatically import data from this source.

Worked example with the default inputs

Annual use of 12,000 kWh, 4.5 peak sun hours and an 80% performance factor imply 12,000 divided by 1,314, or 9.13 kW. This is a simplified capacity relationship, not a roof design.

Interpret the result in a planning record

The solar system size calculator divides annual use by the number of days, peak sun hours and the performance factor you enter. Peak sun hours describe equivalent full-intensity sunshine, not the length of the daylight period. Shade, orientation and seasonal conditions can change production, so the result is sensitive to the assumptions you choose.

Save the input values, units and result together. Change one uncertain input at a time so you can see which assumption moves the answer. A displayed decimal is arithmetic precision, rather than evidence that the measurements or rates are equally precise.

More tools

Read the How to Read a Solar Quote Without Mixing Up the Numbers guide →