Peak Sun Hours Calculator

Peak sun hours are not daylight hours

A peak sun hour is one hour at 1,000 W per square metre — full standard irradiance. A summer day with fifteen hours of daylight might deliver five or six peak sun hours, because early morning and late evening contribute very little. The figure compresses the whole day into an equivalent number of hours at full strength, which is exactly what array sizing needs.

The seasonal swing is the design constraint

In northern Europe, a location averaging four peak sun hours across the year may see six in June and less than one in December. A grid-tied system can average over the year because the grid fills the gaps. An off-grid system cannot, and has to be sized against the worst month or paired with a generator.

Tilt and orientation change the answer

The same location gives different peak sun hours depending on how the panels face. A steeper winter tilt trades summer yield for winter yield, which is often the right bargain off-grid and rarely the right one on a grid-tied roof, where total annual output is what gets paid for.