Peak Sun Hours from Insolation

The sun's irradiance traces a curve across the day — nothing before dawn, a peak near solar noon, nothing after dusk. Peak sun hours collapse that curve into a rectangle of the same area: the number of hours of full 1000 W/m² sun that would have delivered the identical total energy. It is a fiction, and a very useful one, because it turns an integral into a multiplication.

The convention's charm is that the arithmetic disappears. Since the reference irradiance is exactly 1 kW/m², an insolation quoted as 4.5 kWh/m² per day is 4.5 peak sun hours — same number, different name. The collecting area cancels out of the definition, so measuring the energy on one square metre or on a whole array gives the same answer.

Peak sun hours are not daylight hours, and confusing the two is the most reliable way to overstate a solar design by a factor of three. A June day in southern Canada holds sixteen hours of daylight and delivers about five to six peak sun hours. The early and late sun arrives at a shallow angle through a long slanted path of atmosphere and contributes very little; only the hours around noon approach the reference. Annual figures run roughly 3.5–4.5 kWh/m² per day across most of populated Canada and northern Europe, and 5.5–6.5 in the American southwest — a difference of well under a factor of two, which surprises people who expect the desert advantage to be larger.

Two cautions on the data. Solar resource figures are published for a particular surface — global horizontal, direct normal, or global tilted at some angle — and they are not interchangeable; using a horizontal figure for a tilted array understates the winter resource badly. And what the number hides is timing, which makes it the right tool for annual grid-tied yield and the wrong one for sizing an off-grid battery bank, where a December figure of 1.5 hours governs everything and the annual average is a comfortable irrelevance.