Peak Sun Hours by US State (NREL Average)

Why one number decides your array size

A "peak sun hour" is not a clock hour of daylight — it is the amount of sunlight equivalent to one hour at the standard test intensity of 1,000 watts per square metre. It is the honest way to describe a day's solar resource, because the sun is weak at dawn and dusk and strong at noon, and rolling all of that into an equivalent number of full-strength hours is what makes array sizing possible. A location with 5 peak sun hours lets a 100 W panel produce about 500 watt-hours on an average day, before real-world losses.

This table lists NREL-derived geographic averages by US state. Use the figure as the divisor in solar sizing: array watts needed ≈ daily watt-hours ÷ peak sun hours, with a further margin added for panel soiling, wiring and controller losses. The critical judgement is which figure to use. An annual average is fine for a grid-tied system that borrows from the utility in winter, but a year-round off-grid system must be sized against the worst solar month, because December, not the annual mean, is when the batteries run flat.

Feed the number into the Solar Panel Size and Off-Grid System Size calculators. For the whole sizing chain, read how to size an off-grid solar system. These are planning averages; local shading, tilt and orientation change real output.