Peak sun hours explained: why your location changes your array size
Two identical daily loads, sized for two different cities, can need arrays that differ by 50% or more — not because of anything about the load, but because of how many "peak sun hours" each location actually gets.
Peak sun hours measure total daily solar energy compressed into hours at 1,000 W/m² — not daylight length. The same daily load needs a much larger array in a low-peak-sun-hour location like Seattle than in a high one like Phoenix, so array sizing must use your local number, not daylight hours.
Peak sun hours are not daylight hours
A location might see 14 hours of daylight in summer, but "peak sun hours" is a different, smaller number: the equivalent number of hours at a standard reference irradiance (1,000 W/m²) that would deliver the same total daily energy as the location's actual, varying sunlight throughout the day. A cloudy, hazy or high-latitude location can have long days but comparatively few peak sun hours; a clear desert climate can have shorter days but a higher peak-sun-hour figure. It's this number, not day length, that array sizing math actually uses.
Why it moves array size so much
Solar array wattage is sized as daily energy need divided by peak sun hours (with a system-loss factor applied). A location with 3 peak sun hours needs roughly double the array wattage of a location with 6 peak sun hours, for the identical daily load. This is the single biggest location-dependent variable in the whole sizing chain — bigger than chemistry choice, bigger than most efficiency assumptions.
Seasonal swing matters more than the annual average
A location's peak sun hours vary substantially by season — often 2–3x between summer and winter at mid and high latitudes. Sizing against only the annual average leaves a system undersized for exactly the months it needs the most margin. For a system that needs to work year-round, size against the worst realistic month, not the yearly average — the same principle behind why arrays sized to summer numbers struggle every winter.
Finding your number
The solar array size calculator uses peak sun hour data by location (sourced from PVGIS irradiation data) so you don't need to look up a separate table — enter a location and it applies the right seasonal-aware figure to your daily load.
Related tools
Sources & standards
- NREL PVWatts — system-loss and array-output modeling reference.
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — general sizing-practice reference.
- NFPA 70 (NEC) Articles 690 & 706 — PV source circuits and energy storage systems.
Educational content, not an electrical design. Verify with your equipment's datasheet and a licensed installer for anything permanently wired. See the methodology.