Understanding peak sun hours
What a peak sun hour actually measures
A peak sun hour is a unit representing one hour of sunlight at an intensity of 1,000 watts per square meter. It's the standard way solar calculators express a location's total daily solar energy, and it's the single biggest input in most system sizing formulas, including the annual kWh output estimator. A DOE report on federal PV performance notes that solar resource data carries roughly plus-or-minus 10% uncertainty even from high-resolution satellite sources.
Typical peak sun hour ranges across the US
Most US locations fall between 3.5 and 6.5 peak sun hours per day. Sunbelt cities like Phoenix often see 6 or more, while cloudier Pacific Northwest cities like Seattle typically see closer to 3.5 to 4. This single number explains most of the output difference between otherwise-similar systems installed in different parts of the country. National Solar Radiation Database show this exact regional spread visually across the country, making it easy to see at a glance roughly where your own location falls within that national range.
Why latitude alone doesn't tell the whole story
Latitude sets the theoretical maximum for how much sun a location could receive, but local climate determines how much of that potential actually gets captured. A humid, cloudy city and an arid, clear city at the same latitude can see meaningfully different real-world sun hours, which is why this tool asks for both inputs separately rather than relying on latitude alone.
How this differs from daylight hours
No. Daylight hours count the full period from sunrise to sunset, while peak sun hours represent the equivalent time at full, peak-intensity sunlight. A location might have 14 hours of daylight in summer but only 5 or 6 peak sun hours once weaker morning and evening light is accounted for. See the capacity factor calculator for a related metric that captures this same idea from a different angle.
What to do with this number next
Plug your peak sun hours figure directly into the annual output estimator to get a full system-level output estimate. If you're planning an off-grid system rather than a grid-tied one, the off-grid battery bank sizing calculator uses this same peak sun hours figure as one of its core inputs, since there's no utility backup to smooth over a low-sun stretch.