Science & Energy
Peak Sun Hours
Updated 2025-12-16
Definition
The equivalent number of hours per day when solar radiation averages 1,000 watts per square meter, the standard test condition for solar panel ratings. The U.S. average is 4-5 peak sun hours daily; Phoenix hits 6.5, Seattle under 3.5. Daily panel production = panel wattage × peak sun hours × 0.8 system efficiency factor. A 400W panel in Phoenix produces roughly 400 × 6.5 × 0.8 = 2,080 Wh per day, or 760 kWh per year. NREL publishes free peak sun hour data by ZIP code.
Primary source: NREL solar resource maps
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