Peak Sun Hours

Peak sun hours (PSH) is a standardized way to express the total daily solar energy at a location as the equivalent number of hours at full 1,000 W/m² irradiance. It simplifies production calculations by converting the variable sunlight curve throughout a day into a single number that directly multiplies by panel wattage to estimate daily energy output.

For example, a location receiving 5.5 peak sun hours per day does not actually get 5.5 hours of 1,000 W/m² sunshine. Instead, the total accumulated irradiance throughout the entire day — including weaker morning, evening, and partially cloudy periods — adds up to the energy equivalent of 5.5 hours at full intensity. A location might have 10 hours of daylight but only 5.5 peak sun hours because much of that daylight is at reduced intensity.

Peak sun hours directly predict daily energy production. A 400W panel in an area with 5 peak sun hours produces approximately 400W × 5h = 2,000Wh = 2 kWh per day before derating for temperature, wiring, and inverter losses. After real-world losses (typically 15-25%), expect about 1.5-1.7 kWh per day from that panel.

US peak sun hours range from about 3.5-4.0 in the Pacific Northwest and Great Lakes region to 6.0-7.0 in the Desert Southwest. Seasonal variation is significant — winter PSH in northern states can drop to 2-3 hours while summer PSH reaches 6-7 hours, even at higher latitudes.

When sizing a solar system, use your location's annual average peak sun hours for grid-tied systems (net metering smooths seasonal variation) and your worst-month PSH for off-grid systems (which must perform adequately year-round without grid backup).