Peak Sun Hours
The number of hours per day when solar irradiance averages 1,000 W/m² — the key location variable that determines how much electricity your panels will actually produce.
Peak sun hours (PSH) is not simply the number of daylight hours — it is a calculated equivalent representing the total daily solar energy at a location expressed as hours at 1,000 W/m² irradiance. A location receiving 5 kWh/m²/day of solar radiation has 5 peak sun hours per day regardless of whether that energy arrived over 14 actual daylight hours.
PSH values vary dramatically by geography and season. The US Southwest receives 5.5–7 PSH (Phoenix averages 6.5); the Pacific Northwest receives 3.0–3.5 PSH; New England 3.5–4.5 PSH; Florida 4.5–5.5 PSH. NREL's PVWatts calculator provides precise location-specific estimates accounting for tilt, orientation, and local weather patterns.
PSH directly determines system sizing. If a home needs 30 kWh/day and the location has 5 PSH, the raw panel capacity needed is 30 ÷ 5 = 6 kW (before losses). Seasonal variation in PSH also determines battery sizing for off-grid systems — the worst-month PSH drives the design.
Real-World Example
The installer used NREL data showing the San Diego home received 5.8 average annual PSH to calculate that the 9.6 kW system would produce approximately 9.6 × 5.8 × 365 × 0.82 = 16,694 kWh/year.