Solar Insolation Calculator — Peak Sun Hours for Your Location

Peak sun hours (PSH) measure the total daily solar energy available at a location, expressed in a standardized unit: the number of hours during which the sun would need to shine at exactly 1,000 watts per square meter (W/m²) to deliver the same energy as the actual day’s full range of solar intensity. A location might have 14 hours of daylight in summer, but if clouds mean average irradiance is only 500 W/m², the PSH is 14 × 0.5 = 7.0 PSH — not 14. Solar panel output ratings are tested at exactly 1,000 W/m² (STC), so PSH directly tells you how many equivalent “full power hours” your panels produce per day. A 400W panel in Phoenix (6.5 PSH) produces approximately 400 × 6.5 = 2,600 Wh = 2.6 kWh per day (before system efficiency derating).

Tilt angle significantly affects how much solar energy your panels capture. A flat (0°) panel captures Global Horizontal Irradiance (GHI). Tilting the panels toward the sun means they intercept the sun’s rays more directly for more hours of the day. At most US locations, the optimal tilt angle for maximum annual energy production is approximately equal to the site’s latitude. In Denver, CO (latitude 39.7°), the optimal tilt is about 37–38°. Tilting at the optimal angle typically increases annual production by 10–17% compared to a flat mount. Vertical panels (90°) receive only about 65–75% of optimal tilted production for most US locations.

Yes, significantly. In the Northern Hemisphere, south-facing panels (180° azimuth, true south) capture the most annual energy. Panels within ±15° of due south lose less than 1% of annual output. Panels facing due east or due west (±90° from south) produce roughly 13–20% less annually than south-facing panels at the same tilt. East-facing panels produce more in the morning — valuable for time-of-use rates if morning rates are highest. West-facing panels produce more in the afternoon. For maximum total annual energy, south-facing at optimal tilt is always the winner.

Global Horizontal Irradiance (GHI) is the total solar energy received on a horizontal (flat) surface per unit area. This is what “peak sun hours” typically refers to and is the baseline data in this calculator. Direct Normal Irradiance (DNI) is the solar energy received directly from the sun’s disk, measured perpendicular to the sun’s rays. This is the component concentrated solar power (CSP) systems use and is highest in clear, dry climates like deserts. Diffuse Horizontal Irradiance (DHI) is the scattered solar radiation from the sky (excluding direct sun), measured on a horizontal surface. Cloudy regions like Seattle have a high proportion of DHI. For standard flat-panel PV systems, GHI is the most relevant metric.

Seattle and Phoenix have similar summer daylight hours (~15.5 hours in June), but dramatically different solar production. The difference comes from three factors: (1) Cloud cover: Seattle averages 226 cloudy days per year vs. Phoenix’s 85. Phoenix’s June PSH is about 8.1; Seattle’s is about 5.8 — even in summer. (2) Winter performance: Seattle’s December PSH is about 1.3 hr/day; Phoenix’s December is 5.1 hr/day. Seattle’s annual average (3.8 PSH) is only 58% of Phoenix’s (6.5 PSH). (3) Atmospheric clarity: Phoenix’s dry desert air transmits more solar energy than Seattle’s humid marine air. A 10 kW solar system in Phoenix produces about 17,000 kWh/year; the same system in Seattle produces about 9,500 kWh/year.

Step 1: Find your annual electricity consumption in kWh (from your utility bill — typically 8,000–15,000 kWh/year for US homes). Step 2: Divide by (PSH × 365 × system efficiency): System Size (kW) = Annual kWh ÷ (PSH × 365 × 0.78). Example: 12,000 kWh/year in Colorado (5.5 PSH): 12,000 ÷ (5.5 × 365 × 0.78) = 7.66 kW. Step 3: Divide by panel wattage to get panel count: 7,660W ÷ 400W panels = 19.2 → 20 panels. The SurgePV Solar Panel Sizer tool automates all of these calculations when you enter your location and electricity usage.

The PSH data in this calculator comes from the NREL National Solar Radiation Database (NSRDB), which is the US government’s authoritative source for solar resource data. The NSRDB provides hourly and half-hourly values of solar radiation (GHI, DNI, DHI) and meteorological data across the United States. State averages are derived from Typical Meteorological Year (TMY) data from the NSRDB dataset covering 1998–2022. Note that these are state-level averages; actual PSH at a specific location can vary by ±15–25% from the state average due to elevation, local climate patterns, and proximity to coasts. For precise site-specific data, use NREL’s free PVWatts Calculator at pvwatts.nrel.gov.

For off-grid systems, the annual average PSH is misleading — you must size for the worst month (typically December or January in most US locations), not the annual average. Seattle’s PSH ranges from 1.3 hr/day in December to 6.5 hr/day in July. An off-grid system sized for the annual average (3.8 PSH) would be severely undersized in December. Professional off-grid design practice: Size the solar array and battery bank to meet energy needs during the minimum production month. If minimum-month sizing would be prohibitively expensive, plan for supplemental power generation (generator backup) during the worst 1–3 months, and size the solar system for the second or third worst month instead.