Solar Irradiance Energy Calculator
Solar energy output from a photovoltaic system depends on four factors: the total panel area (m2), the solar irradiance reaching the panels (peak sun hours per day or kWh/m2/day), the panel conversion efficiency (the fraction of sunlight converted to electricity), and the system performance ratio (accounting for real-world losses like inverter efficiency, wiring, and temperature). The daily energy output in kWh equals area times irradiance times efficiency times performance ratio. This calculator follows the NREL PVWatts methodology for estimating solar energy generation.
Solar energy output formula
Daily kWh = Area (m2) * Irradiance (kWh/m2/day) * Efficiency (decimal) * PR
Monthly kWh = Daily kWh * 30.44
Annual kWh = Daily kWh * 365
Peak sun hours (PSH) are numerically equal to daily irradiance in kWh/m2/day (since 1 PSH = 1 kWh/m2). Efficiency is entered as a percentage and divided by 100 internally. PR (performance ratio) accounts for all system losses beyond panel conversion efficiency.
Solar resource context
- Phoenix, AZ averages about 6.0 peak sun hours per day, making it one of the best US solar resources.
- Seattle, WA averages about 3.5 peak sun hours per day but solar still makes financial sense with incentives.
- A typical US home uses about 10,500 kWh per year (EIA), requiring roughly 25 to 30 400W panels.
- NREL's PVWatts Calculator provides location-specific, tilt-adjusted solar energy estimates for any US address.
- The federal Investment Tax Credit (ITC) covers 30 percent of solar installation costs for residential systems installed through 2032.
Frequently asked questions
What is solar irradiance?
Solar irradiance is the power per unit area received from the sun, measured in watts per square meter (W/m2). The standard test condition (STC) used for solar panels is 1,000 W/m2. The NREL National Solar Radiation Database provides irradiance data by location across the US.
What is peak sun hours?
Peak sun hours (PSH) is the number of hours per day during which solar irradiance averages 1,000 W/m2. It is equivalent to total daily solar energy in kWh/m2/day. Most US locations receive 3.5 to 6.5 peak sun hours per day depending on latitude, season, and cloud cover.
What panel efficiency should I use?
Standard commercial monocrystalline silicon panels have efficiencies of 18 to 22 percent. Polycrystalline panels are 15 to 17 percent. High-efficiency panels (SunPower, LG) reach 21 to 23 percent. Thin-film panels are 10 to 14 percent. Use the manufacturer's datasheet value.
What is the performance ratio?
The performance ratio (PR) accounts for real-world losses including inverter efficiency, wiring losses, soiling, temperature effects, and shading. A well-maintained system typically achieves PR of 0.75 to 0.85. This calculator uses a default of 0.80.
How can I find peak sun hours for my location?
The NREL PVWatts Calculator (pvwatts.nrel.gov) provides location-specific solar resource data and energy production estimates. The NREL National Solar Radiation Database (nsrdb.nrel.gov) provides hourly irradiance data for the US and other regions.
Official sources
- NREL: PVWatts Calculator.
- NREL: National Solar Radiation Database.
- DOE: Solar Energy Technologies Office.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.