Solar Irradiance Calculator
Solar irradiance is the fundamental input driving how much energy a solar system can produce at a given location — the power of sunlight hitting a given area, averaged over a day and expressed as kWh/m2/day (equivalent to "peak sun hours"). This calculator takes your location's average Global Horizontal Irradiance (GHI) and your panel orientation, then reports the plane-of-array (POA) irradiance actually reaching your tilted panel surface using a representative tilt boost factor. The result plugs directly into the peak sun hours input of our Solar Panel Output Calculator, and for finer geometry tuning, see our Tilt & Azimuth Optimization Calculator.
This is your location's average Global Horizontal Irradiance (GHI), also equivalent to peak sun hours on a flat surface. Look up your specific location via NREL's National Solar Radiation Database or PVWatts tool.
Selecting an orientation auto-fills the Tilt Boost below (still editable).
Representative planning-level estimates. Actual boost varies by latitude and local climate/cloud patterns.
location average horizontal irradiance (kWh/m2/day) × (1 + tilt boost (%) ÷ 100)
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Tilting panels to your latitude angle instead of leaving them flat can capture roughly 15% more sunlight annually, and single-axis tracking can push that advantage to around 27% -- meaningful gains that come purely from geometry, not better technology. A location with 4.5 kWh/m2/day of horizontal irradiance effectively becomes a 5.18 kWh/m2/day resource once panels are properly tilted, a number that then plugs directly into the Solar Panel Output Calculator's peak sun hours input.
How plane-of-array irradiance is calculated
This calculator ties the sunlight actually reaching a tilted solar panel surface to two inputs: the location's average Global Horizontal Irradiance (GHI) and a tilt boost factor representing the panel orientation. One quantity ties the calculation together.
Plane-of-Array Irradiance (kWh/m2/day) = Location Average Horizontal Irradiance (kWh/m2/day) × (1 + Tilt Boost (%) ÷ 100). Global Horizontal Irradiance (GHI) measures sunlight hitting a flat, horizontal surface, expressed as kWh/m2/day when averaged over a day (equivalent to "peak sun hours"). A flat, horizontal panel receives sunlight at an angle for most of the day, spreading the same energy over a larger effective area; tilting the panel to face the sun more directly (roughly toward your latitude angle) concentrates more sunlight per unit of panel area, increasing captured energy. The tilt boost factor captures that gain as a percentage uplift over GHI: 0% for a flat/horizontal mount, roughly 15% for a fixed tilt at latitude (the optimal fixed angle), about 20% for an adjustable seasonal tilt (re-aimed a few times a year), and around 27% for single-axis tracking (which follows the sun east-to-west through the day). At 4.5 kWh/m2/day of GHI and a 15% fixed-tilt boost, that is 4.5 × (1 + 15 ÷ 100) = 4.5 × 1.15 = 5.18 kWh/m2/day of plane-of-array irradiance.
The crucial distinction: GHI measures sunlight on a flat surface, while plane-of-array (POA) irradiance measures sunlight actually hitting your tilted panel surface, which is typically higher than GHI once panels are tilted toward the sun. The POA figure is what a panel actually converts to electricity, which is why it -- not raw GHI -- is the right input to a Solar Panel Output Calculator's peak sun hours field. Two notes on the model. First, the tilt boost factors are representative planning-level estimates; actual boost varies meaningfully by latitude (higher latitudes see larger gains from tilting) and by local climate and cloud patterns (diffuse-dominated climates see smaller gains). Second, this simplified model captures only the orientation uplift and does not separately model azimuth angle, ground albedo, soiling, or shading -- for finer geometry tuning, use a Tilt & Azimuth Optimization Calculator or NREL's PVWatts tool. Data sources: NREL National Solar Radiation Database (NSRDB) and PVWatts tool for location-specific GHI data; solar irradiance tilt boost factors from ASHRAE and PVLIB Python library; plane-of-array irradiance calculation methodology from IEC 61853 and NREL solar resource assessment standards; single-axis tracking performance data from utility-scale solar project case studies and manufacturer specifications.