The number every solar calculator relies on. Check your country's average, then see how much it swings across the year.
Use the selected or annual figure as the "sun hours" input on the Panel, ROI, Off-Grid or EV Charging calculators.
This is a smooth seasonal approximation, not satellite-measured data — real irradiance also depends on local cloud cover patterns, elevation and haze. For an engineering-grade, site-specific number, cross-check against NASA POWER or PVGIS/PVWatts before finalizing a system design; use this tool to quickly sanity-check the ballpark and to feed other Solargrin calculators.
If you want to calculate irradiance beyond this tool's seasonal model, calculating solar irradiance from first principles works like this:
In practice: (1) pull a daily insolation figure in kWh/m²/day for your location and month from NASA POWER, PVGIS, or your national weather/solar agency; (2) that number, divided by the 1 kW/m² reference irradiance used in Standard Test Conditions, directly equals your peak sun hours for that period — this is the shortcut most installers use instead of full radiative-transfer math; (3) adjust for tilt and orientation using the Roof Angle & Tilt Calculator, since a poorly angled panel receives less usable irradiance than the raw horizontal number implies. Calculating irradiance this way, month by month, is exactly what this calculator automates for you.
This tool gives you a climatological average — a typical month's irradiance based on historical seasonal patterns, which is what you want for sizing a system. A true short-term solar irradiance forecast (tomorrow's or this week's expected sunshine) is a different, weather-model-based product used for day-ahead grid dispatch and battery scheduling, not for system design — for that you'd want a service like NOAA's or your national meteorological office's solar radiation forecast, or your inverter/monitoring platform's built-in weather feed. For sizing a new system, the seasonal average here is the more useful number.
Summer falls in December-February south of the equator, so irradiance peaks then instead of in June-August — the calculator flips the peak month automatically based on your country.
Grid-tied systems are usually sized on the annual average, since a low-production month is offset by a high-production one over a full year. Off-grid systems should size around the worst month instead — see the Off-Grid Calculator.
Irradiance is instantaneous power density (kW/m², what's hitting the panel right now); irradiation (or insolation) is that irradiance integrated over time (kWh/m²/day). "Peak sun hours" is a convenient way of expressing daily irradiation in units of hours at the 1 kW/m² reference irradiance.
No — this is a seasonal average for system sizing, not a short-term weather forecast. See the section above for the difference and where to find true forecast data.