Solar Irradiance
Solar irradiance is the instantaneous power per unit area delivered by the sun, measured in watts per square metre (W/m2). It is the primary input for every yield model, determining how much energy a PV system can theoretically produce at any given moment under any given sky condition.
Solar irradiance is the instantaneous flux of solar power arriving at a surface, measured in watts per square metre (W/m2). It is not a fixed number: it varies by time of day, season, cloud cover, atmospheric aerosols, and the angle between the sun and the receiving surface. Every solar yield calculation begins here.
Three components make up the full picture. Global Horizontal Irradiance (GHI) is the total radiation striking a horizontal plane and is the figure reported by most ground stations and satellite datasets. Direct Normal Irradiance (DNI) isolates the beam component arriving straight from the solar disc. Diffuse Horizontal Irradiance (DHI) captures scattered sky radiation that reaches the surface from all directions. GHI equals the sum of DHI and the horizontal projection of DNI.
For a real PV array, the quantity that matters most is plane-of-array (POA) irradiance: the GHI decomposed and recombined for the module's actual tilt and azimuth, then adjusted for ground-reflected and horizon-diffuse contributions. A south-facing 30-degree tilt in central Europe typically receives 10 to 15 percent more annual irradiance than a horizontal surface at the same location. A tracker pushes that gain further by continuously realigning the panel to DNI.
The 1000 W/m2 Standard Test Condition (STC) value is the reference point for module ratings. Field conditions rarely hit STC: most peak hours sit between 700 and 900 W/m2, and thermal derating pushes effective output below nameplate even at those levels.
Data sources for irradiance modeling include NASA POWER, PVGIS, Meteonorm, NREL's NSRDB, and Solargis. Each dataset differs in spatial resolution, temporal averaging, and satellite retrieval methodology, so yield estimates from the same site can vary by 3 to 8 percent depending on the source chosen.
Why it matters for solar installers
Irradiance is the one input the installer cannot control but must get right. An irradiance dataset that overestimates a site's resource by 5 percent inflates the projected yield by 5 percent, which compresses the real payback period and sets up a disappointed customer. solarVis pulls site-specific irradiance data directly into the feasibility analysis and 3D design canvas, translating raw W/m2 values into accurate POA estimates that account for the array's tilt, azimuth, and shading losses before they ever reach the proposal.
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- What is the difference between irradiance and insolation?
- Irradiance is an instantaneous measurement of power, expressed in W/m2. Insolation (or solar irradiation) is irradiance integrated over time, expressed in kWh/m2/day or kWh/m2/year. You read irradiance on a live sensor; you use insolation figures to model annual energy yield.
- What are GHI, DNI, and DHI?
- GHI (Global Horizontal Irradiance) is the total radiation on a flat horizontal surface and includes both direct and diffuse components. DNI (Direct Normal Irradiance) is the beam radiation hitting a surface perpendicular to the sun. DHI (Diffuse Horizontal Irradiance) is scattered sky radiation. Most yield simulations work from GHI, then decompose it into DNI and DHI to calculate plane-of-array irradiance at the module's actual tilt and azimuth.
- What is standard test condition irradiance and why does it matter?
- Standard Test Conditions (STC) define module performance at exactly 1000 W/m2 and 25 degrees Celsius. The rated wattage on every module datasheet is measured at STC. Real-world irradiance is usually below 1000 W/m2, and module temperature is usually above 25 degrees, so actual output is nearly always lower than the nameplate rating.
