← All guides · 2026-07-01
GHI, DNI and DHI Explained: Which Solar Radiation Number Matters?
Three numbers, one sky
GHI (Global Horizontal Irradiance) is the total sunlight hitting a flat horizontal surface. It splits into DNI (Direct Normal Irradiance) — the beam coming straight from the sun's disc — and DHI (Diffuse Horizontal Irradiance) — light scattered by clouds, aerosols and the sky itself. GHI = DNI × cos(zenith) + DHI. On a clear day direct beam dominates; under overcast skies nearly everything is diffuse.
Which one matters for your project?
Fixed PV panels: GHI is your headline number — it correlates almost 1:1 with fixed-tilt energy yield. Tracking PV: DNI gains weight, since trackers follow the beam; high-DNI deserts reward trackers most. Concentrated solar power (CSP): DNI only — mirrors cannot concentrate diffuse light, which is why CSP lives exclusively in deserts. When you compare locations on Sunveyor, the GHI charts are the right basis for standard panel projects.
Where the data comes from
Nobody has a pyranometer on every roof. Modern datasets derive irradiance from geostationary weather satellites: cloud imagery is converted to surface radiation using radiative-transfer models, then validated against ground stations. NASA POWER offers 40+ years globally at ~50 km resolution; PVGIS SARAH3 provides finer European/African/Asian coverage; commercial providers (Solargis, Solcast) reach 1–2 km for bankable studies. Satellite-derived annual GHI is typically within 3–5% of ground truth.
Reading the numbers like a pro
A location's annual average GHI in kWh/m²/day maps roughly to specific yield: multiply by 365 and by a ~0.8 performance ratio to get kWh per kWp. So 5.0 kWh/m²/day ≈ 1,460 kWh/kWp/year. Always check interannual variability too — two sites with the same mean can differ meaningfully in worst-year output, which drives P90 figures banks care about.
Click your location on the satellite map and get irradiation, 40-year history and a production simulation — free.
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