GHI — Global Horizontal Irradiance
Global Horizontal Irradiance (GHI) is the total solar radiation received per unit area on a horizontal (flat) surface. It is the sum of two components: Direct Normal Irradiance (DNI) projected onto the horizontal plane, and Diffuse Horizontal Irradiance (DHI).
The mathematical relationship is:
GHI = DNI × cos(solar zenith angle) + DHI
GHI is the primary metric used for evaluating solar PV resource quality at a site, particularly for fixed-tilt and flat-roof installations. It's measured in W/m² (instantaneous power) or kWh/m² (energy over time).
Typical GHI Values
Annual GHI values (in kWh/m²/year) across the US:
- Southwest US (Mojave Desert): 2,100–2,400 kWh/m²/year
- Mountain West: 1,800–2,200 kWh/m²/year
- Southeast US: 1,600–1,900 kWh/m²/year
- Midwest: 1,500–1,800 kWh/m²/year
- Northeast US: 1,300–1,600 kWh/m²/year
- Pacific Northwest: 1,000–1,400 kWh/m²/year
Germany, the world's fourth-largest solar market, averages only ~1,100 kWh/m²/year GHI — demonstrating that even moderate solar resources can support thriving solar markets.
GHI in SolarScope
SolarScope displays monthly and annual GHI values for your project location, sourced from NASA POWER's ALLSKY_SFC_SW_DWN parameter. GHI is shown in both kWh/m²/day (daily average) and kWh/m²/month formats in the project dashboard.