DNI — Direct Normal Irradiance
Direct Normal Irradiance (DNI) is the solar radiation received per unit area on a surface that is always perpendicular (normal) to the sun's rays. It represents only the direct beam component of sunlight — the parallel rays coming directly from the solar disc — excluding scattered diffuse radiation.
DNI is the most important metric for concentrating solar power (CSP) technologies such as parabolic troughs, solar towers, and dish/Stirling systems, which can only focus direct beam radiation. It's also critical for dual-axis solar trackers that maintain perpendicularity to the sun throughout the day.
DNI vs. GHI
High-DNI locations tend to have clear skies with low aerosol loading and low humidity — desert climates are ideal. The US Southwest (Arizona, Nevada, the Mojave Desert) has some of the world's highest DNI values, averaging over 7.5 kWh/m²/day in peak months.
Humid, cloudy climates have much higher diffuse fractions — DHI makes up a larger share of GHI, and DNI is reduced by scattering. For fixed-tilt PV systems in these climates, the high diffuse component is still useful, making solar viable even where DNI is low.
DNI in SolarScope
SolarScope provides DNI values from NASA POWER (ALLSKY_SFC_SW_DNI parameter) for all project locations. DNI is shown monthly in the analysis dashboard. For fixed-tilt residential and commercial PV, GHI is typically more relevant than DNI; for tracking systems or CSP, DNI is the critical resource metric.