In general, two independent sources of data for Global Horizontal Irradiance (GHI) are available:

1. On-site ground measured data
2. Data derived from satellite instrument measurements

The local irradiance is usually measured by a pyranometer and is very accurate if the equipment is well installed and maintained. When averaged and/or integrated over relatively short periods from as little as a minute to an hour, day, or week a pyranometer is the most accurate solution. But data derived from satellite measurements comes close to the on-site measurement uncertainty over longer time periods such as months and years.

In practice, both satellite and on-site data are often used in parallel and correlated in plant monitoring software (such as SynaptiQ Solar by 3E). However, the varying measurement uncertainties over different time-scales must be borne in mind
Together with 3E we have summarised our best practices in a whitepaper and explain how a combination of both satellite data and on-site measurements with pyranometers works out.