We are working in a solar energy project to locate potential sites for solar panel farms. We are using the Landsat 7 panchromatic band to derive radiance with GRASS using i.landsat.toar (W/m2*sr*µm units). Now, we have access to some ground data from sensors measuring global horizontal irradiance (GHI) which output is in W/m2 units. I would like to compare these datasets for validation.
Is there a way to convert radiance in W/m2*sr*µm to irradiance in W/m2?
I have read in  that radiance = irradiance / π, but I don't completely understand, also, this doesn't take the spectral part (µm) into account.