We could regreen the Sahara by covering less than 0.5% of it in solar panels and desalinating water.
Grassland precipitation - 0.7 meters/year
Solar irradiance (GHI) - 6 kWh/m^2/day
PV efficiency - 23%
Desalination efficiency - 3 kWh/m^3
(2.1 kWh/m^2/year)