This is the specific off grid inverter I'm using - SPF 3000TL LVM-ES. You essentially place it between your loads and grid and it will intelligently switch to

This is the specific off grid inverter I'm using - SPF 3000TL LVM-ES. You essentially place it between your loads and grid and it will intelligently switch to grid when your solar output is too low and your battery voltage is at a certain threshold.