According to The Case for Space Solar Power, 40% overall efficiency with the technology at the time it was written (about a decade ago), with a theoretical maxi

According to The Case for Space Solar Power, 40% overall efficiency with the technology at the time it was written (about a decade ago), with a theoretical maximum of 60%. This was with microwaves from a phased array transmitter and a reference signal from the ground target, and the satellite in geostationary.