TWTA Output vs Input Power
The power figures (120 Watts, 240 Watts, etc) being quoted is OUTPUT power from the TWTA's. I guessing the TWTA's are about 50% efficient, so for 120 Watt OUTPUT, the required DC INPUT power will be 240 Watts. If the total output power from all the TWTA's is 5kW, the DC input power from the batteries and solar panels needs to be 10kW. Assuming the total output power of the solar panels is 13kW, that only leaves 3kW to run the other systems on the satellite (uplink receivers, TT&C transponders, on-board computers, attitude control, etc). Not a lot left over.