The accelerating supply transfers IeV=50 kW into the beam, but the electrons need not be discarded with that kinetic energy. An electrostatic recovery system decelerates the used beam before collection and returns most of its energy to the electric field or supply. The net required power then covers unrecovered beam energy and losses, rather than the whole circulating kinetic-energyflux.
For example, 99% recovery leaves 0.01(50kW)≃500W of unrecovered beam power, plus auxiliary loads. This explains the stated two-order-of-magnitude scale. It does not imply that gun heating, vacuum equipment, magnets and all electrical losses are automatically included in a 500W total plant budget.
