The GTX 780 has a TDP of 250W and thus it needs both a 6-pin and 8-pin power connector to keep the lights on, so to speak. Nvidia recommends a 600W PSUs of good quality. KFA2 blessed the HOF with two 8-pin power connectors and 10+3-phase power system to improve stability and overclocking performance.
In terms of power efficiency, we can report that the GTX 780 HOF can draw slightly more juice (10 to 20W) than the reference GTX 780 Ti. This is quite normal for a factory overclocked card.