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 600 watt PSUs of good quality.
In terms of power efficiency, we can report that the GTX 780 Phantom GLH can draw slightly more juice than the reference GTX 780 Ti. This is quite normal for a factory overclocked card. Overall, performance-per-watt is still good. Recent Geforce cards tend to consume 10 to 15 watts less than competing Radeon cards in a multi-monitor scenario.