Here is Guru3D's power supply recommendation:
GeForce GTX 780 Ti - On your average system the card requires you to have a 550 Watt power supply unit.
GeForce GTX 780 Ti 2-way SLI - On your average system the cards require you to have a 800 Watt power supply unit as minimum.
GeForce GTX 780 Ti 3-way SLI - On your average system the cards require you to have a 1200 Watt power supply unit as minimum.