System Power Supply Requirements for a single NVIDIA Reference Design GeForce GTX 950
• NVIDIA specifies a minimum of a 350 Watt or greater system power supply. (Minimum system power requirement based on a PC configured with an Intel Core i7 3.2GHz 130 Watt TDP processor.)
• the system power supply must also have a maximum combined +12 Volt continuous current rating of 23 Amps or greater
• the system power supply should also have at least one 75-Watt 6-pin PCI Express supplementary power connector. There are some non-reference design cards that require one 150 Watt 8-pin PCI Express supplementary power connector (e.g. EVGA GeForce GTX 950 SSC 2 GB [02G-P4-2957-KR]).
http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-950/specifications