The Radeon R7 250X draws up to 80 Watts.
For a system using a single Radeon R7 250X graphics card AMD specifies a minimum of a 400 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 22 Amps or greater and have at least one 6-pin PCI Express supplementary power connector. (Minimum system power requirement based on a PC configured with an Intel Core i7 3.2GHz 130 Watt TDP processor.)
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the...