power usage is always variable depending on what the video card is doing.
please list your system hardware specs including the power supply being used (specifically need to include brand/model of psu)
I have a 235watts psu running a intel core 2 duo e7500, a 7200rpm 250gb hdd,1 stcik of drr3 ram 4gb and a overclocked 1gb 8400gs and i am replacing the 8400gs and i kno my psu is small but if the r7 240 runs at 12 amps and under 50 watts i think im good form my psu because it has 14 amps on it 1 12v rail