The graphics card, itself, draws at most 300 Watts when it's overclocked and running Furmark Stability Testing. During gaming it reaches 211.28 Watts.
The following review actually measures the card's actual power consumption (It's in German so you may need to run it through a translator):
http://ht4u.net/reviews/2014/sapphire_radeon_r9_280x_vapor-x_im_test/