A 250-watt magnetic ballast sends 250 watts to the light, and uses about 25-50 watts itself, depending on the ballast. That number can vary a lot though, based on the quality and age of the ballast. A digital ballast will also send 250 watts to the light and will pull maybe 10-20 watts for itself, but age doesn't play as large of a factor in its efficiency. Keep in mind these are rough estimates based on my knowledge of certain types of industrial lighting, and they may not translate well to commercial-grade grow lights, but I suppose it's better than nothing.