I've been using cfl bulbs for my first grow and am considering an upgrade to HPS. Now, if I'm using roughly 400 watts with my cfl bulbs, and I get a 400 watt HPS hood, I'm not going to use a bit more electricity am I? Stupid, but I had to make sure.
I don't know too much about indoor cultivation, take what i say by a thread, but I think it's safer to assume you'd use slightly more with 400w hps ... if you had to get bigger fans to control the heat...etc
Slightly more, but your yield will double. 400w of cfl = roughly 27,000 lumens, 250w hps = 28,000 lumens, 400w hps = 55,000 lumens. A 400w ballast uses an additional 30-40w according to this pdf... http://maxigrow.com/pdf/the_truth_about_electronic_ballasts.pdf