Well, I've heard 400 watts left on continiously for a month brings your bill up about only 20 dollars, wouldnt know just yet though, based on what people here have told me.
Overload, I have my 400 HPS plugged into a surge protector which leads right into the a wall socket. along with a small fan plugged into the other slot, so you should be fine.
And area, well your hps, with normal bulb, puts out about 45,000 lumens. 2500-3000 lumens per square foot of grow space; simply remember that and you can determine anything about your space. Hope this helped..
Personally I think you could have a really nice crop if you train your strain and keep the light just where it needs to be, without wasting any of it, get some mylar and keep around where your growing.
-Travis