If I were you my main concern would be the temperature of the are you are growing in. Not only should it be ventilated for the plants sake, but the feds can tell if you have a room with excess radiant heat. A 400w hps puts out a considerable amount of heat. As far as Your electric bill is concerned, i would watch my electrical consumption closely. If you have your own hot water heater in the room perhaps turn it down. Space heaters rack up the bill in a heartbeat as well. Hope this helps.