find out how much it cost for a kilowatt hour and do the math.....if it cost .10 cents per kwh, and you are running 300 watts that .03 cents per hour the lights are on.....times 18 hours during veg period......54 cents a day so about 16 bucks a month for a 300 watt setup....just remeber fans and stuff like that are drwing power too so do the math and figure it out!