Question on Light

I was wondering if a 400w light system would hurt the electric bill? i have a sodium compressed bulb running for 17 hrs.

600W X 18 hours = 10800WH Then we can Convert to KWH: 10800WH/1000 = 10.8KWH (this is how much KWH per day the 600 watt will draw)
We're running it for a month: 1 month = 30 days (or 31 days) 10.8KWH X 30 = 324KHW / month. Now we calculate cost using the rate
of 23.5 cents / KWH. 324 KWH X .$235 = $76.14

just sub in the 400 watt light and how many hours your using and how much ur electric is per kwh. hope it helps