Be easier to just call the company and ask what price your on for each kilowatt hour. I have a day and night rate where the day is say 25pence a kilowatt hour and the night is say 7pence or something similar.
Then i multiply my rate for kilowatt hours by how many kilowatt hours i use - so 600watt for 12 hours of light is 7.2 kilowatts and multiply that by the rate charged per the time of day.
A 600watt hps cost me about £10 or approx 13$ a week to run.
Now thats a gram a week and my elecy is paid - cheap as chips