(Wattage X hrs used per day) divided by 1000 = Kilowatt hrs used per day
multiply this answer by your kilowatt hour price to get cost.
In my case (600 X 24) divided by 1000 = 14.4 kW hrs used
14.4 X $0.07 (my electric cost is 7 cents/kWh) = $1.008 or $1/day running 24/7
Heres a calculator, you just need 3 pieces of info: wattage, hrs used, and your electricity cost per kWh
http://www.citytrf.net/costs_calculator.htm
To figure total electric cost per grow just do the calculation for everything using electricity: other lights, fans, heaters, dehumidifiers etc