watts/1000 gets kwh.
kwh * 24 = kwh/perday * kwh charge = cost per day
cost per day * days this month = cost per month
cost per day * 365 = cost per year
the kwh charge is how much your electric company charges per kwh, im at about 11 cents. so my 2x250 Watt MH would run me...
500 / 1000 = 0.5 kwh
0.5 * 24 = 12 * $0.11 = $1.32 per day
$1.32 * 31 days = $40.92 per month
$1.32 * 365 days a year = $481.8 per year
dont multiply the monthley amount by 12 cause the days a month vary.
Lastley dont forget to add up all the pumps and heaters, extra lights and chillers for your sump(refuge). If you dont know that wattage of something look on the label and take: amps * volts = watts.
I have 500Watts of lights about 50Watts for pumps 350Watts in heaters (which do not run all the time of course) and an extra 60Watts (CFL) for the Refuge.
Best thing to do is buy a Kill-A-Watt (or two) and run that on your power strips for a good average.