Here's the math to find out the cost per kwh
Watts x Hours =Kwh per say 400 watt light times 18 hours = 72 kwh
On the electricty bill it will tell you the cost of kwh around .11 cent per kwh
so .11 cents times 72 kwhs = 792 times 30 days = 23.76 for a month.
I got this from wanta learn web site. hope it helps