For a 100-watt bulb (to use an example), this would be the equivalent of 0.1 kw used per hour. If the bulb runs 7 hours a day, that is 0.7 kw used per day. At the rate of $0.15 per kwh, this is $0.105 spent per day operating the bulb. Multiplying $0.105 by 365 days gives approximately $38.33 to operate a 100W bulb for one year, at 7 hours per day.
