So, just multiply the watts times the hours used to get the watt-hours. (Then divide by 1000 to get the kilowatt-hours, which is how your utility charges you.) Example: 100-watt bulb x 2 hours -- 1000 = 0.2 kWh. On a peak day in 1999, California used 50,743,000 kilowatt-hours.read more >>