Pick up the light bulb, and look on the globe for the watts used by the bulb. Most incandescent bulbs use 25, 60, 75 or 100 watts.
Divide this number by 1,000 either by calculating by hand on paper or with a calculator. For example, if your light bulb uses 75 watts, divide this by 1,000 watts to get the total number of kilowatts for the light bulb just to turn on as .075 kilowatts (kW).
Estimate the amount of time you keep that light bulb on each day, and multiply it by 30 to get a total monthly usage for the light. For instance, if your 75-watt light bulb stays on all day for 10 hours per day, after 30 days, the light would use 300 hours of power.
Multiply the number of hours in a month by the number of kilowatts used by the light to find the total kilowatt hours of usage. For example, .075 kilowatts x 300 hours equals 22.5 kilowatt hours (kWh).
Consult your most recent electricity bill for the the rate per kilowatt hour charged. Multiply this number by the total kilowatt hours used in a month for your light bulb to determine how much it costs you to operate that light bulb. For example, assume a charge of $0.10 per kilowatt hour. Multiply 22.5 kilowatt hours (kWh) x $0.10 per kWh for a total charge of $2.25 per month to operate the 75-watt light bulb for 10 hours per day.
Repeat steps 1 through 5 with the wattage from a florescent bulb you want to use in that same light socket. For example, if you want to use an 11-watt florescent rather than the 75-watt incandescent light bulb, you would use 11 watts/1,000 watts to get .011 kilowatts.
Continue to follow the example for a replacement bulb. Assume that you use this replacement light bulb for the same 10 hours per day. Multiply .011 kilowatts by 300 hours to get a total of 3.3 kilowatt hours per month. Take this number, and mulitply by the example $0.10 per kilowatt hour charge to get a total monthly cost for the florescent light bulb of $0.33 per month.
Take the monthly usage charge or the total kilowatts used by the incandescent light bulb, and subtract the monthly cost or the kilowatts used by the florescent replacement to determine the savings. In the example, the cost savings equals $2.25 - $0.33 for a total savings of $1.97 per month. Subtract the kilowatts used by the florescent bulb from the incandescent usage to see the kilowatts saved. In the example, .075 kilowatts for the incandescent minus .011 kilowatts for the florescent shows the florescent uses .064 kilowatts less than the incandescent.