Mr. Green is busy on his world-wide publicity tour for his new book. In the meantime, here's a Mr. Green classic column from February 2007.
Hey Mr. Green,
I would like to encourage my son-in-law to turn off the lights when he leaves a room. To do that, I would have to show the cost benefit. Can you help? — Ruth in Watertown, Massachusetts
In olden times, a household authority figure would say, "Turn the lights out," and that would be that. But today's contentious whippersnappers apparently need a detailed financial analysis before flipping the switch. Fortunately, the math is on your side. Electricity rates are based on the number of kilowatts used per hour, or kilowatt-hours (1 kilowatt equals 1,000 watts). All you have to do to find the daily cost of operating a lightbulb is multiply its wattage by the number of hours it burns, then multiply that by the kilowatt-hour (kWh) rate printed on your utility bill and divide the result by 1,000.
So if a 100-watt bulb burns for ten hours, and the power company charges ten cents a kilowatt-hour, it costs a dime a day to keep lit. That's about $3 per month, or $36 per year. Leaving a half dozen bulbs burning would waste more than $200 per year. If your son-in-law turns off the lights and puts the annual savings into an account that draws 5 percent interest, in ten years he will have about $2,650, a nice little sum he could invest in some booming alternative energy company.