For decades, most of the lightbulbs in our homes were incandescents. They were cheap to buy and costly to run. Ninety percent of the energy they use is given off as heat, and only 10 percent as light. Maybe that’s why many of us had “Turn out the light!” drilled into us every time we left a room as kids.
Incandescents are being phased out, replaced in large part by CFLs—compact fluorescent lightbulbs. They are cheap to run and more expensive to buy than incandescents. They can last 10 times longer than incandescents, although the more you flip them on and off, the more you shorten their lifespans. So does “Turn out the light!” apply to CFLs?
The Department of Energy proposes a simple rule: Leave your CFL on if you will be out of the room for 15 minutes or less. Otherwise, turn off the light.