Ten thousand hours means leaving the lights on for eight hours a day every day for a little under three and a half years. The three bulbs we used for this chart were an 81-watt GE halogen bulb, a 75-watt GE incandescent bulb, and a 20-watt GE compact fluorescent bulb. We used the manufacturers' estimates for the life of each bulb and priced similar bulbs at the Target store in Bellevue, Wash., in mid-August 2000. The bulbs were equally bright; the incandescent and compact fluorescent were rated at 1,200 lumens, while the halogen came in at 1,220—a negligible difference.

For the cost of electricity, we assumed a per-kilowatt-hour rate of $0.0586, which is the average of the residential rates for these seven locations:

Clark County, Wash., $0.0468

Ames, Iowa, $0.077

Brainerd, Minn., $0 .05

Orlando, Fla., $0.0402

Greenville, Tenn., $0.0560

Washington, D.C., $0.05

Tuscon, Ariz., $0.09

Your rate may also include taxes, service fees, etc.