Hey Mr. Green,

My 10-year-old electric water heater uses about 5,000 kilowatt-hours a year. Should I wait until it dies or replace it now? If the latter, what's the most energy-efficient water heater on the market? —Randi, in Putnam Valley, New York

Holy starry dynamo of night! Your water heater alone uses almost twice as much electricity as my entire house. At New York's average residential rate of 19.6 cents per kilowatt-hour, you could be spending $1,000 a year to feed that power hog. I'd rather take cold showers with Dick Cheney and Mitch McConnell than shell out that much. 

Replace it, pronto, with a tankless, on-demand gas or propane model, or an electric heat-pump type. Depending on where your electricity comes from, conventional electric water heaters can demand even more fossil fuel energy than gas heaters, because two-thirds or more of the fossil energy that makes electricity gets wasted as heat and in transmission. 

For the most efficient models, see the EPA's Energy Star list of tankless and heat-pump units that use one-third as much electricity as conventional electric water heaters. (Heat pumps work like refrigerators in reverse, pumping heat in instead of out.) If you ran a heat-pump model on green power, you'd reduce your net heater emissions to almost zero. You could also install a good old solar water heater. Feel free to donate your annual savings to the Mr. Green Research Fund.