How do I figure out how much energy is used to heat the water in our gas hot water heater? I know that 1 BTU is the energy to heat 1 lb. of water 1°. Do I figure out how many gallons in 1 lb. of water; and then multiply that by the difference in room temperature and 140°? — JH, Maple Grove, MN 

Yes. A gallon of water weighs about 8.3 pounds, so a typical 40gallon hot water heater tank holds 332 pounds of water. To raise that water from its delivery temperature (about 60° F) to its final temperature (about 140° F) takes about 26,560 BTUs.