Depending on the rhythm of the academic year, for the past several decades my household has operated with as few as four and as many as seven adults. Everything has been measured. Refrigerators, the electric dryer, the TV, etc. have separate watt-hour meters. A separate water meter tracks the use of domestic hot water. Ten thousand-hour clocks log the time of operation of oil boiler, circulator pumps, etc. (During one winter we even weighed each stick of wood for the wood stove and logged “pounds of wood per degree day.” But that was too much work.) Each week produces a unique data set. If an occupant leaves for a week, the data will show the extent to which that occupant is a profligate user of hot water. One thing learned from all the household data gathering: The cold-start oil boiler and domestic hot-water storage tank are poor performers. In a typical week the oil burner will consume about 10 gallons of oil to provide about 100 cubic feet of hot water. The implied efficiency is less than 50 percent. A timer was installed that limited hot water use to between 4 p.m. and midnight each day, but this did not help much. At $4.50 per gallon for oil, and 20 cents per kilowatt-hour for electricity, an electric water heater would be a clear winner.