• electric heater, your utility bill won't help because there are far too many other electrical appliances in an average household to separate the heating use. However, there is a solution: you can install a kWh meter on the electric heating power line to record how much you use over a period. Put the meter in place and record your heating power use for a month (or as long as possible, just as long as it's a week or more). If your electric heater is directly wired without a plug, you'll want to get an electrician to do the installation. The good news is that you may not even have to buy the meter, as many libraries and utilities will lend them for short periods.

If none of these methods suit your needs or are not available, there's still another option. You can measure each person's individual hot water use every day for a period and work out the averages from there. To do this you'll need a table of average water use for certain activities (see diagram), adjusted by the flow rates for your particular plumbing setup, and a lot of patience to monitor what everyone does! It's usually easier just to take the "20 gallons a day" estimate and run with it.