Size for your busiest hour
A water heater that is too small runs out during the morning rush; one that is too large wastes energy keeping water hot. For tank heaters, what matters most is not the tank volume itself but the first-hour rating (FHR): how much hot water the heater can deliver in an hour, starting full. It is printed on the EnergyGuide label.
Peak-hour demand
The US Department of Energy suggests adding up the hot water used in your busiest hour:
- Shower: 10 gallons
- Bath: 20 gallons
- Dishwasher: 6 gallons
- Clothes washer, warm or hot: 7 gallons
- Food preparation: 5 gallons, hand dishwashing 4 gallons
Choose a heater whose first-hour rating is at least that total.
Example
A family of four takes two showers between 7 and 8 am while the dishwasher runs and breakfast is cooked:
- Peak-hour demand: 2 × 10 + 6 + 5 + 4 = 35 gallons
- A typical 40-gallon gas heater has an FHR of 60 to 70 gallons and a 50-gallon electric one about 55 to 65, so either covers it
- Suggested tank size for four people: about 60 gallons if baths or long showers are common, 50 gallons otherwise
Tankless water heaters
A tankless (on-demand) heater has no stored water, so it is sized by flow rate and temperature rise:
- Flow = showers running at once × flow per head, for example 1 × 2.0 gpm
- Temperature rise = target temperature − incoming water temperature, for example 120°F − 50°F = 70°F
- Power = flow in L/min × rise in °C × 4.186 ÷ 60, here about 20.5 kW
Whole-house electric tankless units often need 25 to 36 kW and several 40 to 60 A breakers; gas units are rated around 150,000 to 199,000 BTU/h. Groundwater is colder in northern states, so the same unit delivers less flow there in winter.
Choosing the type
- Heat pump water heaters use two to three times less electricity than standard electric tanks, but recover more slowly; choose a slightly larger tank.
- Gas tanks recover fastest.
- Tankless saves standby losses and space but costs more to install.
Set the thermostat to about 120°F to limit scalding and standby losses.