Tank size alone is a poor guide. The Department of Energy sizes a storage heater by its first-hour rating, the hot water it delivers in an hour starting full, and a tankless heater by the flow it can heat through the temperature rise you need.
How to use this calculator
- Choose tank or tankless.
- Tank: think of your busiest hour, often a weekday morning, and enter how many showers, shaves, dishwasher loads and so on happen in it.
- Tankless: enter how many showers, sinks and appliances may run at the same time, and the temperature of your incoming water.
- Shop by the first-hour rating on the EnergyGuide label, or by the flow a tankless heater gives at your temperature rise.
Worked examples
A couple's morning
Water heater: Tank (storage) · Showers in the busiest hour: 2 · Shaves in the busiest hour: 1 · Hand and face washes in the busiest hour: 1 · Food preparation in the busiest hour: 1
- First-hour rating
- 39 gallons
- Look for a rating between
- 39 and 41 gallons
A family's busy hour
Water heater: Tank (storage) · Showers in the busiest hour: 3 · Shaves in the busiest hour: 1 · Hand and face washes in the busiest hour: 2 · Dishwasher loads in the busiest hour: 1 · Food preparation in the busiest hour: 1
- First-hour rating
- 70 gallons
- Look for a rating between
- 70 and 72 gallons
Tankless, two showers at once
Water heater: Tankless (on demand) · Showers running: 2 · Sinks running: 1 · Incoming water temperature: 50 °F
- Tankless heater
- 7.2 gallons per minute
- Temperature rise
- 70 °F
Tankless in a cold-water region
Water heater: Tankless (on demand) · Showers running: 1 · Sinks running: 1 · Dishwasher: 1 · Incoming water temperature: 40 °F
- Tankless heater
- 6.0 gallons per minute
- Temperature rise
- 80 °F
Sizing a tank water heater
DOE's method adds up the hot water used in your busiest hour and looks for a heater whose first-hour rating comes within about 2 gallons of it. Its estimates per use are a shower 15 gallons, a bath 20 gallons, a shave 2 gallons, a shampoo 4 gallons, food preparation 5 gallons, an automatic dishwasher 14 gallons and a warm or hot laundry load 32 gallons.
Those figures are from an older DOE fact sheet; today's efficient dishwashers and washers use less hot water, so your peak may come in lower.
Sizing a tankless water heater
A tankless heater must heat everything that runs at once. ENERGY STAR's sizing figures are 2.5 gallons per minute for a shower, 2.2 gallons per minute for a sink, 1.3 gallons per minute for a dishwasher and 3.3 gallons per minute for a clothes washer. Add up the flows, then find the rise: the outlet temperature, usually 120 °F, less the coldest your incoming water gets.
As a rough guide, DOE puts a gas tankless unit at about 5 gallons per minute and an electric one at about 2 gallons per minute at a seventy-degree rise. Cold incoming water in winter cuts the flow a unit can deliver.
The right temperature
Set the heater to 120 °F: it saves energy and lowers the risk of scalds. The CPSC notes that a serious burn takes about 5 minutes at 120 °F, but only 6 seconds at 140 °F and 2 seconds at 150 °F.
Common questions
What size water heater do I need?
One whose first-hour rating matches the hot water you use in your busiest hour, by DOE's method. The calculator adds it up from the showers, shaves and loads in that hour.
What size water heater for a family of four?
It depends on the busiest hour more than the head count. Three showers back to back already need a first-hour rating near three times 15 gallons; enter your family's morning to see.
How do I size a tankless water heater?
Add up the flow of everything that runs at once and find the temperature rise from your incoming water to 120 °F. Buy a unit rated for that flow at that rise.
What temperature should a water heater be set to?
120 °F, as the Department of Energy and the CPSC recommend, to save energy and prevent scalds.