How to size a tank water heater
The Department of Energy peak-hour worksheet, why first-hour rating beats gallons, and a worked 36-gallon example.
A storage water heater, including a heat pump with a tank, is sized by first-hour rating. That rating is the gallons of hot water the heater can deliver in an hour that starts with the tank fully hot. It includes the stored gallons and what the burner or the elements can reheat during the draws of the test.
The Department of Energy’s sizing guide tells you to estimate the household’s busiest hour and to choose a model whose first-hour rating is within about one or two gallons of that estimate. The gallon number painted in the model name is an input to the rating, not a substitute.
A 40-gallon gas heater with a strong burner can out-deliver a 50-gallon electric heater. The yellow EnergyGuide label is where the rating is printed on conventional storage heaters. Heat pump models may put the figure in the specification sheet instead. Use the figure, whatever cover it is printed on.
The method below is the Energy Saver worksheet. The guide notes that the worksheet is based on standard usage with no extra water-conservation measures, and that it does not estimate total daily use.
It estimates one hour. Daily use is a bill question. The peak hour is a sizing question.
The worksheet
| Use | Average gallons of hot water per use |
|---|---|
| Shower | 10 |
| Shaving | 2 |
| Hand dishwashing or food preparation | 4 |
| Automatic dishwasher | 6 |
| Clothes washer | 7 |
Pick the hour when the house actually stacks its uses. In many homes that is morning. Count only the uses inside that hour, not everything that happens in the day.
The Energy Saver example is three showers, one shave, and one hand-dishwashing: 10 times 3 is 30, plus 2, plus 4, which is 36 gallons. That household should look for a first-hour rating of about 34 to 38 gallons.
Buying a 50-gallon tank because the number feels safer, without checking the rating, can still miss the hour if the burner is weak. Buying a 40-gallon gas heater whose rating is 70 gallons for a 36-gallon peak is a match even though the tank “looks small.”
The shower line is 10 gallons of hot water per use in that worksheet, not 10 minutes under a modern rainfall head. A long shower with a high-flow head uses more. A low-flow head uses less. If your showers are long, raise the shower line honestly instead of pretending the average describes your house.
The clothes-washer line is 7 gallons of hot water, which is plausible for a cold or warm wash on a newer machine and low for an old top-loader run on hot. If you still wash on hot, use the machine’s own hot-water figure.
The worksheet’s footnote is explicit that these are averages without conservation measures. Adjust with numbers you can defend. Do not adjust by doubling everything “to be safe,” or you will buy a tank that costs more, loses more standby heat, and still was not chosen from the rating.
First-hour rating versus recovery
Recovery is the rate at which the heater can reheat a full tank. The storage-tank page shows the arithmetic: gallons per hour equals input Btu per hour times a thermal efficiency, divided by 8.33 times the temperature rise. A 40,000 Btu/h gas burner at an illustrative 0.70 thermal efficiency and a 70°F rise reheats on the order of 48 gallons an hour.
A 4,500-watt electric element, one at a time, lands nearer 26 gallons an hour. The first-hour rating is not identical to “tank gallons plus recovery,” because the federal test uses a specific draw pattern, but the direction is the same.
Gas tanks earn gallon-equivalents with the burner. Electric resistance tanks earn them with volume, because the element is slower. Heat pumps are slower still in pure compressor mode and often need a larger tank to hit the same first hour when they replace a gas burner.
ENERGY STAR’s installation note says to expect different sizing advice for heat pumps, and often to size up when leaving gas.
Read heat pump water heaters before you swap a 40-gallon gas tank for a 40-gallon heat pump and call it equal.
The label’s first-hour rating already did this math under test conditions. Prefer it to a back-of-envelope recovery calculation when both are available. Use the envelope when you are trying to understand why two heaters with the same gallons feel different, or when a used heater has no label.
Do not confuse thermal efficiency in that estimate with uniform energy factor. UEF includes standby loss and a defined draw. It is explained in uniform energy factor.
Fuel, space, and the incoming water
Incoming water temperature changes how hard the heater works but does not change the worksheet’s gallons. Colder groundwater means each gallon delivered at 120°F required more heat, so recovery falls.
The Energy Saver tankless instructions say to assume 50°F incoming water if you do not know better. The same assumption is a fair sanity check for a tank in a cold climate: a heater that barely matched the peak hour on paper may feel short in January.
If you know your groundwater is far below 50°F, favor the higher end of the rating match or a faster burner. Do not invent a precision you did not measure. A thermometer on the cold inlet in winter is a measurement. A memory of a cold shower is not.
Fuel choice changes which heaters are even available at the rating you need. Gas or electric covers vents, circuits, and outages. A rating you cannot power or vent is not a rating you can buy.
Measure the closet, the door swing, and the vent path before you fall in love with a model. Condensing heaters that hit a high rating may need a new plastic vent. That constraint is condensing gas heaters.
After the number
Choose a model whose first-hour rating is within about one or two gallons of the worksheet result, in the fuel you can install, with a uniform energy factor you are willing to pay for after reading the label honestly. Then price the installed job, not the tank on a pallet. The cost guide is the range check.
Confirm the install includes a pan where a leak would cause damage, a correct relief-valve discharge, an expansion tank if the system is closed, and a permit where the city requires one. Those items do not change the first-hour rating and they change whether the new heater is legal. They are listed in permits, pans, and straps.
If the existing heater fades and the worksheet says the rating was always too small, replacement is the fix. If the worksheet says the rating is adequate and the shower still fades, you have a fault, not a sizing problem. Hot water that runs out covers dip tubes, lower elements, and crossed cartridges. Replacing a correctly sized heater that has a broken dip tube is an expensive way to buy a new dip tube.
Checklist: tank sizing
- Busiest hour identified. Uses outside that hour ignored.
- Worksheet gallons written down, adjusted only where your shower or washer clearly differs.
- First-hour rating read from the label or the spec sheet, not from the model’s gallon count.
- Rating within about 1 or 2 gallons of the peak hour.
- Heat pump replacements checked for a possible upsizing versus the gas tank they replace.
- Vent or circuit confirmed possible. Installed price, not pallet price.
- If the old heater meets the rating and still fades, diagnose the fault before you upsize.
Sources
U.S. Department of Energy, Energy Saver, “Sizing a New Water Heater,” including the peak-hour worksheet (shower 10, shaving 2, hand dishwashing or food prep 4, automatic dishwasher 6, clothes washer 7), the worked example totaling 36 gallons, the instruction to match first-hour rating within about 1 or 2 gallons, and the note that the worksheet is not a full-day estimate.
Adapted in that guide from Federal Energy Management Program material. ENERGY STAR, “What Goes into the Cost of Installing a Heat Pump Water Heater?,” on sizing differences when replacing gas. U.S. Department of Energy, Energy Saver, “Which Water Heater Is Right for You?,” for the storage and heat-pump context.