Short answer
Match the first hour rating (FHR) to your peak hour demand. Add up the hot water used in your busiest hour: the Department of Energy worksheet uses 20 gallons per shower, 2 per shave, 3 for hand dishwashing, 7 per dishwasher load and 25 per top loading washer load (15 front loading). Buy a heater whose FHR is at least that total. Three showers, a shave and some dishes come to 66 gallons, so that home needs an FHR of 66 or more.
Your peak hour
How the calculator works
- Gallons per use come from the Department of Energy. Its Energy Saver worksheet lists the average hot water used per shower, shave, hand dishwashing, dishwasher load and washer load (DOE).
- Your busiest hour sets the target. The worksheet adds up one hour only, because that is when a tank runs out. It does not estimate daily use.
- The heater must at least match it. DOE’s advice is to pick a model whose first hour rating at least matches your peak hour demand.
- The tank size shown is a starting point, not a rule. It is our shortcut for where to begin shopping. The first hour rating on the label is what you match.
DOE peak hour worksheet values and its worked example
| Use | Gallons per use | Example: busy morning |
|---|---|---|
| Shower | 20 | 3 showers = 60 |
| Shave (sink running) | 2 | 1 shave = 2 |
| Hand dishwashing or food prep | 3 | 1 = 3 |
| Dishwasher load | 7 | none |
| Top loading washer load | 25 | none |
| Front loading (H-axis) washer load | 15 | none |
| Peak hour demand | 66 gallons |
Gas, electric or heat pump at the same size
The first hour rating depends on how fast the heater can reheat, so fuel matters. A gas burner usually puts heat back into the water faster than electric elements, so an electric tank often needs more gallons to reach the same rating. Heat pump models list their rating in the spec sheet. Compare the three in the gas vs electric guide and the heat pump guide.
When a bigger tank is not the answer
- No room. A taller or wider tank may not fit the closet or the drain pan. Check the size guide for typical dimensions.
- One busy hour, quiet the rest of the day. Spreading laundry or the dishwasher to another hour lowers the demand without a bigger tank.
- A distant bathroom. A small point of use tank near a far sink can cover it without upsizing the main heater.
Questions people ask
What is a first hour rating?
The number of gallons of hot water a storage water heater can supply in one hour, starting with a tank full of hot water. It depends on the tank size, the fuel, and the size of the burner or element. The EnergyGuide label shows it as Capacity (first hour rating).
Is a 40 gallon or 50 gallon tank enough for a family of four?
It depends on how many showers run back to back and whether laundry runs in the same hour, which is why the first hour rating matters more than the gallons. Count your busiest hour in the calculator, then compare real first hour ratings: a 40 gallon gas tank can match or beat a 50 gallon electric one.
Where do I find the first hour rating?
On the yellow EnergyGuide label of conventional gas and electric tanks, in the top left corner, and in the maker’s spec sheet. Heat pump water heaters are not required to carry the label, so check their spec sheets.
Does the calculator account for long or low flow showers?
No. It uses the Department of Energy averages per use. If your household takes long showers or has high flow shower heads, count each long shower as two.
Sources
Last reviewed October 29, 2026. Models, codes and listings change; check your heater’s manual and your local code, and follow the maker’s instructions.