The Cost Desk

Build calculator

Hot water in your busiest hour, for tank water heater sizing

The worksheet the Department of Energy uses for storage water heaters, as a calculator: your busiest hour of hot water use, to set against a heater's first hour rating.

Data last checked . Every source is listed below.

Count what happens at the same time on the busiest morning or evening, not all day.

Hot water in the busiest hour

The Department of Energy worksheet pairs this with a tank water heater whose first hour rating is within a gallon or two of 36 gallons.

Showers
30 gal3 x 10 gallons
Shaves at the sink
2 gal1 x 2 gallons
Hand dishwashing or food prep
4 gal1 x 4 gallons

For information only: not advice, a design or a code ruling. We take care, but our sources can lag and so can we, so a figure may be out of date. Package sizes and prices come from the maker and the store, and codes from the local building authority.

What moves the number

The few things that change the answer the most, in plain terms.

First hour rating
A tank heater's first hour rating is the gallons of hot water it can supply in an hour starting with a full tank. It is printed on the EnergyGuide label and depends on the tank, the burner or element and its size.
Overlap
What counts is use in the same hour, not use all day. Three showers back to back in one morning set the figure far more than the same showers spread over a day.
Not daily use
The worksheet does not estimate total daily use. A big tank with a small burner can hold plenty of water and still run out in a busy hour.
Matching rule, old and new
An earlier version of the Department of Energy page said to look for a first hour rating within a gallon or two of peak demand. A later version says to look for one at least as large. The two readings differ by a gallon or two.

The working

Step by step, with the numbers from the starting example, so you can check it on paper.

  1. Showers: 10 gallons x 3 = 30 gallons.
  2. Shaves at the sink: 2 gallons x 1 = 2 gallons.
  3. Hand dishwashing or food prep: 4 gallons x 1 = 4 gallons.
  4. Busiest-hour demand: 36 gallons.

Try a different case

Each opens the calculator with the numbers filled in.

What we assumed

Where a number is an example, we say so.

  • Checked sourceGallons per use are from the Department of Energy worksheet: shower 10, shaving 2, hand dishwashing or food prep 4, automatic dishwasher 6, clothes washer 7. It assumes no water-saving fixtures.
  • Our estimateBaths, a second dishwasher run and recirculation loops are not on the worksheet and are not counted.
  • Our estimateThe figure is a hot water demand. It is not a recommendation of a model or a size; manufacturers list first hour ratings by model.

Sources

Related pages