Sizing Water Heaters
To properly size a storage water heater, for your family use the water heater’s first hour rating (FHR). The first hour rating is the amount of hot water in gallons the water heater can supply per hour (starting with a tank full of hot water). The first hour rating depends on the tank capacity, source of heat (burner or element); and the size of the burner or element.
The Energy Guide Label lists the first hour rating. The Federal Trade Commission requires and Energy Guide Label on all new conventional water heaters, but not on heat pump water heaters.
After you have completed your version of the example worksheet below, you will want to find a water heater model with a first hour rating that matches within 1 or 2 gallons of your peak hour demand, the daily peak 1 hour hot water demand for your family and home.
Figure the following to estimate your peak hour demand:
- Determine what time of day (morning, noon, evening) you use the most hot water in your home. Keep in mind the number of people living in your home.
- Use the example worksheet below to estimate your maximum usage of hot water during this one hour of the day. This is your peak hour demand. Note: This worksheet does not estimate total daily usage of your hot water.
The example worksheet below shows a total peak hour demand of 88 gallons, therefore this household would need a water heater model with a first hour rating of 86 to 90 gallons. Visit our comparison chart to find the water heater that will fit your needs- GAS water heater chart or ELECTRIC water heater chart.
| Type of Use | Avg. of Hot Water Per Use | Times Used Per Hour | Gallons Used Per Hour |
| Shower | 15 | 3 | 45 |
| Bath | 20 | 1 | 20 |
| Shaving | 2 | 1 | 2 |
| Hand/ Face Washing | 4 | 3 | 12 |
| Shampoo Hair | 4 | 1 | 4 |
| Hand Dishwashing | 4 | 0 | 0 |
| Automatic Dishwasher | 14 | 0 | 0 |
| Food Preparation | 5 | 1 | 5 |
| Automatic Clothes Washer | 32 | 0 | 0 |
| Total Used | 88 |
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