Sizing Storage and Heat Pump (with Tank) Water Heaters

To properly size a storage water heater—including a heat pump water heater with a tank— for your home, use the water heater's first hour rating (FHR). The first hour rating is the amount of hot water in gallons the heater can supply per hour (starting with a tank full of hot water). It depends on the tank capacity, source of heat (burner or element), and the size of the burner or element.

The EnergyGuide Label lists the first hour rating in the top left corner as "Capacity (first hour rating)." The Federal Trade Commission requires an EnergyGuide Label on all new conventional storage water heaters but not on heat pump water heaters. Product literature from a manufacturer may also provide the first hour rating. Look for water heater models 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 home.

Do 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 worksheet below to estimate your maximum usage of hot water during this one hour of the day—this is your peak hour demand. Note: the worksheet does not estimate total daily hot water usage.

The worksheet example shows a total peak hour demand of 46 gallons. Therefore, this household would need a water heater model with a first hour rating of 44 to 48 gallons.

Worksheet for Estimating Peak Hour Demand/First Hour Rating
Use Average gallons of hot water per usage   Times used during 1 hour   Gallons used in 1 hour
Shower 12 ×   =  
Bath 9 ×   =  
Shaving 2 ×   =  
Hands & face washing 4 ×   =  
Hair shampoo 4 ×   =  
Hand dishwashing 4 ×   =  
Automatic dishwasher 14 ×   =  
Food preparation 5 ×   =  
Wringer clothes washer 26 ×   =  
Automatic clothes washer 32 ×   =  
      Total Peak Hour Demand =  


3 showers 12 × 3 = 36
1 shave 2 × 1 = 2
1 shampoo 4 × 1 = 4
1 hand dishwashing 4 × 1 = 4
Peak Hour Demand       = 46

Adapted from information from the Air Conditioning, Heating, and Refrigeration Institute
*The above worksheet assumes no water conservation measures.

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