First hour rating is a calculated amount used to explain the performance abilities of a water heater within the first hour of use when recovered to the thermostat setting.
When determining the first hour rating, one starts with a fully heated tank of water.
Approximate first hour rating can be determined with the following formula:
Tank Capacity x .70 + Recovery = First hour rating.
Example: 40-gallon water heater elements are 5500 watts (electric water heater)
40 (gallons) x .7 (70 percent of the tank capacity)
+ 22.5 (GPH at 80 degree rise in temp)
= 50.5 gallons first hour rating.
Why multiply by 0.70? The amount of water in the tank is multiplied by 70 percent because as water is being used, new cold water is entering the tank and diluting some of the heated water.
The thermostat senses the cold water introduced into the tank and begins the heating process again.
Use the following guide as a quick reference in determining 70% of the tank capacity:
| If your tank capacity is: | 70% of your tank capacity is: |
| 30 gallons | 21 gallons |
| 40 gallons | 28 gallons |
| 50 gallons | 35 gallons |
| 65 gallons | 45.5 gallons |
| 75 gallons | 52.5 gallons |
| 80 gallons | 56 gallons |
| 100 gallons | 70 gallons |
| 120 gallons | 84 gallons |
