How to Determine Water Heater Capacity

You need to determine the capacity of your new electric water heater before installing it in your home. Although medium-sized homes typically use a 65-gallon water heater, some homes require less and some more. The steps below provide the standard formulas to apply in order to choose the correct size water heater your home needs.

Instructions

1. Refer to these standard usage points. Add one point for each person living in the house. Four or less points requires a 40-gallon water heater; five or six points requires a 50-gallon water; seven or eight points requires a 65-gallon water heater and nine or more points requires an 80-gallon water heater. A house with four people and two bathrooms, one washing machine and one dishwasher contains eight points. It requires a 65-gallon water heater.

2. Calculate the wire size for installing the water heater. Use the formula that says wattage equals volts multiplied by amps. For example, 5,500 watts divided by 200 volts equals 27.50 amps.

3. Determine the amp rating in order to calculate the power that each circuit contains. Check the circuit breaker on the main panel. Standard size circuits contain between 15 and 20 amps while heavier circuits have at least 30 amps.

4. Use the formula to determine watts by multiplying the voltage by the amps. For example, 20-amp circuit carrying 100 volts holds 2,000 watts.

5. Find the electrical resistance which controls the current's flow. The electrical resistance causes heat which warms the water in the heater. Use the standard formula of dividing voltage by amps.