What Size Water Heater Do I Need?

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What Size Water Heater Do I Need?

Want to have some fun? Let’s go shopping for a major home appliance! How about a water heater?

But before we go shopping to select a new water heater, first we have to figure out what size water heater we need. Plus, we have to determine which type of water heater we want.

Types of Water Heaters

What Size Water Heater Do I Need?: Storage Tank Water Heater EnergyGuide Label

First Hour Rating on Storage Tank Water Heater EnergyGuide Label

If you are replacing an existing water heater, in most cases your old unit includes a water storage tank. The simplest replacement scenario would be to swap out the old water heater with a similar new unit. However, that might not be possible, especially if your unit fits snuggly in its current space. Recent changes to the U.S. Dept. of Energy water heater manufacturing regulations, to improve energy efficiency, has resulted in larger storage tanks, that take up even more space than they have in the past.

If lack of space for a bigger water tank is an issue, you can consider a wall-mounted tankless water heating system. According to Energy.gov, a tankless model is also the most energy-efficient type of water heater on the market. A new or replacement tankless water heater would provide the long-term lowest operating and repair costs. A tankless system would also have a longer lifetime than a tank-type system. The decades-long savings of a tankless system offset any additional up-front investment.

The type of fuel you use, gas or electricity, is another factor that determines a water heater’s energy efficiency. Gas units are more efficient than electric models. Solar water heaters are emerging as a very energy-efficient option, but the specific environmental requirements, architectural renovations, and up-front costs currently limit candidates for this option. Geo-thermal and heat pump water heaters are also available, under certain conditions. Other situation-specific types of water heaters include utility (or point-of-use) systems and mobile-home systems.

Size Matters

Installing a water heater that is not the proper size can be a costly mistake. A unit that is too large will increase the upfront-cost and your monthly heating bill for many years. A unit that is too small will cause you to run out of hot water, especially during periods of peak demand.

So, how do you determine the proper size water heater for your household? Are your ready to do some math? The water heater that will best meet your needs depends on a few factors that need to be calculated.

Storage Tank Water Heater

For a tank-type water heater, you need to match your demand during the busiest hour of hot water usage with the “First Hour Rating” of the unit you choose. To calculate your demand, consider the amount of hot water your family uses during activities like showering, shaving, washing clothes, and washing dishes by hand or in an automatic dishwasher.

Dept. of Energy 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 10 × =
Shaving (.05 gallon per minute) 2 × =
Hand dishwashing or food prep (2 gallons per minute) 4 × =
Automatic dishwasher 6 × =
Clothes washer 7 × =
Total Peak Hour Demand =

This worksheet example shows a total peak hour demand of 36 gallons. Therefore, this household would need a water heater model with a first hour rating of 34 to 38 gallons.


3 showers 10 × 3 = 30
1 shave 2 × 1 = 2
1 hand dishwashing 4 × 1 = 4
Peak Hour Demand = 36

Adapted from information from the Federal Energy Management Program Energy Cost Calculator

*The above worksheet is based on standard usage with no water conservation measures.

Tankless Water Heater

What Size Water Heater Do I Need?: Tankless Water Heater EnergyGuide Label

Gallons Per Minute Flow Rate on Tankless Water Heater EnergyGuide Label

To determine the proper size tankless water heater you need to meet peak demand, you need to calculate two factors. First, how many gallons per minute (flow rate) of hot water will you need? Second, how many degrees of heat need to be added (temperature rise) to the water flowing through the system?

Your peak demand flow rate is determined by adding up the hot water needs you could have at one time. The “Uses” shown in the worksheet table above apply to this equation. The temperature rise you’ll need will depend on your current ground water temperature, which varies by location, compared to the hot water temperature you need, which varies by use.

A tankless system can be customized to efficiently meet your peak demand needs. Multiple units can be placed to ensure all faucets and appliances have a sufficient flow rate of hot water whenever needed.

Long-Term Cost Matters Too

So, which water heater is right for you? During your decision-making process, remember you will live with and pay for the annual operating cost of your new water heater for perhaps 15 to 30 years. It might serve you well to take a few minutes to calculate the annual operating cost of the models you’re considering buying.

If you would like more information about what size water heater is right for you, just contact us. We’ll be glad to answer any questions about your water heating needs.