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What Tank Water Heater Size Do I Need? [FAQ]

What Tank Water Heater Size Do I Need? [FAQ]
Need a new storage tank water heater? You may be wondering, “What water heater size do I need?”

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Great question (and an important one, too).

A tank that’s too large will run up your energy bills. And one that’s too small will run out out of hot water too quickly, leaving you in a deep freeze during shower time.

So, just like Goldilocks and the three bears, you want to get the size that’s just right.

To do that, do these 2 things:

  1. Find your family’s highest demand of hot water during the day
  2. Find a water heater size that matches that demand of hot water

It’s just like shopping for shoes. Measure your foot size; find the shoe that fits. Simple!

Step 1: Measure your peak hour demand

First, think about the time of day you’re using the most hot water in 1 hour. It’s usually the morning or evening. This is your “peak hour demand.”

Next, estimate how much water you use in that 1-hour period. To do that, use this worksheet from the U.S Department of Energy.


For example, let’s say your highest demand of hot water is in the morning from 6:30 to 7:30. Three people take a shower and 2 people shave. Using this worksheet, you figure out the peak hour demand like this:

  • 3 showers x 10 gallons per shower= 30 gallons
  • 2 shavings x 2 gallons per shaving = 4 gallons
  • 30 gallons + 4 gallons= 34 gallons

That means you need a tank size that can give you at least 34 gallons of hot water within that 1 hour. (And maybe more than 34 gallons in case you have guests.)

OK, so the foot has been measured. Time to find the shoe that fits.

Step 2: Find the first hour rating that matches your peak hour demand

First hour rating (FHR) measures how much hot water a water heater supplies in 1 hour assuming that the entire tank is full of hot water.

Finding this is simple: look on the top left corner of the water heater’s yellow Energyguide label. Or, if you’re shopping online, look in the product details.

Here’s an example:


This electric water heater has a capacity (1st hour rating) of 48 gallons.

Still need help finding the right water heater?

Contact us for advice on the size or type of water heater that fits your family’s needs.

Nicholson Plumbing, Heating, and Air Conditioning gladly serves Framingham, MA and MetroWest. Contact us online to learn more about our services.