What Size Tankless Water Heater Do I Need? (Calculation Explained)

When it comes to the tankless water heater, one of the bigger mistakes is buying a unit that is too weak to satisfy all our hot water needs.

You don’t want one that is too small, nor do you want a tankless heater that’s too big and will needlessly spend energy. Tankless water heater size should be as appropriate to your household hot water needs as possible.

What size tankless water heater do I need?

Here’s the deal:

To properly answer what size tankless water heater do you need, you have to figure out two things beforehand:

  1. What are your maximum hot water needs?
  2. How much water per minute (measured in Gallons Per Minute or GPM) can a certain tankless water heater heat up, and by how many degrees?

To correctly size the tankless water heater, we need to make a rough estimation of our maximum hot water needs at any given point.

Most households have the highest hot water needs from 9 PM to 11 PM. That’s when we shower, brush our teeth using a hot faucet, and might even have a dishwasher running.

We need to tally all this hot water needs up. Here’s a useful table of how much GPMs do different water fixtures require:

Fixture Gallons Per Minute (GPM)
Shower 2.0 – 3.0 GPM
Faucet (kitchen, bathroom) 1.0 – 2.0 GPM
Dishwasher 1.5 – 2.0 GPM
Washing Machine 2.0 – 2.5 GPM

Example: If you’re taking a shower (100% flow and 110˚F hot water) and simultaneously use two faucets (100% flow and 110˚F hot water), you will need at least 5 GPM tankless water heater.

Tankless heaters can deliver anywhere from 2 GPM to 12 GPM of hot water. The 5-10 GPM ones are most appropriate for the majority of households.

Note: For low water needs up to 8 GPM, the electric tankless hot water heaters are appropriate. For bigger needs (8+ GPM), you should choose one of the best gas tankless hot water heaters here

Difference Between Maximum Water Flow And Realistic Maximum GMPs

When looking at different tankless heaters’ specs, you will notice they note the maximum GPMs. In practice, the maximum GMP your tankless heater will realistically achieve can be much lower.

Why the discrepancy?

Because the maximum water flow in GMP is measured by warming up 77˚F water. The inlet temperature of water currently in your pipes matters quite a lot.

In south Texas, for example, inlet water temperature is 77˚F. In Minnesota, for example, the inlet water temperature can be as low as 37˚F. That is an additional 40˚F difference a tankless water heater needs to overcome.

Short calculation: Let’s say we have a tankless heater with a maximum water flow of 10 GPM. In Texas, we can actually get 10 GPM of 110˚F water because the inlet temperature is 77˚F. The heater has to heat water from 77˚F to 110˚F; that’s a 33˚F difference.

On the other hand, the inlet water temperature in Minnesota is 37˚F. To heat water to 110˚F in Minnesota, a tankless heater has overcome a temperature difference of 73˚F degrees instead of a 33˚F difference in Texas.

You’re not from Minnesota or Texas? Here’s an infographic created for Rinnai RU160iP SE+ Series 9 GPM tankless water heater that will give you an idea of what is the maximum water flow in your state (valid of USA):

different ground water temperatures in order to calculate how big of an tankless water heater you need

Here’s another example based on the infographics above: If you live in Florida (77˚F inlet temperature), the Rinnai RU160iP SE+ Series tankless heater will have a maximum water flow of 7.1 GPM. That’s enough to run several showers simultaneously.

On the other hand, if you live in New York (52˚F inlet temperature), the same tankless water heater achieves a maximum water flow of 4.5 GMP. That is the direct result of the difference in inlet temperature.

In New York, the heater has to overcome an additional 25˚F. With the same heater and the same energy expenditure, you will be able to run 2 or 3 showers simultaneously.

Looking At Specifications Sheets

On specification sheets, you will notice that producers either give a maximum water flow number in GMP or maximum electric power in kW. Generally, gas-powered tankless water heaters come with a GMP number, while electric tankless hot water heaters come with a kW number.

As we’ve seen, the maximum GMP is relative. It depends where in the US you live (because that affects the inlet water temperature). Power (measured in kW), however, is absolute. We can compare how powerful different tankless heaters are by comparing their maximum wattage (as we’ve done in the table of the best tankless heaters below).

Let’s look at two examples. The first one concerns what size of a tankless water heater you need if you want to replace, let’s say, the 50-gallon water heater. The second one is based on the number of people living (and using hot water) in your household. For example, how large a tankless water heater does a family of 5 need.

What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater? (Example #1)

Here is how this situation goes: You currently have a 30, 40, 50, or even 80-gallon water heater and want to replace it with a tankless water heater.

The main difference, obviously, is that with a water tank, you have, let’s say, 50 gallons of hot water, and with a tankless, you have on-demand heating of water.

During an average 10-minute shower, for example, you spend about 10 gallons of hot water. If 3 people have a shower, you run a faucet or two, a dishwasher, and so on, you can quickly spend all those 50 gallons.

However, the case with the tankless water heater is different. You don’t have hot water in storage; the powerful heating exchanger in the tankless heater heats the water when you need it with a certain maximum GMP limit.

To replace a 50-gallon water heater, you would, roughly speaking, need:

  • 10 GPM gas tankless heater or at least a 27 kW electric tankless water heater if you live in the northern part of the USA.
  • 7 GPM gas tankless heater or at least an 18 kW electric tankless water heater if you live in the southern part of the USA.

That’s why Rinnai, the world’s best gas tankless heater makes, offers a wide array of models – from 7 GPM to 11 GPM:

choosing size of tankless gas water heater with 7 gpm, 9, gpm, 10 gpm and 11 gpm
Rinnai offers its gas tankless models in several sizes. You can check them all out here.

Be mindful that this is only a rough estimate. The smart thing to do is buy a tankless heater that is a bit more powerful than the estimated needs. Better be safe than sorry.

What Size Tankless Water Heater Do I Need For A Family Of 5? (Example #2)

If 5 people live in a single household, they can use several faucets or showers simultaneously. When picking the size of a tankless water heater, we have to take this into account.

A household hot water consumer that requires hot water fastest is a shower. 5 people can also simultaneously run several hot water faucets, a dishwasher, doing laundry, and so on.

In short, a family of 5 would need a 10 GPM gas tankless heater or 27 kW electric tankless heater if you live in the northern part of the USA, where the input water has a lower temperature. The tankless heater has to work extra hard to bring the water temperature up to 110˚F or 120˚F.

However, if you live in the southern part, the capacity of the tankless water heater can be reduced by as much as 30%. So, for a family of 5 in the southern part of the USA, that would mean a 7 GPM gas tankless heater or an 18 kW tankless heater should be more than enough to satisfy all hot water needs.

11 thoughts on “What Size Tankless Water Heater Do I Need? (Calculation Explained)”

  1. Hi there.
    I live in South Jersey where it looks like the ground water temp is 50F, according to the map.
    It’s a small house with just two people. We use the hot water for showers, dishes and the occasional bath. We do not yet own a washer machine but when we do we will occasionally use hot water in that as well. It will be a small machine, maybe 3cf.

    We want a tankless, electric hot water heather. Any advice?

    Reply
  2. We have 4 kids, oldest beginning to go into teenage years. So HUGE water needs for a family of 6. We have a traditional water tank now and run out of water if we have 3-4 people showering around the same time. We are in a very cold climate. I am assuming we’d need the largest unit?

    Reply
  3. We live in Central Florida and are building a two-story, 4200 s.f. house for a family of four (2 adults, 2 tweens). We will have 4 showers in the house but only 3 of them will be used on a daily basis. Right now, the plan is to have a Rinnai RL94E tankless gas water heater.

    Recently, we were told by one of the vendors that we may need two. Is that really necessary? Seems like an unnecessary expense.

    Thanks.

    Reply
    • Hello there, the Rinnai RL94E has a maximum how water flow of 9.8 GPM. Given that you’re located in Florida, you don’t really need 2 of those. In Boston or New York, maybe, but 9.8 GPM should be enough for a 4-person family.

      Reply
      • Thanks! We asked them about adding a recirculation system and they recommended the RUR 199E, which already has a built-in pump as well as adding a 3/4″ return line. I think having that configuration should be more than sufficient. I have no interest in maintaining 2 water heaters!

        Reply
  4. What size of electric water heater do I need for 2 persons, washing machine, dishwasher and kitchen sink. I live in the Caribbean.

    Reply

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