Do you need some advice on which electric tankless water heater to buy? This guide will help. has been selling tankless water heaters since 2010. We have answered thousands of questions over the years, and distilled it all down to one page, right here for you. Please comment below if you have any feedback.

Quick key to abbreviations. (We like abbreviations. They make the article shorter.)
TWH = tankless water heater
kW = kilowatts
GPM = gallons per minute
NTI = needed temperature increase
TI = temperature increase

The Two Most Common Mistakes Buyers Make When Buying a Tankless Water Heater

We know from experience that buyers often make either of two mistakes:

Tankless Buying Mistake #1

Buying an underpowered TWH, with too few kW. Related:

  • Expecting a TWH to operate like a tank, and that you’ll get nice, hot water at several gallons per minute flow rate with a small, inexpensive TWH (with just a few kilowatts). (They won’t.)
  • Expecting a TWH to give unlimited hot water. Instead, a TWH will provide unlimited heated water (which may or may not be enough temperature increase for you). Big difference.
    Solution: Read our guide below.

Tankless Buying Mistake #2

Buying a powerful TWH with more than enough kW, which can provide plenty of hot water, but the buyer is:

  • not aware that they have insufficient ampere capacity (ampacity) in their electrical panel. They never read the TWH’s specs, or didn’t understand all of it, but clicked “Buy” anyway. This often happens with the popular 27 kW or 36 kW heaters. Solution: Choose a tankless that matches your needed hot water demand. Guide is below.
  • fully aware they they need to upgrade their breaker box, but did not get a quote from an electrician in advance. Then, he arrives to install it, looks at the heater’s kW, and quotes more than $1000 to install it. Ouch! Solution: Get a quote in advance before buying a TWH.

The 4 Things You Must Know to Make the Right Tankless Choice

Replacing your tank water heater with a tankless? Read our buying guide first.

Space saving is one of the main benefits of a tankless water heater

People shopping for an electric TWH must have the following info. With it, you can know which heater to buy.

1. Your coldest water temperature of the year. If you don’t know, then follow these instructions.
1.1.  Is it a really cold week in winter, right now? If so, then run the cold water under a thermometer for the full amount of time you plan to use that point of use. (Why? Because the pipe temperature inside the house/building is affecting the water temperature, and you may or may not get the actual cold water temperature from your water service.)
1.2. If it’s not one of your coldest weeks in winter, then the method above might lead you to underestimate the size of TWH you need when that cold weather arrives. Instead, use this table of Cold Water Inlet Temperatures Selected U.S. Cities. (Do not use the commonly shared maps of average groundwater temperature: Your groundwater temperature may be very different from your cold water temperature, as supplied by the public utility.)

2. The total flow rate of all the ways that you might use hot water simultaneously. Here are the flow rates for common uses:
Shower head (low flow): 1.5 GPM
Shower head (standard flow): 2 GPM
Kitchen sink faucet: 1.5 to 2 GPM
Bathroom/lavatory sink faucet with low-flow aerator: 0.5 GPM
Bathtub faucet: 4 GPM!
Clothes washer: 2 GPM
Dishwasher: 2 GPM (but you probably should not include this in your calculation, since it is unnecessary to use your valuable, available TWH hot water flow rate)

3. The available amperage (amps) in your electrical panel for the TWH

4. Your needed temperature increase (“NTI” for short). Subtract the hot water temperature you seek (usually 105°F) from the temperature in question #1. Example: In Texas during the coldest winter weeks, the incoming water temperature might be about 45°F. Subtract 45°F from 105°F. The difference is 60°F. That’s the NTI.

Gather the Data, and Go Shopping

Do you have a TWH model picked out? Look at the model’s specifications. Find the table or spec showing the temperature increase (“TI” for short, also called temperature rise, temperature delta, delta T) at a given flow rate. Often it is not shown on Amazon or eBay, but you absolutely must have this info to make the correct choice. All of the product descriptions on have TI vs. flow rate data, so you can search for the model you are considering there.

Now, go back to the total simultaneous flow rate that you found in #2 in the previous section above. Find the number on the TWH’s TI table.

Next, we do some simple math to determine whether a TWH will heat water to 105°F.

Let’s say that a TWH you are considering provides a TI of 39°F at a flow rate of 2.5 GPM. Take your flow rate number in #2 above, and with simple math, convert the TI with that flow rate. Example for one low-flow showerhead using 1.5 GPM:
1.5 ÷ 2.5 = 0.6
39°F ÷ 0.6 = 65°F
Therefore, that heater provides a 65°F TI using one low-flow showerhead alone.

Now for example, let’s say a second person begins to shower, also with a 1.5 GPM low-flow showerhead. Will there still be hot water for both? Simply divide your TI by 2. Now, both showers will have a 32.5°F TI. Big difference, right? This is the way tankless works: A heater will heat water with a lower TI at a higher flow rate, and a proportionately higher TI at a lower flow rate). All tankless water heaters operate on the same principle.

Your maximum NTI and water flow rate makes a big difference in output temperature for each point of hot water use, and thus tankless water heater selection, as you can see!

If a TWH doesn’t have a high enough TI using the flow rate number from #2, then:

  • Reconsider if you realistically will use all those points of hot water use simultaneously. For example, can you run the clothes washer at another time instead of when you are showering?
  • Choose a heater with more kW and amps. But remember, any required electrical upgrade can be expensive.

Does all that make sense? Do you know which tankless water heater to look for now? If not, you can contact us with your info in #1 through 4 above (or whatever you can determine in #1, 2, 3 and 4), and we will give you some free recommendations.

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