The Best Tankless Water Heater

Powerful, efficient, and reliable

The 30-Second Review

The best tankless water heater is energy efficient, reasonably powerful, technologically equipped, and comes from a company that supports its customers both during purchasing and installation and down the road. We've found the top gas and electric series from reliable manufacturers; your task is to find the right-sized model for your home. We’re here to help. We’ll walk you through a few simple calculations so you can find your perfect fit.

  • Best Electric Series for Mid-Sized Homes: Electric units aren’t as powerful as gas, but the Stiebel Eltron comes close. The Tempra Plus series offers top power and efficiency, while uniting all the best tech features on the electric tankless market.
  • Best Gas Series for Mid-Sized Homes: Offering excellent power for its price, the K4 series is perfect for the mid-sized household that often runs two showers at once. Plus, high-tech features come standard.
  • Best Gas Series for Larger Homes: If your large household or cold climate requires the mightiest tankless unit on the market, the Professional Prestige series is for you. Top-of-the-line efficiency means these machines work hard to justify their price.

The Best Tankless Water Heaters

An old-fashioned storage tank water heater is the third largest consumer of energy in the average home, constantly guzzling fuel just to keep unused water at temperature. Tankless water heaters are a cost- and energy-efficient alternative, heating water only as needed. And while the upfront costs are relatively higher, the life expectancies of tankless units are up to twice as long as that of tank-types (20 years vs. 10).

We liked the Takagi K4 series for most households running gas. Its mid-range power is enough to supply hot water to several fixtures simultaneously and offers an array of safety features that reduce the risks associated with gas units.

Don’t have a gas hookup? Stiebel Eltron is your best bet for electric. The company specializes in electric models, and has led the industry forward with innovations that save energy and increase power. Their top models can supply ample hot water for a couple of simultaneous showers and a sink.

If you’re looking for the most powerful option out there, check out the Professional Prestige series from Rheem. Representing the next level in gas water heating, these high-efficiency natural gas or propane units can keep up with a washing machine, dishwasher, and shower running simultaneously. Throw in the kitchen sink on full blast, too.

How to Find the Best Tankless Water Heater for Your Home

As mighty as these mini machines are, manufacturers invariably exaggerate their capabilities. They crow that you will “never run out of hot water,” but the truth is you can run out if you overtax the unit’s flow rate — measured in gallons-per-minute (GPM). Purchasing a unit with the right GPM for your household is key.

So, how much hot water do you need? A tankless unit has to quickly heat incoming cold water and supply it to your pipes every time you turn on a faucet or start a load of laundry. The more powerful the water heater is, the quicker it can heat and supply. But a big house in Minneapolis is going to need a lot more power than a condo in San Diego. Using the tools below, we’ll help you choose the unit that’s best for your home.

Figure out your temperature rise.

Start by finding your groundwater temperature. Water temperature varies by region and season. The colder the water coming into your home is, the more your unit will have to work to bring it up to temperature, so it is best to size your unit based on winter conditions.

Average Groundwater Temperature
Northernmost States
40° F
Midwest/Southern States
50° F
Southern California, the Southwest, and Gulf States
60° F
Hawaii and Puerto Rico
75° F

To find the exact water temperature of your area, check with your local utility company.

Then subtract your ground water temperature from 120°. This is your temperature rise.

Tankless units sense incoming water temperature, heating it up to match your water heater’s thermostat setting. The standard is 120° F, unless you’re running a sanitizing cycle in your washing machine. The greater the difference between groundwater temperature and 120° F, the greater the demand placed on your unit.

Next, add up how much hot water your household uses.

Sites like Compact Appliance say to add up the GPM of only the fixtures you plan on using during a time of peak demand — like when your entire family is getting ready in the morning. Buying a tankless unit with a capacity lower than that means you’ll quickly exceed its limits and feel the icy effects.

However, the Bosch representative we spoke to insists you should add up the GPM of every fixture in your house. This way, it’s impossible to run out of hot water. But how often do you run every single fixture at the same time? We recommend a moderate approach:

Add the GPMs of the fixtures you simultaneously use, then round up generously. This is your flow rate.



Bathroom sink






Kitchen sink




Washing machine


*Depending on fixture design and age.
Adapted from The Engineering Toolbox.

Now, put those two numbers together. Your water heater should deliver the flow rate you need at your local temperature rise.

In all the customer reviews we read, the number one complaint was clear: The purchased unit was not as powerful as manufacturers led them to believe. Many manufacturers list the max flow rate — how many gallons of hot water the unit would produce if it was, for example, working with the lukewarm groundwater of the Gulf states. This optimistic GPM will not apply to the vast majority of homeowners.

Robert Dishman, General Manager of Alliance Plumbing in Portland, Oregon, reports that homeowners need to do their research and choose a unit that is “sized and designed correctly for their household hot water demand. Then they’ll be a happy customer.” While it may be tempting to go with a cheaper, lower-GPM model, we garnered from reviews and expert input that these will likely fail to supply the hot water your household needs.

For each of our preferred series, we have provided their GPM capacities at a range of temperature rises, allowing you to see their power as it applies to your climate. If you opt for a different unit, find the same information in spec PDFs on the company website or farther down the page on sites like The Home Depot. It may take a few extra clicks, but finding a unit’s flow rate for real temperatures will give you much more accurate expectations.

Check your fuel type.

Tankless water heaters are available in natural gas, propane, and electric models, but the best choice for you is likely whatever fuel type you’re currently set up for. However, if you are remodeling or building from the ground-up, weighing the benefits and drawbacks of each fuel source is worth your time.

Electric units are cheaper to install and maintain, but their flow rate maxes out at 5.5 GPM. Natural gas and propane units are more common — and more powerful. Some models can pump out water at up to 10 GPM.

Our Picks for the Best Tankless Water Heater

Best Electric Series for Mid-Sized Homes

Stiebel Eltron Tempra Plus The reigning champ of electric water heating, offering stellar technology, sleek design, and reliable service.

It’s only right that Stiebel Eltron should excel in the tankless market: It was the first to manufacture them back in 1927. Of course, tankless water heating has developed substantially since then, but the company maintains its position by continuing to develop new technology.

Few other manufacturers of electric models can come close to Stiebel Eltron in terms of GPM and energy efficiency — all its models boast an energy factor of 0.99, the highest rating among electric tankless. Owning the best of the best comes with a price, however, as Stiebel Eltron surpasses the competition in terms of capacity and cost. Its models typically ring in at $100 to $200 more than units from other brands with equivalent or slightly lower GPM.

So are they worth the upcharge? We say yes, based on power, reliability, cutting-edge technology, and customer support resources.

Stiebel Eltron’s two series, Tempra and Tempra Plus, both come in the same six models: 12, 15, 20, 24, 29, and 36. Like climbing the steps of a staircase, each one increases in power. If you want one to supply your whole house, stick with the 29 and 36 models, which can provide over 4 GPM, but check the chart below to find the model that can deliver your ideal flow rate at your current temperature rise.

In true German fashion, all their models come in the exact same dimensions. At a briefcase-size of about 17 x 15 x 5 inches, installing one in a basement or closet would free up a significant swathe of space. The Tempra and Tempra Plus are sleek too. They feature brushed gray metal covers and detailed control knobs. According to the Stiebel Eltron rep we spoke to, the main difference between the two is that the Plus model uses Advanced Flow Technology. We preferred this option because it means your water will stay warm no matter what, but it also comes with $50 to $200 price jumps.

We appreciated that getting information from Stiebel Eltron was always a straightforward affair. Their customer service phone number actually brings you to customer service, not impenetrable webs of reconnections and dialing 0 for an operator.

Stiebel Eltron Tempra and Tempra Plus Series

80° F
1.0 GPM
1.25 GPM
1.65 GPM
2.0 GPM
2.5 GPM
3.0 GPM
70° F
1.25 GPM
1.5 GPM
1.9 GPM
2.25 GPM
2.75 GPM
3.5 GPM
60° F
1.35 GPM
1.75 GPM
2.25 GPM
2.75 GPM
3.25 GPM
4.0 GPM
45° F
1.82 GPM
2.19 GPM
2.75 GPM
3.64 GPM
4.37 GPM
5.46 GPM

Electric Runner-Up

EcoSmart ECO 27 Tankless Water Heater If your water needs are small, a budget-friendly option from EcoSmart may do the trick.

While moving lower down the price scale means sacrificing some power and reliability, we were impressed with the EcoSmart ECO 27. The ECO 27 costs less than $500, yet its 3 GPM in a 45° F rise is enough to power two low-flow showers at the same time. It is also an Amazon’s Choice product, which means it is highly rated, fairly priced, and available for Prime shipping.

Be aware that as price reduces, so does GPM, and so does your shower duration. Many cheap units ($100 to $200) are really only good for point-of-use installation at a sink.

Best Gas Series for Mid-Sized Homes

Takagi K4 Tankless Water Heater A solid option for most gas-fueled households, offering a mid-high GPM and a plenitude of customer support.

Takagi crafts well-equipped tankless gas units for just about every flow rate, and was the only other manufacturer besides Stiebel Eltron that could confirm its models use flow control technology. We like the K4 series for a mid-sized household, but Takagi also produces gas units capable of supplying more than 8 GPM, even with extremely cold incoming water. When we calculated price per GPM, we found that the K4 offers the most hot water bang-for-your-buck, around $50 less per GPM than its peers. The K4 offers a lot of power, plus nice-to-have features, at a reasonable price.

Pay attention to fuel type. Natural gas and propane models are generally identical in terms of cost, EF, and GPM, but are not interchangeable: Be sure you're buying the right fuel type of any gas unit you select, as well as the correct model for your desired install location — indoor or outdoor.

Standard features include anti-freeze technology, so its outdoor units run smoothly through all seasons, and a Hi-Limit Switch that ensures water temperature stays within safe levels. This is important for gas units, as inaccurately sized gas lines or venting may produce what’s known as “cold water sandwiches” — unexpected bursts of cold water.

We’re happy to report that calling Takagi was quick and painless every time. Within minutes of dialing, a knowledgeable support rep answered all of our questions — without having to look them up or transfer us. We also appreciated the library of resources on Takagi’s website. It’s chock-full of product specifications, manuals, troubleshooting guides, and even a rebate finder. Whether you prefer to call for assistance or want to DIY the solution yourself, it’s clear that Takagi excels in customer support.

Takagi Mid-Efficiency Models

80° F
2.9 GPM
3.8 GPM
3.9 GPM
4.3 GPM
70° F
3.3 GPM
4.3 GPM
4.4 GPM
4.9 GPM
60° F
3.8 GPM
5.1 GPM
5.2 GPM
5.7 GPM
45° F
5.1 GPM
6.6 GPM
6.9 GPM
7.6 GPM

Best Gas Series for Larger Homes

Rheem Professional Prestige Series Virtually endless hot water via one of the most powerful condensing gas units currently on the market.

The Professional Prestige series is able to generate impressive quantities of hot water while maintaining the kind of high efficiency that garners Energy Star approval. A large part of the series’ efficiency is due to condensing technology that prevents hot gas from escaping, reusing it to heat more water instead. This feature costs more upfront, but if your household size demands a high flow rate, condensing ensures that every drop of energy is going toward hot water. A positive side effect of this efficiency is a smaller utility bill.

Rheem’s current top condensing gas model, the Professional Prestige 96, weighs in at a whopping 82 lbs, with hefty dimensions to match — approximately 28 x 19 x 10 inches. If the Stiebel Eltron is a briefcase, the 96 is a piece of checked luggage. But the unit’s heft speaks to its capabilities; the machine boasts a powerful flow rate (8.5 GPM) and an impressive 0.96 energy factor, well above Energy Star’s expectations for gas efficiency.

Stay Tuned The highly anticipated Bosch Greentherm 9000 series promises to displace our Best for Larger Homes pick come Fall. This high-tech series has already been named an honoree of the CES 2017 Innovation Award in Home Appliances.

The Rheem website is far less user-friendly than Takagi’s, requiring a fair amount of snooping to dig up the same information. You have to hunt down individual models to find their specifications and troubleshooting documents, but with enough poking around, you can find what you need to make a selection, install your unit of choice, and keep up with maintenance later on.

The Professional Prestige line is available in both natural gas and propane, and for both indoor and outdoor installation. Again, double check that you are buying the right model for your specifications.

Rheem Professional Prestige Series

Professional Prestige 84
Professional Prestige 90
Professional Prestige 96
80° F
3.8 GPM
4.3 GPM
4.8 GPM
70° F
4.3 GPM
4.9 GPM
5.5 GPM
60° F
5.0 GPM
5.8 GPM
6.4 GPM
45° F
6.7 GPM
7.7 GPM
8.5 GPM

Did You Know?

You can save even more water with an aerator.

If you are looking for more ways to reduce bills and save water, consider installing aerators on faucets to restrict their maximum flow. Low-flow shower heads and sink fixtures also work to moderate output. Old bathroom sink fixtures may be spewing out water at a GPM of 2.2 or more, when 0.5 is all you need.

Recirculate for instant hot water.

Because tankless water heaters need to start the flow of water before the heater can flip on, you may find yourself running cold water for up to a minute. Reduce the waste and the wait time by installing a recirculating pump, or by selecting a unit that comes already equipped (like Rinnai SE+ Series with ThermaCirc360). Recirculating pumps collect cold water as it exits the unit, and brings it back for another pass through the heating element.

Every fuel type has something green to brag about.

  • Natural gas may be a fossil fuel, but it is also the cleanest burning. An MIT study entitled “The Future of Natural Gas” proposes that natural gas may be the bridge to a low-carbon future and reduced oil dependence.
  • Propane, despite a reputation to the contrary, is also a relatively clean-burning fuel due to its low carbon content, and at its increased potency, less powers more, and that equals reduced emissions.
  • Electricity, depending on where you live, may in turn be powered by natural gas; coal; nuclear; or renewables like water, wind, or sun. Unfortunately, according to 2016 data gathered by the US Energy Information Association, around 65 percent of generated electricity is derived from fossil fuels. So while running electric cars and appliances may be emission-free, making electricity is often not. But if you do happen to live in an area that utilizes clean energy sources, opting for an electric tankless water heater is just about as green as your water can get.

Maximize your warranty coverage

There’s a lot of hairsplitting when it comes to warranty coverage; companies accept responsibility for parts, labor, and replacements for different amounts of time — around five years, three years, and one year, respectively. Often, the aspect of the water heater that comes with the longest warranty is the heating chamber (electric) or the heat exchanger (gas); after that: parts.

However, we discovered that many warranties only apply if the unit is professionally installed and, in some cases, regularly serviced.

Manufacturers should make all this information readily available to you as you go through the purchasing process — another reason we prioritized good customer resources in our selection process.

The most common issue affecting tankless water heaters is also one that manufacturers wash their hands of: hard water damage.

And this is unfortunate because, according to Ryan Gardener, a tech support representative with HomePlus Products, “By far, the most common thing that can kill a tankless water heater is excessive hardness.” He is referring to hard water’s tendency to deposit scale over time, and tankless’ vulnerability to it. As the minerals in hard water calcify, scale begins to insulate the heating element, forcing it to work harder and harder to get its job done.

You can run at-home tests to check the mineral composition of your water supply, or you can make an approximation using geological maps. Experts recommend performing active treatment on your tankless unit before calcification becomes a problem. Every six months to a year, flush it with store-bought descaler or clean white vinegar.

The Best Tankless Water Heater: Summed Up

Tankless Water Heater
The Best
Takagi K4
Best Gas Series for Average Households
Stiebel Eltron Tempra Plus
Best Electric Series for Average Households
Rheem Professional Prestige
Best Gas Series for Larger Households