The Best Tankless Electric Water Heaters 2021: Buying Guide & Reviews

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This article today is about the best tankless electric water heaters. The buying guide in the first half will help to give you all of the information you will need to make the most informed purchase decision. To make it easier for you to find the gold standard models, our review section has a range of high rated tankless electric heaters to help you get started right away.

Best Tankless Electric Water Heaters 2021

What is a Tankless Electric Water Heater?

There are two main types of water heater: storage tank heater (or “traditional” heater) and tankless heater.

In a traditional heater, after the cold pipe water is fed into the unit and is heated up to the set temperature by the heating elements (the component that’s responsible for the heating action), the resulting hot water is directed into a separate storage tank. The hot water you enjoy coming out of your showerhead or faucet, comes from that storage tank.

A tankless heater— true to its name— removes the storage tank from the equation. The water is warmed up by the heater and delivered directly to you thereafter. This design can dramatically reduce energy usage and has been proven to be able to save you a hefty sum on your utility bill in the long run. For more in-depth information, see our dedicated article on tankless heaters.

Unlike a traditional heater that’s constantly running to refill the tank and keep the water warm, a tankless heater only turns on when you start using the appliance. Because of this nifty change in the working principle, they have also been referred to by some as “on-demand heaters”.

For an electric-run tankless heater, the main components working to heat the water are the electric heating coils. They are quite similar to the ones you see in hot plates: when a high voltage current is run through them, the coils’ electrical resistance makes them heat up rapidly. The result is continual hot water when cold water passes by the extremely hot coils.

How to Select the Best Tankless Electric Water Heater


In order to properly size your heater, keep in mind two factors: the flow rate and the temperature rise.

A heater’s flow rate is the amount of hot water that the unit can produce at any given time. The most popular unit to measure flow rate is gallon per minute (or GPM.) As an example, a heater with a 2 GPM flow rate means that the unit is capable of generating two gallons of hot water per minute.

To see if a heater’s flow rate can satisfy your household’s demand or not, go around the house and write down all of the appliances that will be using the hot water (taps, sinks, showerheads, etc.) After you have accounted for every single one, refer to the table below and add up the flow demand of everything.

ApplianceAverage Flow Rate
Tub4.0 GPM
Shower1.5 – 2.0 GPM (depending on efficiency)
Washing Machine2.0 GPM
Dishwasher1.5 GPM
Sink1.5 GPM

The final sum is the maximum flow demand of your household.

However, it is very unlikely that all of the appliances in the house would be used in tandem to justify getting a heater with a flow rate matching your family’s maximum demand. 

Consider the water consumption habit of your household and estimate how many appliances are usually used at one time. This approximation should give you a nice idea of how capable your heater should be.

Temperature rise, or ΔT, the difference between the in-flow water temperature and the set water temperature. For example, the average groundwater temperature in the U.S. is 57°F. If you were to set your heater to 110°F, the temperature rise in this case will be 53°F.

This is important to know because it will tell you how efficient your heater needs to be. The greater the difference between the groundwater temperature and your set temperature (or, in other words, the higher the ΔT), the more power the heater must spend. 

ΔT, as a result, can also affect the flow rate. You will see that in colder regions where groundwater is near-frozen, even high performance heaters with an advertised flow rate of 11 GPM can see their water production rate halved to 4 or 5 GPM.

Energy Efficiency Rating

The energy efficiency of a water heater is denoted by its EF rating. EF signifies the amount of hot water that the heater can produce per fuel unit consumed. The higher the EF rating is, the more energy efficient your heater is. Usually, the EF rating of the heater is written right on the front label or the user’s manual.

Another indicator of energy efficiency is the Energy Star label.

Energy Star is a program started by the U.S. Environmental Protection Agency (EPA) in order to recognize and promote energy efficient products with the hope of bringing greater awareness to environmental protection.

For a tankless heater to be qualified for an Energy Star, the heater must be able to have an EF rating equal to or above a certain level. For tankless water heaters, that level is 0.67.

However, energy efficiency shouldn’t be confused with cost effectiveness. The Energy Star label will not tell you how much money you can save using that particular heater instead of another. 

For example, although tankless electric water heaters are dramatically more energy efficient than gas-burning heaters, the cost of electricity in some regions is higher than the cost of gas. As a result, even with all of the convenience, safety, longevity, and eco-friendliness of tankless electric heaters, they can turn out to be more expensive to use than gas heaters.


Electric water heaters are noticeably safer to use than gas heaters because they don’t have to deal with volatile gas and pipelines, which can leak if not properly maintained.

The fact that the heaters we are discussing today are tankless further helps with safety. They don’t have storage tanks and are much less prone to explosion from pressure buildup compared to traditional heaters.

Of course, this is not to say that tankless electric heaters are not without their own safety concerns, such as electrical fires due to improper or defective wirings, for example. However, the risks are quite minimal if the heater is installed and maintained properly.


Fixing a heater can be quite expensive, so it’s always a good idea to make sure yours is covered with a decent warranty. The warranty information predicts the approximate lifespan of the heater as well as how regularly maintenance should be done. The length of the warranty (which usually is a 6, 9, or 12 years term) will give you an idea of how long the manufacturer expects the heater to last. Your plumber can also use this information to estimate how often they should conduct maintenance on the heater.

Keep in mind that most manufacturers will void their warranty for cases of damage due to incorrect installation or lack of proper maintenance. Most incidents relating to rust, corrosion, and leaks are covered as well as faulty parts and quality-related issues. Problems caused by limescale, however, are usually not covered.

The Best Tankless Electric Water Heater in 2021 Reviews

 Below is a short list that includes all of the products that will be featured in this review. Additionally, we also have a comparison table between all of the products to provide a quick overview.

  1. EcoSmart ECO 27Best to Buy in 2021
  2. Stiebel Eltron Tempra 36 PlusBest Value Tankless Electric Water Heater
  3. Rheem RTEX-18 – Best Rheem Tankless Electric Water Heater
  4. EcoSmart ECO 11 – Best Tankless Electric Water Heater For RVs
  5. Rheem RTEX-24 – Best Whole House Tankless Electric Water Heater
NameFlow RateWarranty
EcoSmart ECO 276.5 GPMLimited Lifetime
Stiebel Eltron Tempra 36 Plus7.5 GPM7-year on leakage, 3-year on parts
Rheem RTEX-182.5 GPM1-year on labor, 1 to 6-year on parts, 6-year to lifetime limited warranty
EcoSmart ECO 112 GPMLimited Lifetime
Rheem RTEX-245.9 GPM1-year on labor, 1 to 6-year on parts, 6-year to lifetime limited warranty

1. EcoSmart ECO 27 – Best to Buy in 2021

The EcoSmart ECO 27 has become quite a popular name among consumers. Even though this heater’s price tag is still snugly in the affordable range, the ECO 27 has great performance and reliability that would satisfy most homeowners. Additionally, the compact size and modernized controls are greatly appreciated.

EcoSmart ECO 27 Electric Tankless Water Heater


  • Affordable
  • Great performance
  • High energy efficiency
  • Digital control interface
  • Durable design
  • Comprehensive warranty


  • Underperforms in cold weather

On grounds of performance, the ECO 27 offers a decent flow rate at 6.5 GPM at optimum inflow water temperature (68°F). With 6.5 GPM, you can have most essential household appliances running simultaneously including a showerhead, a washing machine, a dishwasher, and maybe a water saving sink in the house.

In regions with colder groundwater temperature closer to freezing, expect the performance of this heater to cut down by half into the 3 GPM range. This is only enough to supply about one standard showerhead and a sink.

The unit has been favorably reviewed by customers for its modern design and digital control interface. All of the important information about the heater, such as the current set temperature and the inflow water temperature, is displayed on the LCD screen at the front of the unit. Aside from a temperature control knob that you can use to change the set temperature by 1-degree increments, there are a couple of setting buttons for fixing preference.

The self-modulating computer on-board does a very good job at power management. To prevent energy wastage, the heater automatically adjusts its output based on the house’s hot water demand. This is how the unit can achieve an impressive 99.8% energy efficiency rating. In spite of this, the heater doesn’t have an Energy Star certification. 

The ECO 27 requires that you plug it into a 240-volt electric grid at all times and has an amperage draw of 113 amps. It’s got three heating elements, each of which consumes about 27 kilowatts for a grand total of 81 kilowatts between all elements. Keep in mind to discuss the electrical requirements in detail with your plumber and electrician to ensure that the unit is properly installed.

The heater unit also has a number of safety mechanisms in place. There is an overheating and overloading prevention mechanism that will immediately shut down the unit if it detects unsafe temperature or a sudden spike in power.

When properly installed and stored, the unit can go very happily for several years with very minimal maintenance. The parts that come into direct contact with high temperature and high pressure are constructed out of copper or stainless steel, both of which have excellent temperature and pressure endurance as well as rust resistance.

EcoSmart has a limited lifetime warranty on the ECO 27.

2. Stiebel Eltron Tempra 36 Plus – Best Value Tankless Electric Water Heater

If you need maximum performance and the most value for your money, the Stiebel Eltron Tempra 36 Plus is a noteworthy product to consider. It’s got the kind of heating power that makes it comparable to even gas heaters. This, combined with a reliable design that’s easy to use and maintain makes it a value-rich investment if you are looking for one.

Stiebel Eltron Tempra 36 Plus Water Heater


  • Exceptionally powerful
  • Highly energy-efficient
  • Digital control interface
  • Various safety sensors
  • Great warranty policy


  • Expensive unit cost

At its best, the Tempra 36 Plus can output 7.5 GPM of hot water under optimal inflow water temperature (72°F). You can harness this performance to work in tandem four water-saving showerheads (each with a flow rate of 1.5 GPM) as well as one sinks. If you live in colder regions, the Tempra 36 Plus will still be able to provide a sizable hot water supply, enough for two showerheads plus one sink.

The design is straightforward and modern. It has a digital screen to display all of the heater’s operating data and working parameters like set temperature and current settings. It’s equipped with one standard adjusting knob for changing the temperature by 1-degree increments. Memory buttons are available for you to program in the temperature settings that each family member prefers to make switching back and forth easier.

The self-modulating computer helps with power management and the result is impressive: an EF of 0.99. Nearly all of the energy that the heater draws from the grid is going to be used to either heat the water or work the heater’s system. This is how the heater can reduce utility costs.

Aside from self-modulating, the computer also has an Advanced Flow Control function. This can keep water temperature constant and prevents the temperature from suddenly shifting and spiking while you’re using the water. This won’t just eliminate a common cause of annoyance for users, it also saves you a lot of water in the long run.

The heater needs to be plugged into a 240-volt grid and has a maximum amperage draw of 150 amps. All of the heating elements consume in total 36 kilowatts.

The unit has a 7-year warranty period on leakage and a 3-year period on replacement parts.

3. Rheem RTEX-18 – Best Rheem Tankless Electric Water Heater

Among Rheem’s portfolio, the RTEX-18 is the model that has the most optimal performance that would make it a great fit for most small households. Rheem has a reliable track record of making highly efficient, easy to use and install designs. The majority of consumers agree that the company’s gotten all of the above qualities down in the RTEX-18’s design.

Rheem RTEX-18 Residential Tankless Water Heater


  • Adequate performance
  • Durable design
  • Energy-efficient
  • Digital control interface


  • Complicated warranty

The maximum flow rate of the unit is 4.4 GPM under optimal inflow water temperature. It should be able to offer a steady flow to two standard showerheads running in tandem.

The heater unit is compact enough to fit inside of a cupboard and the aesthetic design looks nice enough that even if you leave it out in the open, it won’t impair your interior design. At the front of the unit, you will find an LCD screen that displays the current set temperature and a temperature adjustment knob. You can freely change the temperature in 1-degree increments from 80°F to 140°F.

The RTEX-18 is also equipped with a self-modulating computer that aids in power management, giving it an energy efficiency measurement of a near-absolute 99.8%. 

Not just good looking, the unit is designed to last as well. Vital components like the two heating elements inside are cast from copper, which can withstand high pressure and temperature along with being rust resistant. The unit can operate for several years consecutively without needing replacement parts as a result.

The unit requires a constant supply of 240 volts. The two heating elements inside consume 9 kWs of power each for a total of 18 kWs.

Warranty information about the RTEX-18 is very complicated, however. You have a one-year period on labor and, depending on the part that you need replaced, a warranty on parts ranging from one to six years.

But the real problem is in the limited warranty period. Depending on whether you are the legal owner of the house in which the heater is installed or not, the limited warranty can range from six years to lifetime. If you ever need to use the warranty, it is best to speak with a company representative.

For the full information page about Rheem’s warranty policies, click here.

4. EcoSmart ECO 11 – Best Tankless Electric Water Heater For RVs

For RV-ers, portability, efficiency, and ease of installation and use are the most coveted values in a water heater. The ECO 11 has managed to satisfy every requirement for RV usage with its compact size, high-efficiency heating elements, and simplistic operation, making it one of the better choices to consider on the market.

EcoSmart ECO 11 Electric Tankless Water Heater


  • Affordable
  • Energy-efficient
  • Digital control interface
  • Easy to store
  • Comprehensive warranty


  • Low performance for normal household use

Although the maximum flow rate of the unit still leaves much to be desired from a normal consumer point of view at 2 GPM under optimal inflow water temperature (67°F). For RV-ers, 2 GPM is satisfactory as there aren’t enough spaces on an RV to have multiple showerheads, sinks, and taps anyway. Because of its low flow rate, the ECO 11 is considered to be a point-of-use heater that only supplies water for a single appliance at a time. 

The size of the ECO 11 is compact enough to fit into a small cupboard or be mounted on a small patch on the wall. Its humble size is usually appreciated in RVs, where every inch of floor space (and even wall space) is highly valued.

Operating information is displayed on the front-facing LCD screen, along with a temperature adjustment knob for you to switch up the water temperature. Not just keeping tabs of the heater’s functions, the computer also helps it self-modulate to save energy and achieves an EF of 0.98.

It must be connected to a 240-volt grid and has a maximum amperage of 33 amps. The heating element has a power draw of 8 kW at full capacity. In case of malfunctions, the unit is equipped with overheating and overloading sensors that will automatically shut it down to protect the unit and you.

The EcoSmart ECO 11 has a limited lifetime warranty.

5. Rheem RTEX-24 – Best Whole House Tankless Electric Water Heater

For households that need a medium-size, whole house heater at a performance point between light and heavy duty, the RTEX-24 is not a bad option with its powerful heating capability. This, along with a reliable, modern, and convenient design, gives it the final push needed to make it onto our list as a whole house heater worth considering.

Rheem RTEX-24 Residential Tankless Water Heater


  • Powerful
  • Durable design
  • Energy-efficient
  • Digital control interface


  • Complicated warranty

At its best, the RTEX-24 can provide 5.9 GPM of hot water continuously (under optimal water temperature.) This is enough to provide for two standard showerheads plus a sink running simultaneously. However, since groundwater temperature won’t be the same from one region to another, you’ll need to size your household carefully to make sure the heater can keep up with your demand.

The unit is fairly compact in design and clean in appearance. The LCD screen that constantly displays the set temperature is a great touch. Directly underneath, you will find the temperature adjustment knob.

The heating elements are cast from copper for greater durability and reliability. It’s also equipped with brass tops for easier replacement when necessary.

The RTEX-24 heater requires a constant connection to a 240-volt power grid and the three heating elements consume in total 24 kW while operating at the maximum capacity.

Warranty-wise, the heater is fairly complicated. It’s got one year on labor, and from one to six years on parts (depending on the kind of part that you need to replace). Depending on whether you are the owner of the house that the heater is installed in, the limited warranty period can be anywhere from six years to lifetime.

Money Saving Tips for Tankless Electric Water Heaters

Most people choose to upgrade their heating system to tankless because of the units’ cost effectiveness over traditional heaters. Still, there are a couple more things that you can do in order to save money.

Lower the Thermostat

Most manufacturers set the default temperature for their heater at 140°F. However, most people will be satisfied with 120°F. Try to adjust your heater and take a shower; there aren’t going to be any noticeable differences between these two temperature points. However, what you will definitely notice is a reduction in your utility bill at the end of the month.

High temperature conditions make dissolved minerals in the water flow like calcium and magnesium solidify quicker, quickening the build-up of scales in the heater. The heat can also accelerate corrosion by accelerating the oxidation process between the metal and the oxygen in the water flow.

Use Cold Water Whenever Possible

You don’t need to use hot water all the time. For grooming, washing hands, brushing teeth, or even laundry (during the rinsing cycles) it is highly advisable that you use cold water only. This habit— albeit small— can contribute a lot to saving money on the monthly electricity bill.

Check for Leaky Appliances

A leaky faucet, sink, or showerhead may look as if they are only minor inconveniences, but the lost water volume when added up can become quite significant. If you spot any leaky appliances around the house, either have them fixed or replaced. According to the EPA, 10% of households in America have leaks that waste up to 90 gallons of water or more per day.

Consider Replacing Old Appliances

Old appliances like dishwashers and washing machines that are ten years old or more, even though they still work well for you, can waste a lot of water. Technology has progressed far enough that newer appliances today can be several times more efficient than older ones when it comes to water and energy use. Consider buying new ones (especially ones with an Energy Star label) to further reduce your household water use.

Insulate the Piping

Wrap a thermal blanket or similar insulating materials around the first six feet of the hot and cold water pipes. This practice can help the unit conserve heat and reduce the heater’s workload since it won’t have to work as hard to heat inflow water.


This concludes our best tankless electric water heater buying guide and review! We hope that you have found this article to be helpful in the process of making a purchase decision.

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