The Best Electric Tankless Water Heaters

Advanced Flow Control is the whole reason this one wins. When two showers and a kitchen sink all pull at once, most electric tankless heaters just let the temperature sag. The Tempra throttles the flow a little instead, so your water gets slower rather than colder. Stiebel's documentation and published reviews describe it the same way, and nothing else in this guide offers an equivalent. It's the difference between a weaker shower and a startling one.

At 36 kW, it's sized for water that arrives cold. The colder your incoming water, the more power it takes to reach a comfortable shower temperature. Stiebel rates this unit for roughly 3.5 gallons per minute at a 54-degree rise, which covers two or three fixtures in most of the country. If you live somewhere with a real winter, this is the only heater here we'd trust for a whole house.

The temperature control is precise, and you'll notice it. You set a number on the digital display and the unit holds close to it, within a couple of degrees according to the manufacturer. That matters most in the shower, where a few degrees is the gap between fine and flinching. There's no burner and no fan either, so it runs silently in a utility closet. You won't hear it through a wall.

It's built to be bolted up and forgotten. The Tempra is made in Germany, and reviewers consistently call out the fit and finish as a step above the plastic-bodied competition. Nothing inside moves, so there's no pump or fan to wear out. Paired with the long warranty term, that's a sensible bet for a box you're going to mount on a wall and ignore for a decade.

Flaws but not dealbreakers

The wiring is the real price of admission. This unit can draw up to 150 amps across three 60-amp double-pole breakers. In an older house that usually means a full service upgrade before the heater ever gets mounted, and that work can cost more than the heater itself. Have an electrician look at your panel before you buy. If the answer is no, stop here and read the Rheem entry.

It's the most expensive unit here, and one of the largest. You're paying a real premium over similar-output competitors for the flow control and the build. It also takes up more wall than the flat-panel EcoSmart and Eemax models, which matters in a cramped closet. If your groundwater never drops much, you're buying capability you'll rarely use.

Stiebel Eltron covers the Tempra 36 Plus with a seven-year warranty.

It asks less of your electrical panel than our top pick. Rheem lists three 40-amp double-pole breakers and about 100 amps of draw, with a 200-amp household service recommended. That's still real work, but it's a much easier conversation with an electrician than 150 amps. Plenty of homes can accommodate this one with breaker space they already have. Check your panel anyway before ordering.

You set the temperature to the degree, not to a vague dial position. The RTEX-24 pairs an on/off dial with a digital display and adjusts in one-degree steps between 80 and 140 degrees. If someone in your house likes a scalding shower and someone else doesn't, that precision is genuinely useful. It also makes it easy to dial the output down a notch and shave a little off your bill.

Self-modulation means it only pulls what the moment needs. Rinsing a mug doesn't demand the same power as filling a tub, and this unit doesn't pretend otherwise. It ramps its draw to the actual flow rate, which keeps operating costs and peak demand lower than a fixed-load design. Over years of daily use, that's where the savings from going tankless actually show up.

Service is the quiet advantage. Rheem parts and technicians are available almost everywhere in the country, and the heating elements are field-serviceable rather than sealed in. When an element eventually gives out, a plumber swaps it instead of replacing the whole heater. That's a meaningful difference over the life of a unit you expect to keep for a decade or more.

Flaws but not dealbreakers

Twenty-four kilowatts isn't enough where groundwater runs cold. In a northern climate the RTEX-24 will either throttle your flow to a trickle or hand you lukewarm water during a January shower. Rheem is upfront about this and publishes a performance guide by region — read it before you commit. If you're anywhere with a hard winter, buy the Stiebel instead.

There's no flow control, so peak demand shows up as temperature. Run the dishwasher while someone showers and you can feel the output drift. It's not a dealbreaker for a household that staggers its hot water use, but a busy morning with two bathrooms going will find the limit fast.

It delivers close to three gallons per minute for far less money. EcoSmart configures the ECO 27 for incoming water as low as 37 degrees, which makes it workable for two simultaneous fixtures across much of the country. That's real whole-home capability at a price well below our top pick. For a household of two or three that doesn't run showers back to back, it's usually enough.

The lifetime limited warranty is the strongest coverage in this guide. Most competitors offer somewhere between three and seven years. EcoSmart backs this one for as long as you own it, through authorized service. That's worth something on a component you're building into a wall, and it's the single biggest reason to pick the ECO 27 over a similarly rated unit from another brand.

The flat body fits where a boxier heater won't. At roughly 17 by 17 inches and only a few inches deep, it sits nearly flush against a wall in a utility closet or under a stairwell. Digital temperature control adjusts in one-degree increments, so you're not guessing. If your mechanical space is a converted closet, measure it — this shape usually wins.

It's the easiest whole-home unit here to install yourself. The wiring diagrams are clear and the installed base is enormous, which means someone has already documented whatever you run into. If you're comfortable pulling a circuit and sweating a fitting, you can do this in a weekend. That said, know your limits: this is still high-amperage work.

Flaws but not dealbreakers

Temperature wanders when demand spikes. There's no flow-limiting logic, so two showers at once will thin the heat rather than the stream. In a cold-groundwater region the 27 kW ceiling gets found quickly, and you'll notice it most on the coldest mornings — exactly when you care.

The build feels a grade below the Stiebel and Eemax. The plastic housing looks and sounds cheaper, and reports on EcoSmart's customer service are uneven, which takes some shine off that lifetime warranty. It also still needs heavy dedicated circuits despite the modest price, so check the spec sheet against your panel before you order.

EcoSmart backs the ECO 27 with a lifetime limited warranty through authorized service.

It plugs into a standard 120-volt outlet, and that changes everything. No dedicated circuit, no electrician, no permit in most places. If you're in an apartment or a condo where panel work is off the table, this is the practical answer. The cord runs about three feet, so plan for an outlet inside the cabinet or add one.

It kills the long wait at a far-flung fixture. The 2.5-gallon tank sits right under the sink, so hot water shows up in seconds instead of after thirty seconds of running the tap. Bosch says it can supply up to two sinks at a time. For handwashing, shaving and light dishes, that's exactly the right amount of capacity.

The build is better than the price suggests. The tank is glass-lined, which is the standard defense against corrosion, and Bosch rates thermal efficiency at 98 percent. It mounts on a shelf, a wall or the floor with the included bracket, so you can tuck it wherever the cabinet allows. Installation is a matter of tapping the cold line and plugging it in.

Flaws but not dealbreakers

It's a small tank, so it does run out. Two and a half gallons covers a few minutes of steady use before recovery kicks in, and recovery on 120 volts isn't fast. Don't expect it to fill a mop bucket or run a long rinse cycle.

You're back to paying for standby heat. Unlike everything else in this guide, it keeps a small volume of water warm around the clock, so a little energy leaks out whether you use it or not. The loss is minor at this size, but it's the honest trade for skipping the wiring.