How Many Breakers Does It Take? The Best Electric Tankless Water Heater From 18 to 36 kW

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The best electric tankless water heater for your home depends as much on your electrical panel as on your showers. Whole-home electric tankless units draw serious current: the listings we compared call for two, three or four separate 40-amp double-pole breakers and recommend household service from 150 to 300 amps. Flow matters too, and how much hot water you actually get depends on how cold your incoming water is.

We compared six whole-home and multi-point models from 18 to 36 kW, all in stock and sold by Amazon.com or the brand’s own store: Rheem, Stiebel Eltron, Westinghouse, WINTEMP, ThermoMate and SIVUATEK. For each, we quote the electrical requirements, flow rate, temperature range and warranty exactly as the listing gives them, and we flag where a listing leaves something out.

Read each requirement as a starting point, not a green light. A licensed electrician should confirm your panel, service and wiring before you buy.

Our Top Electric Tankless Water Heater Picks at a Glance

Price key: $ under $50 · $$ $50–$100 · $$$ $100–$175 · $$$$ $175+. Prices change often; check Amazon for today’s price.

1. Rheem RTEX-24 24kW Tankless: Best Overall

Rheem’s RTEX-24 is a 24-kilowatt, 240-volt electric tankless water heater sized for whole-home and multiple-fixture use. It mounts on the wall and heats water only when a hot tap opens, so there is no tank to keep warm. Rheem’s listing says the 24 kW model provides up to 5.9 gallons per minute.

The wiring requirements are spelled out more completely than on most listings: 240V single-phase power, three 40-amp double-pole breakers, three sets of #8 AWG copper conductors with ground, and a recommended minimum 200-amp household electrical service panel. Rheem also tells buyers to choose the right size for the home and verify electrical requirements before installation, and says the units perform as expected when properly sized and professionally installed.

Day to day, digital thermostatic controls set the output in 1-degree increments from 80°F to 140°F. No venting is required, which Rheem notes simplifies installation compared with a gas tankless heater, and the brand says the compact design is 90% smaller than a traditional tank. The listed size is 18.25 by 14.5 by 3.5 inches at 17.8 pounds. The warranty bullet reads 5-year leak and 1-year parts.

It suits homes with a 200-amp panel that want endless hot water for a few fixtures and a well-known brand. Rheem’s smaller RTEX-06 is a point-of-use model for a single sink; this is the whole-home size.

Why we like it

• Complete wiring requirements on the listing
• 1-degree temperature control
• No venting needed

Watch out for

The 200-amp panel recommendation rules out many older homes, and the warranty field and bullet disagree, so confirm coverage with Rheem.

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2. Stiebel Eltron Tempra 24 Trend: Best Warranty

Stiebel Eltron’s Tempra 24 Trend is a 24-kilowatt, 240-volt electric tankless heater the brand pitches for endless hot water for the whole house. It’s part of the Tempra line, which Stiebel Eltron sells in Trend and Plus versions at several kilowatt levels.

The listing’s electrical detail is short: the model requires a minimum recommended electric service of 150 amps. It doesn’t list breaker counts or wire sizes, so those have to come from the installation manual and your electrician. Output water temperature runs from 68°F to 140°F, the widest stated range among our picks.

In use, one electronic switch activates the heater, and Stiebel Eltron says it runs without noise. A digital display shows the temperature, a solid copper heating system does the work, and auto-modulation adjusts power to save energy. Like the other picks, it needs no venting. The listed size is 4.63 by 16.63 by 14.5 inches at 16.1 pounds. Warranty coverage is stated in the bullets as 7-year leakage and 3-year parts.

It suits homeowners who put warranty length and brand history first, have at least 150-amp service and are happy to let an electrician pull the remaining specs from the manual. If you need more or less power, the same Tempra line comes in other kilowatt sizes, so the electrician can match the model to your panel.

Why we like it

• Longest stated leakage warranty
• Widest stated temperature range
• Quiet, switch-activated operation

Watch out for

Without breaker, wire or GPM figures on the listing, you’ll need the manual before you can size the circuit or the fixtures it can run.

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3. Westinghouse 24kW Wi-Fi Tankless (WET-24K-G): Best Smart Tankless

Westinghouse’s WET-24K-G is a 24-kilowatt, 240-volt whole-house electric tankless heater with built-in Wi-Fi. Westinghouse Home Comfort Direct, the brand’s store, sells it, and the bullets refer to it as the APOLO on-demand heater.

The listing gives a maximum flow rate of 5.55 gallons per minute and a starting flow of 0.79 GPM. On the electrical side, it states a current of 100 amps and says the unit can be used with three 40-amp double-pole breakers and three 8 AWG wires, with bottom 3/4-inch NPT water connections. A licensed electrician should match those figures to your panel and local code.

The smart features set it apart day to day. Westinghouse says you can adjust the temperature and monitor usage remotely through an app or by voice with Alexa and Google Home. It mounts flush to the wall at 17 by 13 by 4.3 inches, per the bullets, and the brand pitches running a shower and sink at the same time.

It suits apartments, homes and cabins where someone wants to set the water temperature from a phone, and buyers who already run an Alexa or Google Home setup. Because the listing states no warranty term, ask Westinghouse Home Comfort Direct what coverage applies before you schedule the electrician and plumber.

Why we like it

• App and voice control
• States current, breakers and wire
• 5.55 GPM maximum flow

Watch out for

No warranty term appears on the listing, and the listing says flow must reach 0.79 GPM before it starts heating.

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4. WINTEMP WN36 36kW Wi-Fi Tankless: Best for Large Homes

WINTEMP’s WN36 is the most powerful heater on our list: 36 kilowatts at 240 volts, with a maximum flow rate of up to 8.7 gallons per minute. WINTEMP sells it through its own Amazon store and describes it for homes with three to five bathrooms in mild to moderate climates.

The electrical requirements are the steepest here, and WINTEMP puts them up front. It recommends a 300-amp household electrical service, says the unit draws a maximum of 150 amps, and calls for four 40-amp double-pole circuit breakers and four 8/2 AWG cables with ground. The listing tells buyers to verify their electrical system and consult a licensed electrician before purchasing.

Day to day, self-modulating technology adjusts power to incoming water temperature and flow rate, which WINTEMP says keeps temperatures steady, and built-in Wi-Fi lets you control temperature from a smartphone. Protections listed include leakage, overheat and dry-fire protection. It is ETL certified, per the listing, and backed by a 2-year replacement warranty. The bullet gives dimensions of 13.7 by 21.5 by 5.2 inches with 3/4-inch NPT connections.

It suits larger homes in milder regions that already have, or plan to add, 300-amp service. In a cold-winter area, ask WINTEMP for its flow chart at your inlet temperature before relying on the 8.7 GPM figure.

Why we like it

• Highest flow on the page
• Electrical needs stated in full
• Wi-Fi temperature control

Watch out for

A 300-amp service recommendation and four 40-amp circuits put this out of reach for many homes without an electrical upgrade.

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5. ThermoMate EH180 18kW Tankless: Best for Smaller Homes

ThermoMate’s EH180 is an 18-kilowatt, 240-volt electric tankless heater meant for whole-house, multi-point supply in smaller homes. ThermoMate Official, the brand’s store, sells it, and the listing puts maximum output at 4.4 gallons per minute.

Its stated electrical needs are the lightest among our picks. ThermoMate asks buyers to make sure the main panel has a total capacity of at least 150 amps, and says the unit requires two separate 40-amp double-pole breakers and two sets of 8 AWG copper wire, with standard 3/4-inch NPT water connections. An electrician should still confirm those against your panel’s actual load.

Inside, ThermoMate uses a cast aluminum heating block with iron-chromium-aluminum alloy elements and what it calls a water-and-electricity separation structure, pitched to resist scale and corrosion. A self-modulating chip tracks flow and inlet temperature to hold a setting between 80°F and 140°F. The listing says it is ETL listed to UL-499 and CAN/CSA-C22.2, with anti-leakage, over-temperature, dry-combustion and surge protection. It measures 17.09 by 13.15 by 3.15 inches and weighs 16 pounds, per the bullets.

It suits small homes, apartment closets, cabins and compact bathrooms. ThermoMate says it can run two to three fixtures at once, such as a shower and a bathroom sink, depending on inlet water temperature.

Why we like it

• Two circuits instead of three or four
• Named safety standards
• Self-modulating temperature control

Watch out for

At 4.4 GPM, it’s the smallest-output pick here, and the listing doesn’t state a warranty term.

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6. SIVUATEK TSB-270 27kW Tankless: Best Budget 27kW

SIVUATEK’s TSB-270 is a 27-kilowatt, 240-volt electric tankless heater sold as a whole-house unit through the brand’s own SIVUATEK-US store. The title doesn’t name the brand, but the listing’s brand and seller fields do.

The electrical note is brief: the unit requires three 40-amp circuit breakers and uses a standard 3/4-inch NPT water connection. The listing doesn’t give wire gauge, household service size or a maximum flow rate, so your electrician will need the manual. SIVUATEK claims up to 99.8% thermal efficiency and says it can serve four to five usage points at once, such as kitchen and bathroom.

An LED display sets the water temperature from 86°F to 131°F in 1°F increments, and SIVUATEK says the output stays stable even when water pressure fluctuates. It uses what the brand calls water-electricity separation technology to limit scale, and lists leakage protection, overheating protection and auto shut-off. The brand also says it can install under a sink or on a wall, and claims it can cut water heating costs by up to 50% compared with traditional models.

It suits buyers who want a 27 kW unit without paying name-brand prices and who prefer a lower maximum temperature setting. Plan on asking SIVUATEK for the installation manual before you buy, since the wire size and flow figures your electrician and plumber need aren’t on the listing.

Why we like it

• 27 kW at a budget price
• 1-degree LED control
• Capped top temperature of 131°F

Watch out for

Missing flow, wire and service figures mean you can’t size this unit from the listing alone.

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How We Chose the Best Electric Tankless Water Heaters

We limited our picks to whole-home and multi-point electric tankless heaters that were in stock and sold by Amazon.com or the brand’s own store, then compared what each listing actually says:

• Electrical requirements: breakers, wire size, current draw and recommended household service, quoted as listed.
• Flow: maximum gallons per minute (GPM), starting flow and any climate or inlet-temperature caveat.
• Temperature control: the stated output range and how it is set.
• Safety and certification: ETL or UL references and listed protections.
• Warranty, seller and popularity: the warranty field and bullets, the seller and Amazon sales rank.

Electric Tankless Water Heater Buying Guide

18, 24, 27 or 36 kW: Sizing a Tankless Water Heater Electric Model

More kilowatts means more water heated at once. Our picks run from 18 kW (ThermoMate) through 24 kW (Rheem, Stiebel Eltron, Westinghouse) and 27 kW (SIVUATEK) to 36 kW (WINTEMP). ThermoMate pitches its 18 kW unit for two to three fixtures at once; WINTEMP pitches its 36 kW unit for homes with three to five bathrooms. Count how many showers and taps run at the same time in your home, then size up from there.

GPM at Temperature Rise: Why Climate Matters

A tankless heater adds a fixed amount of heat, so the colder your incoming water, the lower the flow it can bring up to temperature. The listed maximums here, 4.4 GPM (ThermoMate), 5.55 GPM (Westinghouse), 5.9 GPM (Rheem) and 8.7 GPM (WINTEMP), aren’t tied to a stated temperature rise on any listing, and Stiebel Eltron and SIVUATEK list no GPM at all. WINTEMP limits its sizing claim to mild to moderate climates, and ThermoMate says its fixture count depends on inlet water temperature. Before you buy, ask the maker for its flow chart at your winter inlet temperature.

Electrical Requirements: Breakers, Wire and Service Size

This is where electric tankless gets expensive. Every figure below comes from the listing, and none replaces an electrician’s load calculation:

• ThermoMate 18 kW: 2 x 40 A double-pole breakers, 2 sets of 8 AWG copper wire, a main panel of at least 150 A total capacity.
• Rheem RTEX-24: 240V single-phase, 3 x 40 A double-pole breakers, 3 sets of #8 AWG copper with ground, a recommended minimum 200 A service panel.
• Stiebel Eltron Tempra 24 Trend: a minimum recommended service of 150 A; breakers and wire not stated.
• Westinghouse 24 kW: 100 A current, 3 x 40 A double-pole breakers, 3 x 8 AWG wire.
• SIVUATEK 27 kW: 3 x 40 A circuit breakers; wire and service size not stated.
• WINTEMP 36 kW: 150 A maximum draw, 4 x 40 A double-pole breakers, 4 x 8/2 AWG cables with ground, a recommended 300 A service.

If your home has 100- or 150-amp service, some of these units may require a service upgrade. Have a licensed electrician check your panel, available capacity and local code before you order, and pull any permits your town requires.

Best Rated Electric Tankless Water Heater Signals

We don’t use star ratings. Sales rank is one signal: the Rheem RTEX-24 is #1 in Water Heaters, the Stiebel Eltron #21, ThermoMate #36, WINTEMP #39, SIVUATEK #46 and Westinghouse #58. Warranty is another. Stiebel Eltron states 7-year leakage and 3-year parts coverage, Rheem’s bullet says 5-year leak and 1-year parts, and WINTEMP offers a 2-year replacement warranty. Westinghouse, ThermoMate and SIVUATEK don’t state a term.

Hard Water and Filtration

Scale builds up in any water heater on hard water. ThermoMate and SIVUATEK both describe designs pitched to limit scale, but neither replaces treating the water itself. If your water is hard, our best water softener guide compares softeners and descalers, and our best whole house water filter guide covers sediment and point-of-entry filtration that can protect a new heater.

Whole-Home or Point-of-Use?

Every pick here is a whole-home or multi-point unit that needs a 240V supply and multiple 40-amp circuits. If you only need hot water at one sink, a small tankless or mini tank uses far less power; see our best under sink water heater guide for those.

Scald Safety: Set It to 120°F

Several of these heaters can be set as high as 140°F: Rheem, Stiebel Eltron and ThermoMate all list 140°F as their top setting, while SIVUATEK tops out at 131°F. The CPSC’s Avoiding Tap Water Scalds alert urges lowering water heaters to 120 degrees Fahrenheit and says burns will occur with a six-second exposure to 140-degree water. It also warns that even at 120 degrees, a five-minute exposure could result in third-degree burns, so always hand-test water before bathing infants and young children.

Hire a Licensed Electrician and Plumber

Whole-home electric tankless heaters combine high-amperage circuits with pressurized water lines. Have a licensed electrician size the breakers, wire and service to the manufacturer’s installation manual and your local code, and a licensed plumber connect the water lines. Ask whether a permit and inspection are required where you live.

Water Heater Leaks and Your Homeowners Insurance

A tankless heater holds little water, but its fittings still carry full household pressure, and a failed connection can spray or drip unnoticed in a utility room or closet. Check the connections after installation, keep the area visible, and know where your main shutoff is. Homeowners policies often treat sudden, accidental water damage differently from slow leaks or wear, so it helps to know what yours covers. Our Ohio homeowners insurance guide explains the basics.

Frequently Asked Questions

What is the best electric tankless water heater for a whole house?

The Rheem RTEX-24 is our top pick. It ranks #1 in Amazon’s Water Heaters category, lists up to 5.9 GPM and spells out its wiring: three 40-amp double-pole breakers, #8 AWG copper and a recommended 200-amp panel.

How many amps does a tankless electric water heater need?

It varies by size. Among our picks, the 18 kW ThermoMate needs two 40-amp breakers and a 150-amp panel, the 24 kW Rheem needs three 40-amp breakers and recommends 200 amps, and the 36 kW WINTEMP needs four 40-amp breakers and recommends 300-amp service. A licensed electrician should confirm.

Is the best tankless electric water heater good in cold climates?

Colder inlet water lowers how much flow a heater can bring up to temperature. WINTEMP limits its sizing claim to mild to moderate climates, and ThermoMate says fixture count depends on inlet temperature. Ask the maker for a flow chart at your winter inlet temperature.

Which electric tankless water heater has the longest warranty?

Stiebel Eltron’s Tempra 24 Trend lists 7-year leakage and 3-year parts warranties in its bullets. Rheem’s bullet says 5-year leak and 1-year parts, though its warranty field reads “Limited lifetime warranty,” so confirm with Rheem.

Can I install an electric tankless water heater myself?

These units need multiple high-amperage 240V circuits and often a large service panel. Hire a licensed electrician for the wiring and a licensed plumber for the water lines, follow the manual, and check whether your town requires a permit.

The Bottom Line

For most homes with a 200-amp panel, the Rheem RTEX-24 is the best electric tankless water heater to start with: the top-ranked unit here with its wiring spelled out. The Stiebel Eltron Tempra 24 Trend leads on stated warranty, the Westinghouse 24kW adds app and voice control, and the WINTEMP WN36 brings the most flow for big homes with 300-amp service. Smaller homes on a 150-amp panel should look at the ThermoMate EH180, and the SIVUATEK 27kW offers 27 kW at a budget price. Whatever you choose, have a licensed electrician confirm your panel first.

Replacing a water heater and wondering how your policy handles leaks or failures? Call Jeff Smith’s State Farm office at (740) 354-7747 or get a free coverage review.