Best Electric Tankless Water Heaters (2026) by Climate

For most all-electric homes, the best electric tankless water heater is the Stiebel Eltron Tempra 24 Trend, as long as a licensed electrician confirms your panel can take two 50-amp double-pole breakers. Stiebel recommends at least 150-amp service for it. Electric tankless flow falls as the incoming water gets colder: Stiebel’s table gives the Tempra 24 6.05 GPM from 77°F water but 2.52 GPM from 39°F water, at 105°F out. For cold ground water in a house with 300-amp service, look at the Tempra 36 Trend. In a warm climate the EcoSmart ECOS 27 is still a good unit while it is in stock. For one sink, the EcoSmart ECO 8.

Plumbing Picks is reader-supported. As an Amazon Associate, we earn from qualifying purchases at no extra cost to you.

Our pick for most all-electric homes: the Stiebel Eltron Tempra 24 Trend. Check the Stiebel Eltron Tempra 24 Trend on Amazon

Gas or electric? If the house already has gas, our gas tankless water heater picks cover that side, and the best tankless water heater guide compares gas, propane and electric by situation. This page is for homes without gas, or for a small unit at one bathroom or sink. Not sure tankless is right at all? See tank vs tankless and heat pump vs tankless. For one sink on a 120-volt supply, see under-sink water heaters.

Electric Tankless Water Heaters Compared

Every figure is the maker’s own: Stiebel’s spec sheet and flow table for the Tempra units, EcoSmart’s spec sheets and zone table for the ECOS 27 and ECO 8. The two makers list different incoming-water temperatures, so the coldest column compares Stiebel’s 39°F with EcoSmart’s 37°F. Every linked unit was sold and shipped by Amazon and in stock when we checked on September 29, 2026. The ECOS 27 was down to its last unit, with more on the way, so check before you order.

How to Size an Electric Tankless Water Heater

Both makers size the same way, and it works for any brand. Stiebel says the right Tempra depends largely on the ground-water temperature and the number of bathrooms. EcoSmart’s sizing guide uses three steps:

1. Add up what runs at the same time. EcoSmart’s average flow rates are 2 GPM for a shower, 1.5 for a kitchen faucet, 1 for a bathroom faucet, 0.5 for a low-flow faucet, 2 for a dishwasher and 3 for a washing machine. One shower and a kitchen faucet together come to 3.5 GPM.
2. Find your ground-water temperature. EcoSmart’s map runs from Zone 1 (37°F ground water) to Zone 9 (77°F); the coldest zones are in the north, per the map. Ground water temperature varies with the season, so size for winter.
3. Pick the model that meets your total at that temperature.

What each pick runs at once, by incoming water temperature. Flow is the maker’s figure at 105°F out; what it runs is our arithmetic with EcoSmart’s fixture flows (and 1.5 GPM for a low-flow shower, from EcoSmart’s ECO spec sheet).

No unit on this page runs two 2 GPM showers at once from the coldest water in its maker’s table (our arithmetic). Set the heater hotter and flow drops: at 113°F out, Stiebel gives the Tempra 24 2.21 GPM from 39°F water, against 2.52 GPM at 105°F.

Here is Stiebel’s flow table for the Tempra Trend range, in GPM at 105°F out:

*The Tempra 29 Trend sits between the 24 and the 36, on three 40-amp breakers with 200-amp service recommended by Stiebel. It is not linked because two stock checks on September 29 disagreed on who was selling it on Amazon.

And here is EcoSmart’s flow table, in GPM at a 105°F set point:

*The ECOS 18 is shown for comparison only. It showed 2 left on Amazon when we checked on September 29, so it is not linked. Our tankless size calculator, in the related guides below, runs the same GPM and temperature-rise math for any unit.

Best Electric Tankless Water Heater for 2 Bathrooms

Two bathrooms usually means two showers running at once, and that is where an electric tankless runs short first. Stiebel’s sizing guide answers it by incoming water temperature. It counts a shower as 1.5 GPM at 105°F, a kitchen sink as 1 GPM at 105°F and a bathroom faucet as 0.5 GPM at 90°F, and shows how many fixtures each Tempra can supply at the same time:

So with 1.5 GPM shower heads, a Tempra 24 runs two showers at once where incoming water is 47°F or warmer on Stiebel’s map, and colder than that it takes the Tempra 36, for which Stiebel recommends at least 300-amp service, or the Tempra 29 (200-amp, not linked; see the note above), which Stiebel’s guide also shows running two showers down to 37°F. Stiebel says the map temperatures are zone averages that may vary by season and exact location, so if your water runs colder than your zone in winter, size from the colder column. The guide’s GPM figures do not always match Stiebel’s flow table above (5.9 against 6.05 GPM for the Tempra 24 from 77°F water, 8 against 6.61 GPM for the Tempra 36), so use it for the fixture count, not to compare GPM. Stiebel also says to go up one model on 208-volt power.

With standard 2 GPM heads, two showers need 4 GPM. In Stiebel’s flow table the Tempra 24 reaches that only in the 77°F column and the Tempra 36 from the 59°F column up, and EcoSmart’s table gives the ECOS 27 4.3 GPM from Zone 6 (62°F) up (our arithmetic). Fitting 1.5 GPM shower heads is the simplest way to let a smaller unit cover two bathrooms.

I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.

Electrical Requirements: Breakers, Wire and Service

An electric tankless heater draws more current than almost anything else in a house. These are the makers’ own figures at 240 volts. They are the starting point for your electrician, not a replacement for one.

• Stiebel’s footnotes matter. Its copper conductors must be rated 75°C or higher and sized to keep voltage drop under 3% under load, so a long run from the panel can need heavier wire than the minimum. The type of cable matters too: the National Electrical Code (section 334.80 and Table 310.16) holds ordinary NM house cable (Romex-type) to the amp rating of a 60°C wire, even though its wires are rated 90°C. That is 40 amps for 8 AWG copper, less than the 50 amps on each Tempra 12, 24 or 36 circuit. State and local codes can change that rule, so your electrician picks the cable or conduit and the wire size. Stiebel sizes overcurrent protection at 100% of load because it treats tankless heaters as a non-continuous load. Its service minimums depend on the total electric load at the house, and it calls all of these its recommendations, to be checked against local codes.
• EcoSmart’s basis. EcoSmart sizes its breakers per the NEC for intermittent duty and calls for wire with 75°C insulation on the ECOS 27. Its spec sheets warn that household electric service panel upgrades are often required when converting from an electric tank to an electric tankless water heater.
• Grounding. Stiebel says the appliance must be properly grounded, and EcoSmart says to connect the unit’s ground terminal to the grounding terminal of the service or supply panel with a continuous copper wire.
• 208 volts. Some buildings are wired for 208 volts instead of 240. Stiebel lists the Tempra 24 at 18 kW on 208 V, and its flow from 39°F water drops from 2.52 to 1.89 GPM. Our 240 V vs 208 V guide explains why the same heater makes less heat. EcoSmart’s spec sheets list the ECOS 27 and ECO 8 at 240 V.
• Who installs it. Stiebel’s manual says the unit must be installed by a licensed electrician and plumber. EcoSmart recommends a licensed and qualified electrician and plumber, and the ECO 8’s warranty requires both. Your town may also require a permit and inspection for the new circuits and the heater; the electrician and plumber handle those.

Have a licensed electrician confirm your panel capacity before you order. The question is not only whether there is room for two or three more double-pole breakers, but whether the service can carry the added load with everything else in the house running.

What the House Needs Before You Buy

• Water pressure. Stiebel lists a 30 psi minimum and 150 psi maximum for the Tempra. EcoSmart lists 30 to 150 psi for the ECOS 27, with 35 to 80 psi as the optimal range, and a 25 psi minimum for the ECO 8. If your pressure is low, see how to fix low water pressure first.
• Enough flow to switch on. Stiebel lists 0.37 GPM to activate the Tempra 12, 0.50 GPM for the Tempra 24 and 0.77 GPM for the Tempra 36. For ECOS 27, EcoSmart’s April 2024 sheet and myRheem manual list 0.3 GPM activation, while its media-site ECOS manual lists 0.5 GPM. Plan for 0.5 until EcoSmart confirms the exact unit if you need very low-flow use; do not assume guaranteed start-up at 0.3. The separate ECO 8 specification lists 0.3 GPM.
• No vent. EcoSmart says venting is not required, and Stiebel lists no venting required for the Tempra, so an electric unit can go where a gas unit cannot.
• A frost-free spot. Stiebel says to install the Tempra in a frost-free area and to drain it if it may see temperatures below 36°F; not doing so voids its warranties. EcoSmart says not to install its units where they may freeze, and freeze damage is not covered.
• Room around it. EcoSmart asks for 14 inches above the ECOS 27 and 12 inches above and below the ECO 8, with combustible materials kept 24 inches away. Stiebel asks for at least 5 inches on all sides for servicing.
• A plan for leaks. Stiebel’s warranty says the heater must be installed so that a leak will not damage the area. EcoSmart’s ECO 8 warranty calls for drip pans and drains on installations above ground level.
• Relief valve. Stiebel says the Tempra is not required to have a temperature and pressure relief valve, and EcoSmart says one is not required for most installations. Your inspector or local code may still require one; Stiebel says to put it on the hot water outlet side.
• Scald setting. Both makers warn that water over 125°F can cause severe burns. Stiebel says homes with small children, disabled or elderly people may need the setting at 113°F or lower, and the Tempra Trend can be set to cap its output at 109°F. EcoSmart ships its units set at 120°F, says those homes may need 120°F or lower, and recommends a mixing valve to lower the temperature at the tap (its ECO manual names one meeting ASSE 1017).

Warranty Terms and Their Conditions

All three warranties leave labor to the owner. Stiebel’s and EcoSmart’s ECOS warranty texts do not mention registration, but their manuals do, so register the unit when it is installed.

Hard water shortens the list of what is covered. Stiebel’s manual says damage from scale or a high mineral count is not covered, and recommends clearing scale from faucet aerators, the unit’s filter screen and the heating canisters from time to time. EcoSmart says hard water calls for more frequent maintenance, that flushing should be done by a qualified service technician, and that improper maintenance will void the warranty.

Best Electric Tankless Water Heater for Well Water or Hard Water

On a well, or on hard city water, the water decides how long the heating elements last. Stiebel’s warranty does not cover damage from scale or hard water, and EcoSmart’s ECO 8 warranty excludes liming and sediment buildup (table above), so plan the treatment and the flushing before you buy.

• Test the water first. Stiebel’s water-quality bulletin says to test your water if you do not already know its quality. A water hardness test kit gives you the number to start from.
• Treat it without salt, per Stiebel. Stiebel says a non-salt-based water conditioner may prolong the unit’s life, and recommends its own Scale TACler Plus (part 692500) for that. Our salt-free water softener guide covers that type of treatment.
• Plan the flush. Depending on water hardness, Stiebel says scale may need flushing from the unit every six months to a year. It recommends service valves or braided stainless steel flex connectors on both pipes, so the unit can be isolated and a descaling solution pumped through it. On the EcoSmart units, EcoSmart leaves the flush to a qualified service technician.
• Clean the inlet screen. Both makers’ instructions call for a filter screen at the cold inlet. Stiebel’s manual says the cold connection must be disconnected periodically to clean it, so it calls for easily detachable connections such as braided steel flex connectors. Its bulletin adds faucet aerators and shower heads to the cleaning list.
• Run it cooler. Stiebel says an output temperature of 110°F or below helps minimize scaling.
• Purge the air after pump or plumbing work. EcoSmart says that after any maintenance that may let air into the pipes, turn the heater’s power off and purge the air before it powers up, or the element can be permanently damaged. Its restart bulletin adds that air pockets, sediment and debris can collect while the water is off, and says to run the hot taps fully open for 5 minutes with the power off before switching it back on with the water still running.

Where Electric Tankless Falls Short

Cold ground water is the limit. From 39°F water, Stiebel’s biggest Tempra makes 3.78 GPM, and EcoSmart’s ECOS 27 makes 2.7 GPM from 37°F water, so no unit here runs two 2 GPM showers at once in the coldest zones (our arithmetic). The Department of Energy says gas-fired tankless heaters produce higher flow rates than electric ones. Other 36 kW models we checked on Amazon on September 29 (EcoSmart ECO 36, Rheem RTEX-36) were third-party listings, low on stock or refurbished, so none is linked. In a cold-winter state with no gas, compare a heat pump water heater before you commit, and see gas vs electric water heaters for the trade-offs.

FAQ

Can an electric tankless water heater run a whole house?

In a warm or mild climate, yes. Stiebel rates its 24 kW Tempra 24 Trend at 6.05 GPM from 77°F water and 3.63 GPM from 59°F water, at 105°F out: three showers at once in warm water, or a shower and a kitchen faucet in mild water (our arithmetic). From 39°F water it makes 2.52 GPM, one shower at a time (our arithmetic), so a cold-climate house needs a much larger unit and service, or a different kind of water heater.

How many amps does an electric tankless water heater need?

It depends on the size. Stiebel lists 2 x 50 amps for the 24 kW Tempra 24 Trend on two 50-amp double-pole breakers, and 3 x 50 amps for the 36 kW Tempra 36 Trend on three. EcoSmart lists 112.5 amps for the 27 kW ECOS 27 on three 40-amp double-pole breakers, and 33 amps for the 8 kW ECO 8 on one 40-amp breaker. Have a licensed electrician confirm that your panel and service can carry it.

Will a 200 amp panel handle an electric tankless water heater?

Sometimes. Stiebel recommends at least 150-amp service for the Tempra 24 Trend and 300 amps for the Tempra 36 Trend; EcoSmart recommends 200 amps for the ECOS 27. Whether your 200-amp service can carry one depends on what else is on it, and Stiebel says its minimums depend on the total electric load. That is a question for a licensed electrician.

Do electric tankless water heaters need to be vented?

No. EcoSmart says venting is not required, and Stiebel lists no venting required for the Tempra. That is one of the main differences from a gas tankless heater.

What size electric tankless water heater do I need?

Add up the flow of the fixtures you run at the same time (EcoSmart uses 2 GPM for a shower, 1.5 for a kitchen faucet and 1 for a bathroom faucet), find how cold your ground water gets in winter, and pick a model that delivers that flow at that temperature in its maker’s table. Our tankless size calculator does the math.

Will an electric tankless water heater work on 208 volts?

Stiebel’s Tempra will, with less heat. Stiebel lists the Tempra 24 Trend at 18 kW on 208 volts instead of 24 kW on 240, and its flow from 39°F water drops from 2.52 to 1.89 GPM. EcoSmart lists the ECOS 27 and ECO 8 at 240 volts. Check which voltage your building has before you size.

What size electric tankless water heater do I need for 2 bathrooms?

For two 1.5 GPM showers at once, Stiebel’s sizing guide puts the Tempra 24 Trend where incoming water is 47°F or warmer and the Tempra 36 Trend (or the unlinked Tempra 29) where it is 42°F or colder. With 2 GPM shower heads, two showers need 4 GPM, which Stiebel’s flow table gives the Tempra 36 from 59°F water and the Tempra 24 only from 77°F water (our arithmetic). Check the panel first: Stiebel recommends at least 150-amp service for the Tempra 24 and 300-amp for the Tempra 36.

Do I have to register the warranty?

Yes, register it. Stiebel’s manual says to register within 90 days of purchase to activate the standard warranty. EcoSmart’s manuals say its warranties are void without registration within 30 days, and the ECO 8’s lifetime warranty also requires installation by a licensed electrician and plumber within 30 days of receipt.

Why Trust Plumbing Picks

Plumbing Picks is written by a licensed Georgia plumber — journeyman license #JP507711 — and the license is verifiable through the Georgia Secretary of State’s license verification portal. Sizing, electrical and warranty figures come from the maker spec sheets, manuals and warranties listed below, except the house-cable limit, which comes from the National Electrical Code and a cable maker’s spec sheet, also listed below. Product details come from the Amazon listing or the maker, not from hands-on testing. Nobody pays to be included here. Where a link goes to Amazon we may earn a commission at no extra cost to you. More on who writes this and how we pick.

Sources

• Stiebel Eltron: Tempra Trend and Plus technical specifications, Tempra flow table, Tempra Trend and Plus installation manual, electric tankless limited warranty, Tempra Trend product page
• Stiebel Eltron sizing guide and water-quality technical bulletin: Tempra Trend and Plus sizing guide, tankless electric water heaters and water quality
• EcoSmart ECOS 27: ECOS 18 and 27 spec sheet and sizing guide, ECOS 27 electrical requirements, ECOS use and care manual (0.3 GPM activation), alternate ECOS manual on EcoSmart’s media site (0.5 GPM activation), ECOS limited warranty, ECOS 27 product page
• EcoSmart ECO 8: ECO 8 and 11 spec sheet and sizing guide, ECO use and care manual, ECO limited lifetime warranty, ECO 8 product page
• U.S. Department of Energy: Tankless or demand-type water heaters (archived June 30, 2026)
• House-cable limit: National Electrical Code (NFPA 70), section 334.80 and Table 310.16; Southwire Romex NM-B cable spec sheet

Bottom Line

For most all-electric homes, the Stiebel Eltron Tempra 24 Trend: two 50-amp breakers and 150-amp service, per Stiebel, and 3.63 GPM from 59°F water. For cold water in a house with 300-amp service, the Tempra 36 Trend. The EcoSmart ECOS 27 is still a good warm-climate unit while it is in stock, and the Tempra 12 Trend covers one bathroom in a warm climate. For one sink, the EcoSmart ECO 8. Size by your coldest month and the most fixtures you run at once, and let a licensed electrician confirm the panel and service and run the circuits.