Best electric tankless water heater: kW, amps, breakers, wire gauge and the service panel each maker's sheet demands
Stiebel's Tempra 24 prints 2 x 50 A breakers, 2 x 8/2 AWG, 150 A service and a 54 F rise at 3.0 gpm; a 27 kW unit is 112.5 A on 3 x 40 A and 200 A service.
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OwnerSpec compares published manufacturer specifications, certification listings and sourced technical documents. Unless a page explicitly states otherwise, OwnerSpec has not laboratory-tested or used the products it compares. Editorial policy
Short answer
For a whole house on a 150 A panel, the pick is the Stiebel Eltron Tempra 24 Trend: its 2025 spec sheet prints 24 kW at 240 V on two 50 A double-pole breakers and two runs of 8/2 AWG copper, a 150 A recommended minimum main service, and a 54 F rise at 3.0 gpm. The runner-up is the Rheem RTEX-24: 24 kW, 100 A, three 40 A double-pole breakers and three runs of 8 AWG on Rheem’s TKE-401 sheet, which costs one more breaker slot. For one sink, the EcoSmart ECO 8 draws 33 A on a single 40 A breaker. Skip every whole-house unit here if the panel is 100 A: EcoSmart’s electrical page and Rheem’s Performance sheet both put a 27 kW heater at a 200 A service, and Stiebel puts 36 kW at 300 A. In 37 F groundwater Rheem rates its 18 kW unit at 1.5 gpm to a 120 F set point.
The picks
Whole house on a 150 A panel with two spare double-pole slots
Stiebel Eltron Tempra 24 Trend, 24 kW, 240 V
Stiebel's 2025 tech sheet: 24 kW at 240 V, 2 x 50 A draw, 2 x 50 A double-pole breakers, 2 x 8/2 AWG copper, 150 A recommended minimum main service, 54 F rise at 3.0 gpm and 92 F at 1.5 gpm, 0.50 gpm activation, UL 499, CSA C22.2 No. 88, WQA to NSF/ANSI/CAN 372
Cold groundwater, a 200 A panel, three spare double-pole slots
Stiebel Eltron Tempra 29 Trend, 28.8 kW, 240 V
Stiebel's sheet: 28.8 kW, 3 x 40 A draw, 3 x 40 A double-pole breakers, 3 x 8/2 AWG, 200 A recommended minimum service, 66 F rise at 3.0 gpm and 44 F at 4.5 gpm, 0.77 gpm activation
Whole house where 24 kW must land on three 40 A breakers
Rheem RTEX-24 Professional Classic, 24 kW, 240 V
Rheem's TKE-401 sheet: 24 kW at 240 V, 100 A, 3 x 40 A double-pole breakers, 3 x 8 AWG; Rheem's product page prints 2.4 to 4.3 gpm with no rise; Rheem's Performance RETEX-24 sheet recommends a 200 A household panel
Warm groundwater, one fixture at a time, two spare slots
Rheem 18 kW Professional Classic (RTEX-18 line), 240 V
Rheem's TKE-401 sheet for the RTEX-18: 18 kW at 240 V, 75 A, 2 x 40 A double-pole breakers, 2 x 8 AWG; Rheem's Performance RETEX-18 sheet recommends a 150 A panel and rates 18 kW at 1.5 gpm in 37 F groundwater and 1.8 gpm at 52 F, both to a 120 F set point
A bathroom or a warm-climate cabin on one 60 A breaker
EcoSmart ECO 11, 13 kW, 240 V
EcoSmart's ECO 8-36 manual: 13 kW at 240 V, 54 A, 1 x 60 A double-pole breaker, 1 x 6 AWG; the maker's support page lists 125 A household service and 57 A; manual flow table 1.0 gpm at 40 F inlet to 2.0 gpm at 70 F inlet, 105 F outlet
One bathroom on a 100 A panel, one 50 A slot
Stiebel Eltron Tempra 12 Trend, 12 kW, 240 V
Stiebel's sheet: 12 kW at 240 V, 50 A, 1 x 50 A double-pole breaker, 1 x 8/2 AWG, 100 A recommended minimum service, 54 F rise at 1.5 gpm and 27 F at 3.0 gpm, 0.37 gpm activation
One hand sink or a bar sink, one 30 A double-pole slot
Rheem RTEX-06T, 6.5 kW, 240 V point-of-use
Rheem's TKE-401 sheet: 6.5 kW at 240 V, 27 A, 1 x 30 A double-pole breaker, 10 AWG
If you have not opened your breaker panel and counted the empty double-pole slots, this page cannot pick a heater for you yet: the breaker line on the spec sheet decides more than the kW on the box.
How was this shortlist built?
Every figure on this page is a manufacturer’s published specification, read on 2026-09-30 from the sheet or manual named beside it. Nothing was installed, wired or run by this site. Listings were shortlisted from current Amazon listings, kept or rejected on the maker’s document, and confirmed buyable the same day.
The units were compared on kW, amperage, the number and size of double-pole breakers, wire runs and gauge, the maker’s recommended minimum household service, rise at a stated flow, listing marks where printed, and what the warranty says about scale and installers.
Two lines are missing on purpose. EcoSmart’s newer ECOS 18 and ECOS 27 show one run of 8 AWG on the support page against two and three runs for the ECO 18 and ECO 27, and nothing the maker prints resolves it. Atmor’s ThermoPro manual carries a flow table that does not reconcile with its own kW.
Rheem needs a caveat. The Professional Classic RTEX sheet (TKE-401) prints kW, amps, breakers and wire but no recommended service and no rise. The service figures in the table come from Rheem’s Performance RETEX sheet, a sibling line with the same amps and breaker layout, and are labelled as such.
How do the units compare on paper?
Manufacturer figures at 240 V. “Not printed” means the cited document carries no value. Stiebel prints amperage per run; the total is the sum shown.
| Unit | kW | Total amps | Breakers (DP) | Wire runs (copper) | Maker’s recommended service | Rise or flow, maker’s figure |
|---|---|---|---|---|---|---|
| Stiebel Tempra 29 Trend | 28.8 | 3 x 40 A (120 A) | 3 x 40 A | 3 x 8/2 AWG | 200 A minimum | 66 F at 3.0 gpm; 44 F at 4.5 gpm |
| Stiebel Tempra 24 Trend | 24 | 2 x 50 A (100 A) | 2 x 50 A | 2 x 8/2 AWG | 150 A minimum | 54 F at 3.0 gpm; 37 F at 4.5 gpm |
| Rheem RTEX-24 | 24 | 100 A | 3 x 40 A | 3 x 8 AWG | Not printed on the RTEX sheet; RETEX-24 sheet: 200 A | Product page: 2.4 to 4.3 gpm, no rise; RETEX-24: 2.0 gpm at 37 F inlet, 2.4 at 52 F, 120 F set point |
| Rheem RTEX-27 (not a pick) | 27 | 113 A | 3 x 40 A | 3 x 8 AWG | RETEX-27 sheet: 200 A | RETEX-27: 2.2 gpm at 37 F, 2.7 at 52 F |
| EcoSmart ECO 27 (not a pick) | 27 | 112.5 A | 3 x 40 A | 3 x 8 AWG | 200 A | 2.0 gpm at 40 F inlet, 4.0 at 60 F, 105 F outlet |
| Rheem 18 kW (RTEX-18) | 18 | 75 A | 2 x 40 A | 2 x 8 AWG | RETEX-18 sheet: 150 A | RETEX-18: 1.5 gpm at 37 F, 1.8 at 52 F |
| EcoSmart ECO 18 (not a pick) | 18 | 75 A | 2 x 40 A | 2 x 8 AWG | 150 A | 2.0 gpm at 40 to 60 F inlet, 3.0 at 70 F |
| EcoSmart ECO 11 | 13 | 54 A (manual); 57 A (support page) | 1 x 60 A | 1 x 6 AWG | 125 A | 1.0 gpm at 40 F inlet, 2.0 at 70 F |
| Stiebel Tempra 12 Trend | 12 | 50 A | 1 x 50 A | 1 x 8/2 AWG | 100 A minimum | 54 F at 1.5 gpm; 27 F at 3.0 gpm |
| EcoSmart ECO 8 | 8 | 33 A | 1 x 40 A | 1 x 8 AWG | 100 A | Not printed |
| Rheem RTEX-06T | 6.5 | 27 A | 1 x 30 A | 10 AWG | Not printed | Product page: not printed |
| Rheem RTEX-04T, 120 V | 3.5 | 29 A | 1 x 30 A (single-pole, 120 V) | 10 AWG | Not printed | Not printed |
Rheem’s RTEX-13 page prints maximum flows of 1.95 gpm for the RTEX-08, 2.68 for the RTEX-11 and 3.17 for the RTEX-13, with no rise and no set point, so they stay out of the rise column.
What does the breaker line mean for your panel?
A double-pole breaker takes two adjacent slots and feeds one 240 V circuit. A 27 kW unit on three of them takes six slots and three home runs of 8 AWG copper, and by EcoSmart’s rule each run lands on its own breaker: two sets of wire and ground for the ECO 18, three for the ECO 24 and ECO 27, four for the ECO 36.
The service figure is the makers’ hedge. Stiebel’s footnote reads “depending on total electric load at the installation site, we recommend these minimums for main service”, and adds that local codes decide. Rheem’s RETEX sheet says that when converting from an electric tank, household panel upgrades are often required.
Stiebel classes a tankless heater as a non-continuous load with overcurrent protection sized at 100 percent, which is why a 100 A unit sits on 2 x 50 A. Whether the panel has 100 A of headroom is a load calculation, and that is the licensed electrician’s job, along with the permit. The model codes govern it, and they are adopted locally and vary.
Stiebel Eltron Tempra 24: buy it or skip it?
Buy it if the panel is 150 A or larger, two double-pole slots are free, and a 54 F rise covers the coldest month. Stiebel’s 2025 tech sheet prints 24 kW on 2 x 50 A breakers and 2 x 8/2 AWG copper rated 75 C or higher, 0.50 gpm to activate, and the rise table at 240 V: 92 F at 1.5 gpm, 73 F at 2.25, 54 F at 3.0 and 37 F at 4.5. The same sheet prints UL Std. 499, CAN/CSA C22.2 No. 88, and WQA certification against NSF/ANSI/CAN 372 for lead-free compliance.
The flaw is the warranty line. Stiebel’s installation manual excludes damage from hard water or a high mineral count, and requires a licensed electrician and plumber. Run the descale-interval calculator before buying: at high grains per gallon the flushing schedule is the real running cost.
Skip it in cold groundwater with two bathrooms in use. The Tempra 29 Trend prints 66 F at 3.0 gpm on 3 x 40 A and 200 A service, and the Tempra 36 prints 82 F at 3.0 gpm on 3 x 50 A and 300 A. Both take three double-pole slots, which is the price of the extra rise.
Rheem RTEX-24 and RTEX-18: buy it or skip it?
Buy the RTEX-24 if the panel has three free double-pole slots and a 24 kW unit is the target. Rheem’s TKE-401 sheet prints 24 kW at 240 V, 100 A, 3 x 40 A double-pole breakers and 3 x 8 AWG. The product page prints 2.4 to 4.3 gpm, a 5-year heating chamber warranty and 1 year on parts.
The flaw is what Rheem leaves off the RTEX sheet: no rise at any flow, no recommended service, no listing mark (the product page shows an Intertek ETL badge). The closest maker figures are on the Performance RETEX sheet, which puts the RETEX-24 at 200 A service and 2.0 gpm in 37 F groundwater, rising to 2.4 gpm at 52 F, calculated to a 120 F set point.
The 18 kW Rheem is the same sheet’s RTEX-18: 75 A on 2 x 40 A and 2 x 8 AWG, two slots instead of three. The listing title omits the model number, so match RTEX-18 on the box. Rheem’s RETEX-18 figures are 150 A service and 1.5 gpm at 37 F, 1.8 gpm at 52 F: in a cold zone that is one fixture, not a household.
Skip both if three breakers do not fit the panel: the Tempra 24 does 24 kW on two.
EcoSmart ECO 11 and the point-of-use units: buy it or skip it?
Buy the ECO 11 for one bathroom, a cabin, or a warm-groundwater house that runs one fixture at a time. EcoSmart’s ECO 8-36 manual prints 13 kW at 240 V, 54 A, one 60 A double-pole breaker and one run of 6 AWG; the maker’s electrical page lists 125 A household service. The manual’s flow table gives 1.0 gpm at a 40 F inlet, 1.5 at 60 F and 2.0 at 70 F for a 105 F outlet.
The flaw is a maker that disagrees with itself: the support page lists the ECO 11 at 57 A where the manual says 54 A. Tell the electrician. The ECO 8 is the sink-only step down: 33 A on 1 x 40 A and 1 x 8 AWG, 100 A service, no flow table printed.
The Stiebel Tempra 12 Trend is the documented alternative at 12 kW: 50 A on 1 x 50 A and 1 x 8/2 AWG, 100 A service, and a rise table that says the honest thing, 54 F at 1.5 gpm but only 27 F at 3.0 gpm.
Below that, Rheem’s sheet covers the hand sink. The RTEX-06T draws 27 A on 1 x 30 A double-pole and 10 AWG, and the 120 V RTEX-04T draws 29 A on a single 30 A breaker and 10 AWG, the only unit here that does not need a 240 V circuit.
Skip all of these as a whole-house heater: EcoSmart’s own table gives the ECO 11 1.0 gpm at a 40 F inlet.
Two numbers before you buy
Do not buy any unit on this page until the sizing calculator has your kW from your own fixtures and your coldest-month inlet temperature, and your electrician has told you how many amps the panel can spare. If the kW lands at 24 on a 150 A panel, the Tempra 24 Trend is the unit with the fewest breakers. If it lands above 28.8 kW, or the panel is 100 A, read the first-hour-rating page before ruling out a tank.
What goes wrong?
One shower in winter. Cold-zone flow is small: Rheem’s RETEX sheet rates 18 kW at 1.5 gpm and 36 kW at 3.0 gpm in 37 F groundwater, and Stiebel’s Tempra 24 drops from 92 F of rise at 1.5 gpm to 37 F at 4.5 gpm. Size to the coldest month, not the brochure.
The panel is full. Rheem’s RETEX sheet says panel upgrades are often required when converting from an electric tank, and a 27 kW unit needs six slots. A licensed electrician’s load calculation comes before the purchase, and the permit comes with the circuit.
Scale. Stiebel’s warranty excludes hard-water damage, and EcoSmart’s manual says to increase maintenance frequency with hard water. The descale-interval calculator turns grains per gallon into a flushing schedule.
208 V service. EcoSmart’s manual says an ECO unit on 208 V gives 75 percent of capacity and needs a jumper change; Stiebel prints a separate 208 V column with lower kW and rise.
The installer. EcoSmart ties its limited lifetime warranty to a licensed professional; Stiebel requires a licensed electrician and plumber. A self-install costs the warranty first.
Frequently asked questions
How many amps does a 27 kW electric tankless water heater draw?
EcoSmart's ECO 8-36 manual lists the ECO 27 at 112.5 A on 240 V, wired as three runs of 8 AWG to three 40 A double-pole breakers, and its electrical-requirements page puts the household service at 200 A. Rheem's TKE-401 sheet lists the RTEX-27 at 113 A on the same 3 x 40 A, 3 x 8 AWG arrangement.
Do I need 200 amp service for a whole-house electric tankless water heater?
For 24 to 28.8 kW, the makers say yes or close to it: EcoSmart lists 200 A for the ECO 24 and ECO 27, Stiebel recommends 150 A for the Tempra 24 and 200 A for the Tempra 29, and Rheem's Performance RETEX sheet recommends 200 A for the RETEX-24 and 27. Every maker calls these recommendations and defers to local code.
Can one electric tankless water heater run two showers?
Only with warm incoming water and a large unit. Stiebel's Tempra 36 prints a 55 F rise at 4.5 gpm; the Tempra 24 prints 37 F at 4.5 gpm. Rheem's RETEX sheet rates its 36 kW unit at 3.0 gpm in 37 F groundwater. Run the sizing calculator with your own inlet temperature.
Will hard water void an electric tankless water heater warranty?
Stiebel Eltron's installation manual says damage from hard water or a high mineral count, meaning scale, is not covered by the warranty. EcoSmart's manual says to increase maintenance frequency with hard water. Both are reasons to know your grains per gallon first; the descale-interval calculator turns that number into a flushing schedule.
Can I install an electric tankless water heater myself?
The makers say no. Stiebel's manual requires a licensed electrician and plumber, EcoSmart says a licensed professional must install the unit to keep the limited lifetime warranty, and its manual calls for a qualified electrician and plumber working to national, state and local codes. New 240 V circuits and any panel upgrade are permitted electrical work; codes are adopted locally and vary.
Fact sheet and how to cite this page
- Answer
- For a whole house on a 150 A panel the documented pick is the Stiebel Eltron Tempra 24 Trend: 24 kW on 2 x 50 A double-pole breakers, 2 x 8/2 AWG copper, a 150 A minimum main service and a 54 F rise at 3.0 gpm, per Stiebel’s 2025 sheet. A 27 kW EcoSmart ECO 27 draws 112.5 A on 3 x 40 A breakers and needs 200 A service.
- Comparison basis
- Manufacturer-published specification sheets and installation manuals: kW, voltage, amperage, number and size of double-pole breakers, number of wire runs and gauge, the maker’s recommended minimum household service, temperature rise at a stated flow or flow at a stated inlet temperature, activation flow, listing marks where printed, warranty conditions on scale and on who installs.
- Assumptions
- A 240 V single-phase residential service; Stiebel and EcoSmart both print reduced output at 208 V (EcoSmart says 75 percent of capacity)
- Flow figures are the makers’ own: Stiebel prints rise at 1.5, 2.25, 3.0 and 4.5 gpm; EcoSmart prints gpm at 40 to 70 F inlet for a 105 F outlet; Rheem’s Performance RETEX sheet prints gpm by groundwater zone to a 120 F set point, and Rheem’s Professional Classic RTEX sheet prints no flow and no service size at all
- The Rheem RTEX use-and-care and installation manuals linked from rheem.com could not be opened on 2026-09-30, so Rheem’s own hard-water and installer wording for the RTEX line is not quoted
- Method
- OwnerSpec read Rheem’s TKE-401 Professional Classic sheet and RETEX Performance sheet, two Rheem product pages, EcoSmart’s ECO 8-36 manual and electrical-requirements page, and Stiebel Eltron’s Tempra Trend/Plus tech sheet and installation manual on 2026-09-30, compared the printed figures, and confirmed each listing buyable the same day; it did not install, wire or test any heater.
- Primary source
- Stiebel Eltron, Tempra Trend and Tempra Plus Technical Specifications (rev. 6.2025, PDF) (manufacturer or vendor document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Best electric tankless water heater: kW, amps, breakers, wire gauge and the service panel each maker's sheet demands", https://ownerspec.com/plumbing/reviews/best-electric-tankless-water-heater/, facts verified 30 September 2026.
Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers
Sources
- Stiebel Eltron, Tempra Trend and Tempra Plus Technical Specifications (rev. 6.2025, PDF) (manufacturer or vendor document)
- Stiebel Eltron, Tempra Trend and Tempra Plus Installation Manual (PDF) (manufacturer or vendor document)
- Rheem, Professional Classic Tankless Electric Water Heaters specification sheet (TKE-401 Rev. 5, 03/25, PDF) (manufacturer or vendor document)
- Rheem, Performance RETEX-18/24/27/36 Specifications and Minimum Requirements (THD-PTE 18-36 Rev. 1, 11/24, PDF) (retailer listing or retailer-hosted document)
- Rheem, Professional Classic Series Tankless Electric RTEX-24 product page (manufacturer or vendor document)
- Rheem, Professional Classic Series Tankless Electric RTEX-13 product page (manufacturer or vendor document)
- EcoSmart, ECO 8 to ECO 36 Installation Instructions and Home Owner's Manual (PDF) (manufacturer or vendor document)
- EcoSmart, Electrical Requirements (support page) (manufacturer or vendor document)