Stiebel Eltron Water Heaters
The only brand in this category that publishes the number you need. That is why it ranks first.
Heat & Tap earns a commission on qualifying purchases made through the Amazon links on this page, at no extra cost to you. Commission does not decide what we recommend or how we rank it. Full disclosure.
Stiebel Eltron is the one electric tankless brand here that publishes maximum temperature rise at four flow rates, on both 240 V and 208 V. The Tempra 24 Plus gives a 54°F rise at 3.00 GPM on 240 V and 41°F at 208 V. It needs two 50 A breakers, 100 A of draw and a minimum 150 A service.
Most of this site's rankings come down to performance figures. This one comes down to documentation, and we think that is the right call: a unit you can size correctly before buying is worth more than one with a bigger number and no way to check it.
The table that earns the ranking
| Flow | Rise at 240 V (24 kW) | Rise at 208 V (18 kW) |
|---|---|---|
| 1.50 GPM | 92°F | 82°F |
| 2.25 GPM | 73°F | 54°F |
| 3.00 GPM | 54°F | 41°F |
| 4.50 GPM | 37°F | 27°F |
Four data points at two voltages. With that table and a thermometer in your cold tap in January, you can establish before purchase exactly what this unit will deliver in your house. No other electric brand in our registry makes that possible.
Quick picks
Tap any row to read the full assessment below, or go straight to Amazon from the right-hand column. We earn a commission on qualifying purchases at no extra cost to you.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | Stiebel Eltron Tempra 24 PlusThe electric tankless to buy if you are going to buy one at all: it publishes its full rise-versus-flow curve, and the Advanced Flow Control throttles flow instead of letting the temperature sag. | Whole-house electric, warm inlet | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 2 |  EcoSmart ECO 27The biggest electric unit most homes can actually feed: 27 kW across three 40 A breakers, 6.6 GPM maximum, and a lifetime limited warranty on the unit. | Maximum electric output | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 3 |  Rheem Professional Classic RTEX-24The value electric unit: 24 kW, 5.9 GPM rated flow and a digital set point in 1°F steps, from a brand whose parts your local supply house actually stocks. | Electric tankless on a budget | Check price on Amazon No live price feed on this site, so we show none. #ad |
The picks in full
Pick 1 — Whole-house electric, warm inlet
Stiebel Eltron Tempra 24 Plus
The electric tankless to buy if you are going to buy one at all: it publishes its full rise-versus-flow curve, and the Advanced Flow Control throttles flow instead of letting the temperature sag.
| Output at 240 V | 24 kW |
|---|---|
| Output at 208 V | 18 kW |
| Breakers | 2 x 50 A double-pole |
| Wire | 2 x 6 AWG copper |
| Current draw | 100 A |
| Minimum electrical service | 150 A |
| Rise at 240 V | 54°F at 3.0 GPM; 73°F at 2.25 GPM; 37°F at 4.5 GPM |
| Rise at 208 V | 41°F at 3.0 GPM; 54°F at 2.25 GPM; 27°F at 4.5 GPM |
| Maximum flow | 8 GPM |
What is good
- Publishes rise at four flow rates on both voltages, which is the only honest way to size an electric tankless
- Advanced Flow Control cuts flow to hold the set temperature rather than delivering lukewarm water
- No venting, no gas line, no condensate
- Small enough to go in the cabinet it replaced
What is not
- 100 A of draw and a 150 A minimum service rules out a lot of older houses
- At 208 V it loses a quarter of its output, and plenty of homes are nearer 208 than 240
- Running cost follows the electric rate, which is usually worse than gas per BTU
Skip it if: Skip it if your inlet water drops to 45°F in winter and you want two showers at once. At a 73°F rise it delivers 2.25 GPM — one shower, nothing else. Cold-inlet climates are where electric tankless disappoints people.
No live price feed on this site, so we show none. #ad
Specifications above from Stiebel Eltron Tempra Plus technical specifications (PDF); Stiebel Eltron Tempra Plus submittal sheet (PDF). We have not tested this unit ourselves.
Pick 2 — Maximum electric output
EcoSmart ECO 27
The biggest electric unit most homes can actually feed: 27 kW across three 40 A breakers, 6.6 GPM maximum, and a lifetime limited warranty on the unit.
| Output | 27 kW at 240 V, single phase |
|---|---|
| Current draw | 113 A |
| Breakers | 3 x 40 A |
| Wire | 3 x 8 AWG |
| Maximum flow | 6.6 GPM |
| Minimum activation flow | 0.3 GPM |
| Dimensions | 17 in x 17 in x 3-5/8 in |
| Weight | 14.7 lb |
| Connections | 3/4 in NPT |
| Warranty | Lifetime limited |
What is good
- Highest output of any single electric unit here, so it holds temperature at higher flow
- Lifetime limited warranty on the unit is unusual at any price
- 3-5/8 inches deep — it genuinely disappears on a wall
- 0.3 GPM activation flow is the lowest on this page
What is not
- 113 A of draw needs three circuits and, in practice, a 200 A service
- The published flow is a maximum, not a rise-qualified figure — you have to do the rise math yourself
- No buffer, so a sudden flow change is felt at the shower head
Skip it if: Skip it if your panel is 100 A or 125 A. Three 40 A circuits and 113 A of draw is a service upgrade, and a service upgrade usually costs more than the entire water heater.
No live price feed on this site, so we show none. #ad
Specifications above from EcoSmart ECO 27 technical data (BIMobject). We have not tested this unit ourselves.
Pick 3 — Electric tankless on a budget
Rheem Professional Classic RTEX-24
The value electric unit: 24 kW, 5.9 GPM rated flow and a digital set point in 1°F steps, from a brand whose parts your local supply house actually stocks.
| Output | 24 kW at 240 V |
|---|---|
| Rated flow | 5.9 GPM |
| Breakers | 3 x 40 A |
| Service required | 200 A |
| Thermal efficiency | 99.8% |
| Set point range | 80 - 140°F in 1°F increments |
What is good
- Parts and warranty support through a brand with national distribution
- Digital set point in single degrees rather than a notched dial
- 99.8% thermal efficiency: effectively all the electricity lands in the water
- Cheaper than the German-built alternatives at the same kW
What is not
- Rheem quotes a rated flow rather than a published rise-versus-flow curve
- Three 40 A circuits and a 200 A service requirement
- No flow control, so high flow shows up as lower temperature
Skip it if: Skip it if you want to size from published data. Rheem lists a rated GPM without the temperature rise it was measured at, so you cannot check the number against your own inlet temperature the way you can with the Tempra.
No live price feed on this site, so we show none. #ad
Specifications above from Rheem RTEX-24 / RTEX-27 product page; Pace Supply listing, RTEX-24, 240 V 24 kW 100 A. We have not tested this unit ourselves.
Advanced Flow Control: the right failure mode
The Tempra Plus range throttles the flow to hold the set temperature rather than letting the temperature sag at higher flow. That is a meaningfully better behavior than the alternative: a slightly weaker shower is a minor inconvenience, and a shower that goes cool is the thing people call a plumber about.
It also means the unit's published figures describe what you actually experience, rather than a theoretical maximum you only reach with warm inlet water.
The electrical requirement, stated plainly
| Specification | Value |
|---|---|
| Output | 24 kW at 240 V; 18 kW at 208 V |
| Breakers | 2 x 50 A double-pole |
| Wire | 2 x 6 AWG copper |
| Current draw | 100 A |
| Minimum electrical service | 150 A |
| Maximum flow | 8 GPM |
A 150 A minimum service is less demanding than the EcoSmart ECO 27's 113 A across three circuits, and Rheem's stated 200 A requirement for the RTEX-24. It is still a substantial ask, and a 100 A panel does not accommodate it. Breaker and wire size.
Where Stiebel Eltron is weaker
- Lower maximum output than the EcoSmart ECO 27. If raw kilowatts are the goal, this is not the unit.
- Price. German-built and priced accordingly against the retail-focused competition.
- Thinner US service network than Rheem's national distribution, which matters for a part in year eight.
- It is still an electric resistance appliance. In a cold-inlet climate no amount of good documentation makes 2.25 GPM at a 73°F rise into a whole-house heater.
Our verdict
If you are buying an electric tankless water heater, buy this one — not because it is the most powerful, but because it is the only one you can size with confidence before the money leaves your hand. In a category where the standard practice is a maximum GPM figure with no stated rise, publishing a four-point curve at two voltages is a meaningful act of good faith.
And read the 208 V column before you order. Best electric tankless water heaters.
What goes in with it
A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.
- Flush kit
The annual descale is not optional on a tankless unit, and you need a pump, hoses and a bucket.
- Descaler solution
What actually dissolves the scale once the kit is hooked up.
- Recirculation pump
Fixes the cold-water delay that makes new tankless owners call a plumber.