Verdict
Head-to-head · Best Tankless Water Heaters

Rheem RTEX-13 vs Stiebel Eltron Tempra 36 Plus

Which is the better buy? Side-by-side on rating, price, strengths, and watch-outs — with the published ratings we averaged to get there.

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The short answer

Stiebel Eltron Tempra 36 Plus comes out ahead by a clear margin (4.1 vs 4.5). The gap is mostly about All-electric homes in moderate climates that need whole-house hot water without gas or venting. — read the strengths below before deciding.

Rheem RTEX-13
Ranked #5 in Best Tankless Water Heaters
Rheem RTEX-13
2 sourcesall derived$271.48as of Aug 1

The Rheem RTEX-13 is the best bang for the buck, Bob Vila's value choice and Reviewed's budget electric unit, praised for a '99 percent efficiency rating' at the lowest price in this group. At about 3.17 GPM it's a point-of-use heater designed, as Reviewed notes, 'to support one to two appliances in a single location, such as a shower in the bathroom.' It won't run a whole house. For a remote bathroom, an addition, or a booster, its compact size, efficiency, and low cost make it the obvious affordable choice.

Strengths
  • Lowest price of the group — excellent point-of-use value
  • Roughly 99% energy efficiency with self-modulating control
  • Compact 13 kW unit easy to install near a single fixture
Watch-outs
  • Only ~3.17 GPM — supports one to two fixtures, not a whole home
  • Not suitable as a primary whole-house heater in most homes
  • Flow drops further with cold inlet water
Stiebel Eltron Tempra 36 Plus
Higher ratedRanked #1 in Best Tankless Water Heaters
Stiebel Eltron Tempra 36 Plus
3 sources2 derived$712.73as of Aug 1

The Stiebel Eltron Tempra 36 Plus is the clear frontrunner for whole-home electric tankless heating, good for up to 7.5 GPM from its 36 kW heating element. This Old House rated the Stiebel Eltron electric line 4.4/5, and Bob Vila named it best electric, both praising the silent operation and the Advanced Flow Control that dynamically reduces flow rather than letting cold-water sandwiching happen when demand exceeds capacity. It needs serious electrical service and performs best in moderate climates. For an all-electric home, it is the top whole-house pick.

Strengths
  • Best whole-home electric option, up to 7.5 GPM at 36 kW
  • Advanced Flow Control throttles flow to prevent cold-water sandwiching
  • Silent operation with no combustion, venting, or CO risk
Watch-outs
  • Needs substantial electrical service — often a panel upgrade
  • Flow rate drops in cold climates with high temperature rise
  • No federal gas tax credit (electric)

Derived means the reviewer published no score, so one was inferred from their written verdict. Everything else is a number the publisher printed. How we rank.

How they stack up

Rheem RTEX-13

The Rheem RTEX-13 is the smallest and cheapest unit here, a point-of-use heater at about 3.17 GPM versus the whole-home flow of the Stiebel Eltron Tempra 36 Plus and EcoSmart ECO 27, and far below the gas Rinnai RU199iN and Takagi T-H3-DV-N. It's not a whole-home replacement, but it's the value leader for single-fixture jobs.

Stiebel Eltron Tempra 36 Plus

The Stiebel Eltron Tempra 36 Plus is the best whole-home electric unit, with a 7.5 GPM flow rate that beats the EcoSmart ECO 27 and Rheem RTEX-13 but trails the gas Rinnai RU199iN and Takagi T-H3-DV-N. Its Advanced Flow Control is a feature neither the EcoSmart nor the Rheem offers, trading a brief flow reduction for consistently hot water.

Specs side-by-side

SpecRheem RTEX-13Stiebel Eltron Tempra 36 Plus
FuelElectricElectric
Flow RateUp to ~3.17 GPMUp to 7.5 GPM
Efficiency~99%
Power13 kW36 kW
BTU2.3-2.3 GPM
TypeElectricTankless Electric
InstallationIndoor, point-of-use
Dimensions8.25"W x 12.62"H
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