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Head-to-head · Best Tankless Water Heaters

Rheem RTEX-18 Electric Tankless Water Heater 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

Rheem RTEX-18 Electric Tankless Water Heater comes out ahead by a clear margin (4.5 vs 3.5). Rheem RTEX-18 Electric Tankless Water Heater is best for: small homes, apartments, additions or small buildings with modest hot-water demand and at least a 150A electrical panel. Read the strengths below before deciding.

Rheem RTEX-18 Electric Tankless Water Heater
Higher ratedRanked #1 in Best Tankless Water Heaters
Rheem RTEX-18 Electric Tankless Water Heater
1 source$427.9as of Oct 16

The Rheem RTEX-18 is an 18 kW, 240V electric tankless heater that Bob Vila's Glenda Taylor installed in a small commercial building and rated 4.8 out of 5. It mounts flat on a wall at 13.9 by 17 by 3.6 inches and delivered hot water at the sink within a few seconds. It is a small-capacity unit that needs two 40A double-pole breakers and a 150A service panel, so it suits light demand rather than a house with several bathrooms.

Strengths
  • — Compact 13.9 x 17 x 3.6 inch wall-mounted body, per Bob Vila
  • — Hot water reached the faucet in a few seconds in Bob Vila's installation
  • — Temperature set with a single front knob; Bob Vila settled on 120 degrees F
Watch-outs
  • — Requires a 150A service panel and two 40A double-pole breakers
  • — Small capacity: Bob Vila advises a higher-capacity model for a large home with several bathrooms
  • — Professional installation suggested; plumbing parts such as hoses and manifolds are bought separately
$427.9 · Check Price on AmazonFull review →
Stiebel Eltron Tempra 36 Plus
Ranked #4 in Best Tankless Water Heaters
Stiebel Eltron Tempra 36 Plus
1 sourceall derived$852.76as of Oct 14

The Stiebel Eltron Tempra 36 Plus is a 36 kW whole-house electric tankless heater with Advanced Flow Control, which Stiebel Eltron says slightly reduces flow when demand exceeds what the heater can handle so the temperature holds. In a video review after 10 months, Foray Fog found it cut his water-heating cost and gave his family of five unlimited hot water, but at about 3 GPM a bath plus a shower left the shower at a drizzle.

Strengths
  • — Cut Foray Fog's water-heating cost from about $61 to about $27 a month versus two 40-gallon electric tanks
  • — Unlimited hot water for his family of five when showers are staggered
  • — Reduces flow to hold the set temperature when demand is too high, per Stiebel Eltron and Foray Fog
Watch-outs
  • — Foray Fog sees about 3 GPM at most; a bath plus a shower drops the shower to a drizzle
  • — No app or Wi-Fi monitoring, which Foray Fog missed when tracking down a line-voltage problem
  • — Heavy electrical load: Foray Fog moved other loads to a second panel and could not add a fast EV charger
$852.76 · Check Price on AmazonFull review →

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-18 Electric Tankless Water Heater

At up to 4.4 GPM in the warmest zone, per Bob Vila's spec table, the RTEX-18 is a smaller electric unit than the Stiebel Eltron Tempra 36 Plus (7.5 GPM) and well below the rated flow of the gas Rinnai RU199iN (11 GPM) and Takagi T-H3-DV-N (10 GPM). Unlike the gas units it needs no venting, but it does need two 40A double-pole breakers.

Stiebel Eltron Tempra 36 Plus

At 36 kW it is the largest electric unit here, above the 27 kW EcoSmart ECO 27 and the 18 kW Rheem RTEX-18, and it has Advanced Flow Control, which Stiebel Eltron says is exclusive to its Tempra Plus models. Foray Fog saw about 3 GPM in real use, well below the rated flow of the gas Rinnai RU199iN (11 GPM) and Takagi T-H3-DV-N (10 GPM).

Specs side-by-side

SpecRheem RTEX-18 Electric Tankless Water HeaterStiebel Eltron Tempra 36 Plus
Power18 kW36 kW
Voltage240V—
Maximum Flow Rate4.4 GPM (warmest zone)—
Dimensions13.9 x 17 x 3.6 inches16.54"W x 14.52"H
Breaker SizeTwo 40A double-pole breakers—
Minimum Service Panel150 amps—
Water Temperature Range80 to 140 degrees F—
FuelElectricElectric
Flow Rate—Up to 7.5 GPM
Type—Tankless Electric
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