Stiebel Eltron Vs Rheem Electric Tankless Water Heaters

Why This Comparison Matters

Electric tankless water heaters don't have the brand variety of gas units. In the whole-house electric tankless category, Stiebel Eltron and Rheem are the two brands contractors actually spec and homeowners actually buy. They overlap at a few kilowatt levels (24 kW and 36 kW), but their engineering philosophy, warranty structure, and technology are fundamentally different.

If you're replacing a 40- or 50-gallon electric tank with a tankless unit — or building new and want to skip the gas line — this page will help you pick the right unit the first time.

Head-to-Head: Brand Overview

Technology: What's Actually Different Inside

Stiebel Eltron's Approach

Stiebel Eltron uses a true thermostatic control system. A microprocessor continuously reads both the incoming water temperature (via a thermistor) and the flow rate (via a turbine-style flow sensor), then adjusts power to the heating elements via triacs (solid-state switches) to deliver a precise output temperature. The Tempra Plus models add Advanced Flow Control — a patented feature that automatically reduces water flow if demand exceeds the unit's heating capacity, so the output temperature never drops. You get slightly less water volume, but the shower stays hot.

The Tempra Trend models use the same microprocessor and flow sensor technology but do not include Advanced Flow Control. They cost less and perform identically at normal demand levels — the difference only shows up when someone opens a second fixture that pushes the unit past its capacity.

Rheem's Approach

Rheem RTEX units use a self-modulating system with digital thermostatic controls. The unit adjusts power output based on demand, and the user sets temperature in 1°F increments via external controls and a digital display. If demand exceeds capacity, the water temperature drops — there's no automatic flow reduction. The advantage is simplicity: fewer electronic components, lower cost, and a very low activation flow rate of 0.3 GPM across all models (Stiebel requires up to 0.77 GPM on larger units).

Model-by-Model Lineup

Here's where the lineups overlap and where they don't. Stiebel offers more granularity at the low end; Rheem fills the 27 kW gap that Stiebel skips.

Notice that at 24 kW, Stiebel uses two breakers (2 × 50A) while Rheem uses three (3 × 40A). Same total power, different wiring. At 36 kW, Stiebel uses three breakers while Rheem uses four. Fewer breaker runs means less labor and conduit on install — but 40A breakers are more common in existing panels than 50A breakers. Talk to your electrician about what's already in your panel.

GPM by Climate Zone

This is the table that actually matters for sizing. Flow rates below are calculated to a 105°F set point. Your zone depends on your ground water temperature — colder water means the unit has to work harder, which reduces maximum flow rate.

Rheem RTEX — GPM by Ground Water Temperature Zone

Stiebel Eltron Tempra — Max Temp Rise at Key Flow Rates (240V)

Stiebel publishes temp rise curves rather than zone-based GPM. To figure your usable GPM: subtract your ground water temp from 105°F to get the rise you need, then find which model delivers that rise at your desired flow rate.

Example: Ground water at 52°F (Zone 4), target output 105°F → you need a 53°F rise. At 3.0 GPM (one shower + one faucet), the Tempra 24 delivers 54°F rise — just enough. The RTEX-24 in Zone 4 delivers 3.1 GPM. Both work. The Stiebel has a slight edge because Advanced Flow Control would throttle flow before the temperature dropped if someone opened a third fixture.

Warranty Comparison

This is where Stiebel pulls away definitively.

That 3-year parts warranty matters. If a triac, flow sensor, or circuit board fails in year two, Stiebel covers the part. Rheem doesn't. On a unit that costs $500–$850, replacing a circuit board out of warranty can run $150–$250 in parts alone.

Electrical Requirements: The Hidden Cost

The sticker price of an electric tankless is only part of the cost. The electrical work — new breakers, wire runs, and potentially a panel upgrade — is often the bigger expense. Here's what each brand requires at the 24 kW level, the most common whole-house size:

At the same kilowatt output, Stiebel needs fewer breakers, fewer wire runs, and a smaller minimum panel. On a retrofit where the existing panel is already crowded, this can be the deciding factor. Fewer wire runs also means less labor — typically $200–$400 savings on install depending on run length and local rates.

Pricing Reality

Street prices from PlumbersCrib (authorized dealer pricing, not MAP-inflated MSRP):

Stiebel costs more. The question is whether the longer warranty, fewer breaker runs, and Advanced Flow Control justify the premium. For a single-family homeowner who plans to stay in the house, we'd say yes — the warranty savings alone can pay back the difference if a part fails in years 2–5. For a rental property or a quick flip, the Rheem's lower upfront cost is hard to argue with.

Budget tip: The Stiebel Eltron Tempra Trend series offers the same kW range and thermostatic control as the Plus — minus Advanced Flow Control — at a lower price point. The Tempra 24 Trend runs about $649, which closes the gap with Rheem significantly.

The Verdict: When to Pick Each Brand

Our Honest Take

If a homeowner calls us and says "I want an electric tankless for my primary residence and I'm willing to spend what it takes to get the best unit" — we recommend the Stiebel Eltron Tempra 24 Plus or Tempra 29 Plus depending on climate. The warranty is better, the technology is more sophisticated, and the electrical install is simpler.

If a contractor calls and says "I need to spec 12 units for a rental development and I need them under budget" — we recommend the Rheem RTEX-24. It's a proven unit, the price is right, and when you're buying in volume the savings add up fast.

Both brands are solid. Both will last. The right answer depends on the project — and that's why we carry both.