Rheem RTEX-18 18kW Electric Tankless Water Heater vs EcoSmart ECO 27 27kW Electric Tankless Water Heater

The Rheem RTEX-18 and EcoSmart ECO 27 are the two best-selling whole-house electric tankless water heaters in the United States. They are not the same product class. The decision between them is not really a brand decision — it is a question of which one your house can actually run, and what the groundwater temperature is where you live.

The 30-second answer

The ECO 27 will reliably serve a 2.5-bathroom home in any US climate. The RTEX-18 will reliably serve a 2-bathroom home in the southern half of the country and a 1-bathroom home in the northern half. If you cannot afford a service panel upgrade and you live in the North, neither of these is the right product — you should be shopping gas or heat-pump instead.

Electrical reality check (read this first)

This is the single most-skipped step in the buying process and the reason most "tankless doesn't work" reviews exist. Before you compare features, compare what your house can power.

The Rheem RTEX-18 is rated for 18 kW. It pulls 75 amps at 240 volts on a single feed and requires two 40-amp double-pole breakers. To install it cleanly you need at least 10–15 amps of headroom on a 100-amp panel after everything else in your house is wired. In practice that means most homes with a 100A panel will fail the load calculation and need either an upgrade or an aggressive load-shed of an electric dryer or range.

The EcoSmart ECO 27 is rated for 27 kW. It pulls 112.5 amps at 240V across three 40-amp double-pole breakers (six breaker slots). The math says you cannot run this unit on anything smaller than a 200-amp service. There are no exceptions and no workarounds — three separate feeds is non-negotiable, and any electrician who tells you otherwise should not be working on your house.

So: RTEX-18 → 100A panel may work after load calc · ECO 27 → 200A panel required.

Groundwater temperature changes the math

Tankless capacity is rated as a temperature rise. To deliver 105°F shower water from 45°F incoming groundwater (typical northern winter), the unit must heat the water by 60°F. From 65°F incoming (typical southern winter), the same shower needs only a 40°F rise. The same physical unit delivers nearly 50% more gallons-per-minute in the South.

Manufacturer charts at a 60°F rise: RTEX-18 ≈ 2.0 GPM · ECO 27 ≈ 3.0 GPM. At a 40°F rise: RTEX-18 ≈ 3.0 GPM · ECO 27 ≈ 4.6 GPM. A single typical shower runs 2.0–2.5 GPM. The RTEX-18 cannot simultaneously run two showers anywhere in the country. The ECO 27 can do two showers in the South and just barely two showers in the North.

Self-modulation behavior

Both units self-modulate — meaning they reduce power draw when demand is low — but the implementations differ. The Rheem uses a thermistor-driven modulation that adjusts in roughly 0.5 kW increments. The EcoSmart uses what the company markets as "patented self-modulating technology" with finer-grain control, in roughly 0.25 kW increments. In practice the difference is not noticeable at the tap, but the EcoSmart's modulation is slightly more efficient at very low draws (handwash use) — adding up to maybe $8/year in energy savings.

Installation, plumbing, and physical footprint

The Rheem RTEX-18 measures roughly 14" × 10" × 4" deep and weighs 13 lbs. The EcoSmart ECO 27 measures roughly 17" × 17" × 4" deep and weighs 15 lbs. Both wall-mount and both have 3/4" NPT inlet/outlet. Either can replace a 50-gallon tank and free up roughly 18 square feet of floor space.

The Rheem ships with a copper heat exchanger; the EcoSmart ships with a stainless-steel heat exchanger. Stainless tolerates harder water with less scaling, which matters in well-water areas. If your water hardness is above 10 grains/gallon, the EcoSmart will need descaling roughly half as often as the Rheem (every 18–24 months vs every 9–12 months on hard water).

Warranty and serviceability

Rheem RTEX-18: lifetime on the heat exchanger, 5 years on the electronics. Service is handled through Rheem's plumber network — finding a tech who has seen one before is easy.

EcoSmart ECO 27: lifetime on the heat exchanger and the electronics (the only major brand offering lifetime electronics). Service is handled through EcoSmart directly — you call the company, they ship you the part, you replace it yourself or hire a local electrician. Most failures are flow sensor or thermistor, both user-replaceable in under 30 minutes.

The EcoSmart warranty is on paper better. In practice the Rheem is easier to get a same-day plumber for because more techs have worked on them.

10-year cost comparison

Assuming a household of four, southern climate, 65 gallons/day at 105°F:

• Rheem RTEX-18: $549 unit + $850 install (two 40A breakers, 6-AWG run) = $1,399 up-front. ~$28/mo electric heating cost. 10-year total ≈ $4,759.
• EcoSmart ECO 27: $629 unit + $1,200 install (three 40A breakers, three 8-AWG runs) = $1,829 up-front. ~$27/mo electric heating cost. 10-year total ≈ $5,069.

The ECO 27 costs about $310 more over a decade, primarily in install. The energy difference is rounding error. You are not paying for efficiency — you are paying for headroom.

Who buys which

Buy the Rheem RTEX-18 if: you live in the Sun Belt, have a 1–2 bathroom home, currently have an electric tank and a 100A panel, and your household runs showers one at a time. This is the right answer for the largest single buyer demographic — a southern retiree couple replacing a leaking 40-gallon electric.

Buy the EcoSmart ECO 27 if: you have a 200A panel (or are doing an upgrade anyway), you live anywhere north of Tennessee, and you have 2+ bathrooms with overlapping morning shower use. Also the right answer if you have well water (the stainless heat exchanger pays for itself in reduced descaling).

Buy neither if: you are in the North with a 100A panel and not upgrading. You cannot get reliable whole-house performance from electric tankless under those constraints — gas tankless (Rinnai or Navien) or a heat-pump tank (Rheem ProTerra) will outperform both options here.