Rheem RTEX-18 18kW Electric Tankless Water Heater

The Rheem RTEX-18 Electric Tankless Water Heater is Rheem's mid-range electric tankless — 18,000 watts, 4.4 GPM peak flow at 35°F rise, 0.99 UEF, 5-year heat-exchanger warranty. At about $369–$449 retail, it's a sub-$400 path into the Rheem residential tankless category. The right pick for warm-climate single-bathroom installs, ADU/garage apartments, point-of-use applications, or supplemental hot water alongside an existing tank.

Headline specifications

• Type: electric tankless, point-of-entry or point-of-use
• Power: 18,000 watts (3 × 6kW elements)
• Electrical service: 240V, 3 × 30-amp circuits (total 90 amps)
• Max flow rate: 4.4 GPM at 35°F rise
• Min activation flow: 0.4 GPM
• UEF: 0.99
• Adjustable temperature: 80°F – 140°F via digital display
• Warranty: 5-year heat exchanger, 1-year electronics
• Dimensions: 17" H × 14.5" W × 3.625" D (compact wall-mount)

Who this model is for

The RTEX-18 is calibrated for:

• Warm-climate whole-house single-bathroom installs (Sun Belt with 70°F+ inlet water)
• ADUs, garage apartments, in-law suites needing electric tankless without major electrical work
• Supplemental hot water for distant fixtures (workshop, pool house)
• Off-grid or solar installations where electric matters more than gas access

For whole-house electric tankless in cold-inlet regions, step up to the EcoSmart ECO 27 (27 kW) or Stiebel Eltron Tempra 36 Plus.

Where it beats the alternatives

Vs EcoSmart ECO 18 ($399): Rheem and EcoSmart spec equivalent at 18 kW. Rheem ships at slightly lower price and has better digital controls; EcoSmart has lifetime warranty on heat exchanger (vs Rheem's 5-year). Choose Rheem for controls and brand-channel availability; EcoSmart for warranty depth.

Vs RTEX-13 ($289): RTEX-13 is the 13 kW step-down at $80 less, but flow drops to 3.2 GPM. RTEX-18's 4.4 GPM is worth the upgrade for most 1-bathroom installs.

Vs Stiebel Eltron Tempra 24 ($749): Tempra is premium-tier German build at 24 kW. Better controls, better modulation, longer warranty (7-year). Rheem's price is half. For budget-tier installs, Rheem; for premium electric tankless, Stiebel.

Where it falls short

5-year heat exchanger warranty is short. Most other tankless heaters offer 10+ years.

4.4 GPM is the limit. Two simultaneous demand points will drop temperature. Cold-inlet regions drop effective flow to about 2.5 GPM in winter.

Electrical service requirement is real — 3 × 30-amp circuits totals 90 amps. Older panels can struggle. Get a load calculation before purchase.

Install considerations

Three dedicated 30-amp 240V circuits. 10 AWG copper. Standard water connections. Wall-mount, no floor footprint. Permit typical.

Install cost: $500–$900 with existing electrical capacity; $1,500–$3,000 if panel/service upgrades needed.

Maintenance

• Inlet filter clean every 6 months
• Annual descaling in hard-water regions
• Element check at year 5 and year 10

Bottom line

The Rheem RTEX-18 is the budget Rheem electric tankless — adequate for single-bathroom whole-house in warm climates, ADUs, or supplemental use cases. For whole-house cold-climate use, step up to the EcoSmart ECO 27. For longer warranty exposure, the EcoSmart 18 matches at the same price. Click through to Amazon for live pricing; get an electrician's load check before purchase.