18 kW or 27 kW Tankless Water Heaters? Compared by Specs
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When comparing an 18 kW vs 27 kW tankless water heater, you face a choice between moderate and high-capacity electric heating. Both units are whole-home, 240V electric models designed for on-demand hot water. The primary difference lies in their power output and resulting flow rates. The 18 kW unit offers a steady 4.4 GPM, while the 27 kW model delivers up to 6.6 GPM. This wattage gap affects how many fixtures you can run simultaneously. Your electrical panel capacity and desired shower flow will determine the right fit for your home.
Key differences
| Aspect | Rheem 18kW 240V Electric Tankless Water Heater | Thermomate 27kW 6.6 GPM Electric Tankless Water Heater |
|---|---|---|
| Wattage | 18 kW | 27 kW |
| Flow Rate | 4.4 GPM | 6.6 GPM |
| Heating Element | Copper immersion | Cast aluminum alloy |
| Temperature Control | ±1°F accuracy | 80°F to 140°F range |
| Certification | not listed | ETL |
| Weight | 14.8 pounds | 19 pounds |
Which one should you choose?
Choose Rheem 18kW 240V Electric Tankless Water Heater if
- You need a lower flow rate of 4.4 GPM, which is sufficient for smaller households or single-bathroom setups.
- Your electrical panel has limited capacity, as the 18 kW unit draws less power than the 27 kW model.
- You prefer a lighter unit that is easier to mount on the wall, weighing 14.8 pounds compared to 19 pounds.
Choose Thermomate 27kW 6.6 GPM Electric Tankless Water Heater if
- You require a higher flow rate of 6.6 GPM to support multiple simultaneous fixtures, such as two showers and a sink.
- You want a unit with ETL certification for verified safety standards against UL-499 and CAN/CSA-C22.2.
- You need a cast aluminum heating block that resists scale buildup and corrosion better than copper immersion elements.
Verdict: Choose the 18 kW model if your household has modest hot water demands and limited electrical capacity. Select the 27 kW model if you need higher flow rates for multiple bathrooms or prefer the durability of a cast aluminum heating element. Both units provide endless hot water, but the 27 kW version supports more simultaneous use.
18 kW vs 27 kW Heater Specs
Wattage and power
The Rheem unit offers 18 kW of heating power, while the ThermoMate model delivers a robust 27 kW. This significant difference in wattage directly impacts how much hot water each device can produce. The higher wattage in the ThermoMate allows it to handle greater simultaneous demand. You get more raw heating capacity with the 27 kW model. The Rheem provides sufficient power for standard household needs. However, if you require higher energy output for larger homes or multiple fixtures, the ThermoMate has the advantage. The extra 9 kW makes a practical difference in performance.
Flow rate capacity
Flow rate determines how many fixtures can run at once. The Rheem 18kW unit provides up to 4.4 GPM. In contrast, the ThermoMate 27kW unit supports up to 6.6 GPM. This means the ThermoMate can supply water to more showers or sinks simultaneously without pressure drops. The Rheem is suitable for one or two fixtures. The ThermoMate handles three or four fixtures better. If your household has high water usage, the higher flow rate of the ThermoMate is beneficial. It ensures consistent hot water during peak times.
Physical dimensions
Size matters for installation space. The Rheem measures 3.5 x 14.5 x 18.5 inches. The ThermoMate is 17.09 x 13.15 x 3.15 inches. The Rheem is taller but narrower in some dimensions. The ThermoMate is flatter and slightly wider. Both are compact and wall-mounted. The Rheem has a smaller footprint in height. The ThermoMate is thinner in depth. You should measure your installation area carefully. The Rheem fits well in vertical spaces. The ThermoMate suits areas with limited depth. Both fit standard residential closets.
Weight and mounting
Weight affects wall mounting requirements. The Rheem weighs 14.8 pounds. The ThermoMate weighs 19 pounds. The ThermoMate is heavier due to its cast aluminum heating block. You need sturdy wall supports for both units. The Rheem is lighter and easier to handle during installation. The ThermoMate requires secure mounting to support its weight. Both units are designed for wall mounting. Ensure your wall can support the load. The lighter Rheem is slightly easier to install alone. The heavier ThermoMate may need two people for safe placement.
Temperature control accuracy
Precision in temperature control enhances comfort. The Rheem features an external digital thermostatic control with LED display. It offers ±1°F accuracy. The ThermoMate provides temperature stability from 80°F to 140°F. It uses smart modulation for steady output. The Rheem gives you exact degree adjustments. The ThermoMate focuses on consistent flow temperature. Both units prevent temperature fluctuations. The Rheem allows precise manual tuning. The ThermoMate automates stability through its chip. You get better manual control with the Rheem. The ThermoMate relies on automated regulation.
Which model fits your needs?
Users needing high flow for multiple fixtures
The ThermoMate 27kW unit delivers a maximum flow rate of 6.6 GPM. This higher output supports running three to four plumbing fixtures simultaneously. Its cast aluminum heating element and self-modulating chip maintain stable temperatures between 80°F and 140°F. You get continuous hot water even during peak usage times in your home.
Buyers prioritizing precise digital temperature control
The Rheem 18kW model features an external digital thermostatic control with an LED display. It offers precise temperature accuracy within plus or minus 1 degree Fahrenheit. This level of control allows you to set exact water temperatures without fluctuations. The self-modulation technology adjusts power to meet your specific hot water demand efficiently.
Shoppers with limited electrical panel capacity
The Rheem 18kW unit requires less electrical infrastructure than the higher-wattage alternative. You do not need to ensure your main panel has 150 Amps or install three separate 40 Amp breakers. This makes the installation process simpler for homes with standard electrical setups. It provides reliable whole-home heating with fewer circuit requirements.
Quick picks
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | Rheem 18kW 240V Electric Tankless Water Heater | Single bathroom or light whole-home use | Check price of Rheem 18kW 240V Electric Tankless Water Heater on Amazon |
| 2 | Thermomate 27kW 6.6 GPM Electric Tankless Water Heater | multi-bathroom homes with strong electrical panels | Check price of Thermomate 27kW 6.6 GPM Electric Tankless Water Heater on Amazon |
The products compared
Rheem 18kW 240V Electric Tankless Water Heater
Best for: Single bathroom or light whole-home use
- Wattage
- 18 kW (what this guide is about)
- Fuel Type
- Electric
- Flow Rate
- 4.4 GPM
- Voltage
- 240 V
- Temperature Control
- ±1°F
- Dimensions
- 3.5 x 14.5 x 18.5 inches
View on Amazon: Rheem 18kW 240V Electric Tankless Water Heater Affiliate link
This Rheem unit delivers 18 kW of power at 240 volts, supporting a flow rate of up to 4.4 GPM. It features external digital thermostatic control with ±1°F accuracy and copper immersion heating elements. The compact design measures 3.5 x 14.5 x 18.5 inches and weighs 14.8 pounds. It suits single-bathroom setups or light whole-home use where demand stays moderate.
Pros
- 18 kW output at 240V for efficient electric heating
- Precise ±1°F temperature control via external digital display
- Field-serviceable copper immersion heating elements
- Compact dimensions of 3.5 x 14.5 x 18.5 inches
Cons
- Lower 4.4 GPM flow rate limits simultaneous multi-fixture use
- Heavier 14.8-pound build compared to lighter point-of-use models
- Requires 240V electrical circuit, limiting installation locations
Good fit if
- You need precise temperature control for a single bathroom
- You have a 240V circuit available for installation
- You want field-serviceable copper heating elements for longevity
Skip if
- You require higher flow rates for multiple bathrooms
- You only have a 120V electrical outlet available
- You need a lighter unit for easy wall mounting
Verdict: Rheem RTEX-18 18kW Electric Water Heater fits single-bathroom households needing precise temperature control and reliable copper heating elements. It does not support high-demand multi-fixture usage due to its 4.4 GPM limit.
Thermomate 27kW 6.6 GPM Electric Tankless Water Heater
Best for: multi-bathroom homes with strong electrical panels
- Wattage
- 27 kW (what this guide is about)
- Fuel Type
- Electric
- Flow Rate
- Up to 6.6 GPM
- Installation Location
- Indoor
- Voltage
- 240 V
- Temperature Control
- 80°F to 140°F
View on Amazon: Thermomate 27kW 6.6 GPM Electric Tankless Water Heater Affiliate link
The ThermoMate 27kW unit delivers up to 6.6 GPM of hot water on 240V power. It uses a cast aluminum heating element to resist scale and corrosion. The smart modulation chip adjusts output to keep temperatures stable between 80°F and 140°F. This model requires three 40-amp breakers and significant electrical capacity. It fits well in compact spaces like apartments or small homes needing multi-point supply.
Pros
- 27 kW wattage supports up to 6.6 GPM flow rate
- Cast aluminum heating block resists scale and corrosion
- Smart modulation maintains stable temperature from 80°F to 140°F
- Compact dimensions of 17.09 x 13.15 x 3.15 inches
Cons
- Requires 150 Amp main panel capacity
- Needs three separate 40 Amp double-pole breakers
- Uses 8 AWG copper wiring for installation
Good fit if
- You have a 150 Amp electrical panel
- You need hot water for multiple fixtures simultaneously
- You want a compact unit for tight spaces
Skip if
- You cannot install three 40 Amp breakers
- Your home panel has less than 150 Amps
- You only need a single fixture supply
Verdict: ThermoMate 27kW Electric Tankless Heater suits homes with robust electrical infrastructure that need high flow rates for multiple bathrooms. It is ideal for users who prioritize consistent temperature stability and space-saving design over easy DIY installation.
Specifications side by side
| Spec | Rheem 18kW 240V Electric Tankless Water Heater | Thermomate 27kW 6.6 GPM Electric Tankless Water Heater |
|---|---|---|
| Fuel Type | Electric | Electric |
| Flow Rate | 4.4 GPM | Up to 6.6 GPM |
| Wattage | 18 kW | 27 kW |
| Installation Location | not stated | Indoor |
| Voltage | 240 V | 240 V |
| Temperature Control | ±1°F | 80°F to 140°F |
| Certification | not stated | ETL Listed |
| Dimensions | 3.5 x 14.5 x 18.5 inches | 17.09 x 13.15 x 3.15 inches |
| Use | Whole House | not stated |
| Weight | 14.8 pounds | 19 pounds |
| Size | Small | not stated |
| Color | Gray | Gray |
| Model | RTEX-18 | EH270 |
Frequently asked questions
Can an 18 kW tankless water heater serve a whole house?
Yes, but it is best suited for single-bathroom households or light whole-home use. The Rheem RTEX-18 model has a flow limit of 4.4 gallons per minute. This capacity handles one fixture comfortably but struggles with high-demand multi-fixture usage. You should verify your household's simultaneous hot water needs before choosing this lower wattage option.
What is the maximum flow rate for a 27 kW electric heater?
The ThermoMate EH270 delivers a maximum flow rate of 6.6 gallons per minute. This higher output supports running three to four plumbing fixtures simultaneously. It is ideal for multi-bathroom homes with strong electrical panels. The increased wattage allows for greater water volume while maintaining temperature stability during peak usage times.
How do the physical dimensions of these units compare?
Both models are compact and gray, but their shapes differ. The Rheem RTEX-18 is wider at 14.5 inches but thinner at 3.5 inches. The ThermoMate EH270 is taller at 17.1 inches and square in footprint at 13.2 inches by 13.2 inches. You should measure your installation space carefully to ensure either unit fits your available area.
Which unit is lighter and easier to mount?
The Rheem RTEX-18 weighs 14.8 pounds, making it lighter than the ThermoMate EH270 at 19 pounds. Both units are small and designed for wall mounting. The Rheem model may be slightly easier to handle during DIY installation. However, the ThermoMate requires a robust electrical infrastructure regardless of its weight.
Do both heaters operate on standard 240V power?
Yes, both the Rheem RTEX-18 and ThermoMate EH270 operate on 240V electricity. They are both whole-home electric tankless water heaters. This standard voltage is common in residential settings. You do not need special industrial power supplies for either model, though higher wattage units may require upgraded electrical panels.
What heating element technology do these models use?
The Rheem RTEX-18 uses reliable copper heating elements for precise temperature control. The ThermoMate EH270 features a cast aluminum heating element. Both designs support on-demand hot water generation. The ThermoMate also includes a self-modulating chip to maintain consistent temperature stability under varying flow conditions.