18 kW vs 11 kW Tankless Water Heaters in 2026: Key Differences
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Choosing between an 18 kW vs 11 kW tankless water heater depends on your hot water needs. Both units are electric and operate at 240V. They use self-modulating technology to adjust power based on demand. The main difference is output capacity. The 18 kW model supports whole-house use with higher flow rates. The 11 kW unit is designed for point-of-use applications like single sinks or small bathrooms. Select the power level that matches your household usage.
Key differences
| Aspect | Rheem 18kW 240V Electric Tankless Water Heater | Thermomate 11 kW Electric Tankless Water Heater |
|---|---|---|
| Wattage | 18 kW | 11 kW |
| Flow Rate | Up to 4.4 GPM | 2.15 GPM at 35°F rise |
| Recommended Use | Whole House | Point of Use |
| Water Connections | 3/4 inch NPT | 1/2 inch NPT |
| Temperature Control | ±1°F accuracy | Max 131°F |
| Dimensions | 14.5 x 18.5 x 3.5 inches | 7.09 x 7.09 x 10.6 inches |
Which one should you choose?
Choose Rheem 18kW 240V Electric Tankless Water Heater if
- You need hot water for multiple fixtures simultaneously, such as a shower and a kitchen sink.
- You require a higher flow rate of up to 4.4 GPM for whole-house supply.
- You prefer precise digital temperature control with ±1°F accuracy.
Choose Thermomate 11 kW Electric Tankless Water Heater if
- You are installing the unit for a single sink, bathroom, or office break room.
- You have limited space and need a compact unit measuring 7.09 inches wide.
- You want a maintenance-free design with a cast aluminum heating chamber.
Verdict: The 18 kW model is the practical choice for households needing consistent hot water across multiple outlets. The 11 kW unit is better suited for specific, low-flow locations where space and efficiency are priorities. Match the wattage to your actual fixture count to avoid pressure drops.
18 kW vs 11 kW Heater Comparison
Wattage and Power
The Rheem unit delivers 18 kW of power, while the ThermoMate provides 11 kW. Both operate at 240V, meaning they draw different amperage levels. The 18 kW model requires a higher electrical capacity to support its increased output. The 11 kW model draws 46 amps and needs a double-pole 50Amp breaker. You must check your home’s electrical panel before installing either unit. Higher wattage generally means faster heating and greater capacity for multiple fixtures. Lower wattage suits smaller, isolated applications. This difference defines the core capability of each device.
Flow Rate Capacity
Flow rate determines how much hot water you get simultaneously. The Rheem model supports up to 4.4 GPM. This allows you to run a shower and a sink at the same time. The ThermoMate offers 2.15 GPM at a 35°F temperature rise. This flow is sufficient for a single bathroom or point-of-use sink but may struggle if another faucet opens. If you need whole-house coverage, the higher flow of the Rheem is essential. For a single tap, the lower flow of the ThermoMate is adequate.
Physical Dimensions
Size affects where you can mount the unit. The Rheem measures 14.5 inches wide, 3.5 inches high, and 18.5 inches long. It is a flat, rectangular box that fits against a wall. The ThermoMate is more compact, measuring 7.09 inches wide, 10.6 inches high, and 7.09 inches long. Its square footprint saves horizontal space. The ThermoMate weighs 11 pounds, while the Rheem weighs 14.8 pounds. Both are lightweight, but the ThermoMate’s shape may fit better in tight cabinets or small utility closets.
Temperature Control Precision
Thermostat accuracy impacts shower comfort. The Rheem features an external digital thermostatic control with LED display. It offers ±1°F accuracy, allowing you to set a specific temperature precisely. The ThermoMate states a maximum temperature of 131°F but does not list a specific accuracy rating or digital precision feature. If you need exact temperature settings to avoid scalding or lukewarm water, the Rheem’s stated accuracy is superior. The ThermoMate relies on its self-modulating technology to adjust heat, but lacks the same specified control granularity.
Heating Element Construction
Build quality influences longevity and maintenance. The Rheem uses durable copper immersion heating elements. These are field-serviceable, meaning you can replace them if they fail. The ThermoMate uses a cast aluminum heating chamber with the element separated from water lines. This design aims to prevent leaks and scale buildup, claiming maintenance-free operation. Copper conducts heat efficiently and is standard in many heaters. The aluminum chamber offers a different approach to corrosion resistance. You may prefer serviceable copper parts or a sealed aluminum system based on your maintenance preferences.
Intended Use Case
Your application dictates which unit fits your needs. The Rheem is labeled for whole-house use. Its 18 kW power and 4.4 GPM flow support multiple bathrooms. The ThermoMate is marketed for point-of-use applications, such as a bathroom, small sink, or office break room. It supports up to 3 faucets theoretically, but flow drops significantly if more than one is open. If you need hot water for an entire home, the Rheem is the clear choice. For a single remote sink, the ThermoMate is sufficient and more space-efficient.
Which Tankless Heater Fits Your Home
Whole-house needs with high flow
The Rheem unit delivers 4.4 GPM, supporting multiple simultaneous uses. Its 18 kW output handles whole-house demands. The copper immersion elements provide durable heating. You get precise temperature control with ±1°F accuracy. This model suits homes requiring consistent, high-volume hot water for showers and appliances.
Point-of-use or low-flow tasks
The ThermoMate offers 2.15 GPM, ideal for single sinks or small bathrooms. Its compact 7.09-inch width fits tight spaces. The cast aluminum chamber prevents leaks and scale. It requires a 50-amp breaker but saves floor space. This unit is practical for dedicated point-of-use applications where high flow is unnecessary.
Precise temperature control is critical
The Rheem features an external digital thermostat with LED display. It maintains temperature within ±1°F for stability. The self-modulating technology adjusts power to meet demand precisely. This level of control ensures consistent water temperature. You benefit from advanced regulation that prevents fluctuations during use.
Quick picks
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | Rheem 18kW 240V Electric Tankless Water Heater | Whole-house hot water with 240V power | Check price of Rheem 18kW 240V Electric Tankless Water Heater on Amazon |
| 2 | Thermomate 11 kW Electric Tankless Water Heater | single faucet point-of-use installations | Check price of Thermomate 11 kW Electric Tankless Water Heater on Amazon |
The products compared
Rheem 18kW 240V Electric Tankless Water Heater
Best for: Whole-house hot water with 240V power
- 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, making it suitable for whole-house use. It reaches up to 4.4 GPM with ±1°F temperature accuracy. The compact 3.5 x 14.5 x 18.5 inch design fits tight spaces. You get external digital control and copper immersion elements. It handles higher demand than 11 kW models.
Pros
- 18 kW output for whole-house hot water
- 4.4 GPM flow rate capacity
- ±1°F temperature control accuracy
- Compact 3.5-inch depth profile
Cons
- Requires 240V electrical connection
- No remote control or app mentioned
- Heavier at 14.8 pounds
Good fit if
- You need whole-house hot water
- You have 240V electrical access
- You want precise temperature control
Skip if
- You only have 120V outlets
- You need point-of-use heating
- You want a lighter unit
Verdict: Rheem 18 kW Electric Water Heater fits homes needing high flow rates and precise temperature control. It suits buyers with 240V power who want a compact, durable whole-house solution.
Thermomate 11 kW Electric Tankless Water Heater
Best for: single faucet point-of-use installations
- Wattage
- 11 kW (what this guide is about)
- Fuel Type
- Electric
- Flow Rate
- 2.15 GPM
- Voltage
- 240 V
- Certification
- ETL
- Dimensions
- 10.6 x 7.09 x 7.09 inches
View on Amazon: Thermomate 11 kW Electric Tankless Water Heater Affiliate link
This 11 kW unit delivers up to 2.15 GPM at a 35°F rise, making it suitable for point-of-use applications like a single bathroom or small sink. It operates on 240V and requires a 50A double-pole breaker. The compact gray design measures 10.6 inches high and weighs 11 pounds. It features an ETL certification and cast aluminum heating elements for durability.
Pros
- 11 kW heating power with 2.15 GPM flow rate
- Compact 10.6-inch height fits tight spaces
- ETL certified with overheating protection
- Cast aluminum heating chamber resists scale
Cons
- 11 kW output is lower than 18 kW competitors
- Requires a 50A double-pole breaker
- Max temperature limited to 131°F
Good fit if
- You need hot water for one faucet
- You have a 50A circuit available
- You want a compact point-of-use unit
Skip if
- You need to supply multiple showers
- You only have a 40A breaker
- You require higher flow rates
Verdict: Thermomate 11kW Electric Tankless Heater is a compact solution for low-flow needs. It fits well in tight spaces where a 50A breaker is already installed.
Specifications side by side
| Spec | Rheem 18kW 240V Electric Tankless Water Heater | Thermomate 11 kW Electric Tankless Water Heater |
|---|---|---|
| Fuel Type | Electric | Electric |
| Flow Rate | 4.4 GPM | 2.15 GPM |
| Wattage | 18 kW | 11 kW |
| Voltage | 240 V | 240 V |
| Temperature Control | ±1°F | not stated |
| Certification | not stated | ETL |
| Dimensions | 3.5 x 14.5 x 18.5 inches | 10.6 x 7.09 x 7.09 inches |
| Use | Whole House | not stated |
| Weight | 14.8 pounds | 11 pounds |
| Size | Small | not stated |
| Color | Gray | Gray |
| Model | RTEX-18 | ET Mini |
Frequently asked questions
Can an 11 kW tankless heater supply whole-house hot water?
No, the 11 kW Thermomate unit is designed for single faucet point-of-use installations. It lacks the output capacity to serve multiple fixtures simultaneously. For whole-house needs, you require a higher wattage model like the 18 kW Rheem. The 11 kW size is ideal for tight spaces or specific low-flow applications rather than full home coverage.
What is the physical size difference between these two units?
The 18 kW Rheem model is larger, measuring 18.5 inches in length and 14.5 inches in width. In contrast, the 11 kW Thermomate is significantly more compact at 7.09 inches by 7.09 inches. The smaller unit weighs 11 pounds, while the larger one weighs 14.8 pounds. Choose the smaller model if you have limited installation space.
Do both heaters require the same electrical setup?
Yes, both units operate at 240V. However, their power consumption differs due to wattage. The 18 kW model draws more current than the 11 kW unit. You must ensure your electrical panel can handle the amperage required by the specific heater you choose. The 11 kW unit fits well where a 50A breaker is already installed.
Which heater is better for a bathroom sink?
The 11 kW Thermomate is better suited for a single bathroom sink. It is optimized for point-of-use installations where flow rates are lower. Its compact dimensions allow for easy mounting near the sink. The 18 kW Rheem is overkill for this task and is intended for whole-house demands with higher simultaneous usage.
How does wattage affect the flow rate of hot water?
Higher wattage generally supports higher flow rates. The 18 kW Rheem unit delivers 4.4 GPM, supporting multiple simultaneous uses. The 11 kW model provides less output, making it suitable for single faucet use. If you need hot water for a shower and a sink at the same time, the 18 kW unit is the necessary choice.
Are both units electric and self-modulating?
Yes, both the Rheem and Thermomate heaters are electric and use self-modulating technology. They adjust power output based on your immediate hot water demand. This ensures energy efficiency by not heating water when it is not needed. Both models offer precise temperature control to maintain consistent water temperatures during use.