The best tankless hot water heater for whole-home electric use needs enough power to run several fixtures at once. In this head-to-head comparison, we look at two high-capacity electric models: the ThermoMate 27kW and the Rheem RTEX-24. Both mount on the wall, save space, and deliver continuous hot water.
Best Tankless Hot Water Heater: Top Picks Compared
Our top picks for tankless hot water heaters are the ThermoMate 27kW Tankless Water Heater 240V 6.6 and the Rheem RTEX-24 24kW 240V Electric Tankless Water.
Pros
- Exceptional space efficiency compared to bulky tank heaters, ideal for compact living spaces and tight closets
- Intelligent self-modulating chip delivers consistent temperature without annoying hot-cold cycling
- Robust construction with corrosion-resistant materials built to last longer than standard electric heaters
- ETL listed with comprehensive safety features including dry-combustion proofing and lightning protection
- Significant long-term savings through elimination of standby energy loss inherent to traditional tanks
Cons
- Requires substantial electrical infrastructure, 150+ Amp main panel capacity with three separate 40 Amp breakers, limiting compatibility with older homes
- Performance varies significantly based on inlet water temperature, less effective in cold climates for whole-home use
- Installation complexity requires professional electrician familiar with high-amperage circuits and proper wire gauging
The ThermoMate 27kW is a high-capacity electric tankless water heater designed for homeowners and small commercial users seeking on-demand hot water without the bulk of traditional storage tanks. This unit excels for apartments, small homes, cabins, and multi-fixture applications where space is premium and continuous hot water supply is essential.
Performance is the standout feature here. With a 6.6 GPM flow rate and self-modulating technology, this heater intelligently adjusts power consumption based on real-time demand, maintaining stable temperatures between 80-140°F without the frustrating temperature fluctuations common in cheaper models. The 99% thermal efficiency delivers measurable savings, cutting water heating costs by up to 60% by eliminating the standby energy losses of tank heaters that continuously reheat stored water throughout the day.
Build quality reflects premium engineering with a heavy-duty cast aluminum heating block and iron-chromium-aluminum alloy elements. The advanced water-electricity separation structure prevents scale accumulation and corrosion, addressing common failure points in electric tankless units. ETL certification to UL-499 and CAN/CSA-C22.2 standards ensures the unit meets rigorous safety protocols, backed by 100% factory testing.
The primary limitation is electrical requirements. This unit demands 150+ Amp service capacity with three separate 40 Amp double-pole breakers and 8 AWG copper wiring. Many older homes lack this infrastructure, requiring expensive panel upgrades. Additionally, performance in cold climates diminishes since inlet water temperature directly impacts heating capacity, making it better suited as a point-of-use heater in frigid regions rather than a true whole-home solution.
For buyers with adequate electrical service seeking space efficiency and long-term savings, the ThermoMate 27kW represents solid value. Its compact 16-pound wall-mounted form factor, self-modulating intelligence, and durable construction justify the investment for appropriate applications. Verify electrical compatibility before purchasing.
Power and Performance: 27 kW power output, 240V AC operation, 6.6 GPM flow rate, maximum temperature 140°F, maximum pressure 150 psi
Physical Dimensions: 13.15 inches wide, 17.09 inches tall, 3.15 inches deep, weighs 19 pounds, wall-mounted installation
Electrical Requirements: Requires 3 x separate 40 Amp double-pole breakers, 3 x sets of 8 AWG copper wires, minimum 150 Amp main panel capacity
Water Connections: Standard 3/4 inch NPT connections for inlet and outlet
Safety and Certifications: ETL listed to UL-499 and CAN/CSA-C22.2 standards, includes anti-leakage, over-temperature cutoff, dry-combustion proofing, and surge/lightning protection
Special Features: Self-modulating technology, advanced water-electricity separation structure, touch control display, 99% thermal efficiency
Pre-Installation Checklist: Verify your home's main electrical panel has minimum 150 Amp capacity. Confirm you can dedicate three separate 40 Amp double-pole breakers. Check that your home's water supply uses standard 3/4 inch NPT connections. Consider hiring a licensed electrician familiar with high-amperage circuits for proper installation.
Electrical Installation: This unit requires professional installation. Three separate 40 Amp circuits with 8 AWG copper wiring must be run from your main panel to the heater location. Improper wiring creates serious fire and electrocution hazards. Do not attempt DIY electrical work unless you are a licensed electrician.
Water Connection: Mount the unit on a wall near your main water supply line. Use standard 3/4 inch NPT fittings to connect inlet and outlet. Ensure proper water flow direction. Consider installing a water softener upstream if you have hard water to extend component lifespan.
Professional Installation Recommended: Due to complex electrical requirements and safety implications, professional installation by a licensed electrician and plumber is strongly recommended. Most municipalities require permits for high-amperage electrical work and tankless water heater installation.
Pros
- Unlimited hot water supply, eliminating the cold water runs that plague tank heaters with multiple users
- Significant space savings with compact 17.6 x 18.25 inch footprint ideal for apartments and small homes
- Lower energy consumption translates to measurable monthly utility bill reductions
- Precise temperature control via LED display with 1-degree increment adjustments
- No venting required, reducing installation complexity and cost versus gas models
Cons
- Requires substantial electrical infrastructure: 200-amp household service panel and three 40-amp double-pole breakers minimum
- Performance significantly decreases in cold climates with low groundwater temperatures, particularly during winter months
- Some users report inconsistent temperature output and insufficient heating capacity for simultaneous multiple fixture use
The Rheem RTEX-24 is a 24kW electric tankless water heater engineered for homeowners seeking unlimited hot water without the bulk of traditional storage tanks. This unit delivers 5.9 gallons per minute and is designed for both single point-of-use and multiple fixture applications. It's ideal for households in moderate to warm climates with adequate electrical infrastructure and a commitment to energy efficiency.
The standout feature is its ability to provide continuous hot water on demand while consuming energy only when hot water is actively being used. The 99.8% energy efficiency rating translates to real savings on monthly utility bills, with users reporting reductions of $30 or more monthly. The digital LED display offers precise temperature control in 1-degree increments from 80 to 140 degrees Fahrenheit, giving you exact control over your water temperature preferences.
Design-wise, the RTEX-24 is impressively compact at just 17.63 inches wide and 18.25 inches tall, roughly 90% smaller than conventional tank heaters. The stainless steel construction and wall-mount capability free up valuable floor space in utility rooms, basements, or kitchens. Installation is straightforward compared to gas tankless models since no venting is required, though professional installation is strongly recommended.
The primary limitation is electrical demand. This unit requires three 40-amp double-pole breakers and a minimum 200-amp household electrical service panel. Additionally, performance varies significantly based on groundwater temperature in your region. Homes in cold northern climates may experience reduced flow rates or insufficient heating during winter months, while those in warm climates with 70+ degree groundwater will see optimal performance.
Overall, the Rheem RTEX-24 is a reliable, space-saving solution for continuous hot water in the right conditions. If your home has adequate electrical capacity and you live in a moderate climate, this heater delivers excellent value through energy savings and unlimited hot water. Verify your groundwater temperature and electrical infrastructure before purchasing to ensure success.
Power and Electrical: 240V single-phase power, requires three 40-amp double-pole breakers, three sets of #8 AWG copper conductors with ground, 200-amp household service panel recommended, 24 KW power output.
Performance: Flow rate of 5.9 GPM, maximum pressure 150 PSI, temperature range 80-140 degrees Fahrenheit with 1-degree increment controls, 99.8% energy efficiency rating.
Physical Dimensions: Width 17.63 inches, height 18.25 inches, depth 3.5 inches, weight 17.8 pounds, wall-mounted installation type, stainless steel material construction.
Warranty and Included Components: 5-year leak warranty with limited lifetime parts coverage, includes fittings and aerators, LED display for temperature monitoring.
Is This Right for You? Before purchasing, verify two critical factors: your home's groundwater temperature and electrical capacity. Check your local groundwater temperature using USGS maps or your water utility. If your area has groundwater below 55 degrees Fahrenheit, this electric tankless heater will struggle during winter months. Consult a licensed electrician to confirm your home can accommodate the 200-amp service panel requirement and three 40-amp breaker installation.
Climate Considerations: This heater performs optimally in moderate to warm climates with groundwater temperatures of 70+ degrees Fahrenheit. Homes in North Florida, Southern California, Texas, and similar regions will experience excellent performance. Northern climates with cold groundwater may see reduced flow rates or insufficient heating, particularly during winter when you need hot water most.
Installation Requirements: Professional installation is highly recommended despite the lack of venting requirements. Factor installation costs into your total investment. Ensure adequate space near your point-of-use application, as proximity reduces energy loss and improves efficiency. Plan for descaling maintenance every 1-2 years depending on water hardness in your area.
Usage Expectations: This unit supports multiple simultaneous fixtures, such as a shower and sink running concurrently. However, simultaneous use of multiple high-demand applications may reduce temperature or flow. Review Rheem's performance guide for your specific region and usage patterns to confirm this model meets your household's hot water demands.
Our Best Tankless Hot Water Heater Picks Reviewed
The ThermoMate 27kW and the Rheem RTEX-24 suit different buyers, and the comparison below shows where each one wins.
- ThermoMate 27kW: This tankless hot water heater delivers up to 6.6 GPM with 99% thermal efficiency. Its higher power rating helps it supply multiple fixtures at once and handle colder incoming water.
- Rheem RTEX-24: This 24kW unit delivers about 5.9 GPM of continuous hot water. Rheem says it saves up to 90% of the space of a traditional tank, and the brand is well established in water heating.
Which Tankless Hot Water Heater Should You Buy?
Choosing the right tankless hot water heater comes down to choosing the ThermoMate 27kW and choosing the Rheem RTEX-24.
- Choose the ThermoMate 27kW if you have higher demand, colder groundwater, or several bathrooms.
- Choose the Rheem RTEX-24 if you prefer a well-known brand and your demand fits within about 5.9 GPM.
How to Choose the Best Tankless Hot Water Heater
Choosing the right tankless hot water heater comes down to flow rate, temperature rise, electrical requirements, and efficiency.
- Flow Rate: Add up the flow rates of fixtures used at the same time. Showers use about 2 GPM each.
- Temperature Rise: In cold climates, incoming water is colder, which reduces how much flow a unit can heat to your target temperature.
- Electrical Requirements: High-capacity electric units need 240V and large breakers, often more than one. Have an electrician confirm that your panel can support a tankless hot water heater of this size.
- Efficiency: Electric tankless models are highly efficient because almost all energy goes into the water.
- Controls: Digital temperature controls help maintain steady water temperature.
Installation Tips
- Upgrade your electrical service if needed.
- Install the unit close to major fixtures to reduce wait times.
- Add isolation valves for easier maintenance.
- Hire licensed professionals for electrical and plumbing work.
For more options, read our guides to the best tankless hot water heaters and the best home water heater.
Sizing for Whole-Home Use
The ThermoMate 27kW and Rheem RTEX-24 are large electric units designed to supply several fixtures at once. A helpful formula is BTUs per hour equal 500 times gallons per minute times the temperature rise. One kilowatt equals about 3,412 BTUs per hour, so a 27kW unit can raise about 4.1 gallons per minute by 45 degrees, while a 24kW unit handles about 3.6 gallons per minute at the same rise. In cold climates, expect lower flow at shower temperature.
Electrical Requirements
Units this size usually need several high-amperage circuits and substantial panel capacity, often 150 amps or more for the home’s service. Check the manual and have an electrician confirm your panel can support the load.
Permits and Professional Installation
Many areas require a permit and inspection when a water heater is installed or replaced. Gas connections, venting, and high-amperage electrical work should be handled by a licensed plumber, gas fitter, or electrician.
Descaling
Flush the unit once a year, or more often with hard water, using isolation valves and a descaling solution.
Common Mistakes to Avoid
Do not size by maximum GPM alone. Avoid installing without confirming panel capacity, and plan for annual maintenance when choosing a tankless hot water heater.
Final Verdict on the Best Tankless Hot Water Heater
For high demand, the ThermoMate 27kW is the best tankless hot water heater in this comparison, with 6.6 GPM and 99% thermal efficiency. The Rheem RTEX-24 is an excellent choice for homes that fit within 5.9 GPM and want a trusted brand.
Tankless Hot Water Heater FAQs
Common questions from buyers include “How many showers can a 27kW tankless heater handle?”, “Do electric tankless heaters need a panel upgrade?”, and “Are electric tankless heaters efficient?”.
How many showers can a 27kW tankless heater handle?
Usually two to three, depending on climate and shower flow rates.
Do electric tankless heaters need a panel upgrade?
Often, yes. High-capacity units require large breakers and heavy wiring.
Are electric tankless heaters efficient?
Yes. They are typically 98% to 99% efficient.
What size breaker do I need?
It depends on the model. Check the manufacturer’s requirements.
How long do tankless heaters last?
Many last 15 to 20 years with regular maintenance.
For more home gear, browse our best water heaters category.
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