A self-modulating tankless water heater adjusts its heating output to the water flow and temperature, rather than heating a full storage tank. The five electric models below cover different needs: the Rheem 18kW is best for buyers seeking a higher-capacity unit, the ThermoMate 27kW is designed for multiple fixtures, and the smaller ThermoMate and mini models are better suited to point-of-use installations. Before choosing, compare flow needs, incoming water temperature, and electrical service requirements.
| Product | Best suited for | Listed flow or use |
|---|---|---|
| Rheem 18kW | Higher-capacity electric use | Up to 4.4 GPM |
| ThermoMate 4.5kW | One sink | 0.26–1.0 GPM |
| ThermoMate 8kW | Point-of-use applications | Up to 1.56 GPM at 35°F rise |
| Mini 3kW 120V | Low-flow, single-point use | One point of use |
| ThermoMate 27kW | Several fixtures, subject to conditions | Up to 6.6 GPM |
Rheem 18kW Tankless Electric Water Heater
Best for: Buyers looking for a higher-capacity electric unit with adjustable output and a digital temperature control. The listed maximum flow is up to 4.4 gallons per minute, though actual delivered flow depends on the temperature rise required and household water conditions.
The Rheem has an external digital thermostatic control with an LED display and listed temperature accuracy of plus or minus one degree. Its self-modulating technology adjusts power to meet hot-water demand. The product information also specifies durable copper immersion heating elements and field-serviceable components.
Why it was selected: Its listed flow capacity is higher than the point-of-use models in this guide, and its digital control and self-modulation are relevant to shoppers seeking a whole-home-style electric option.
Limitation: The stated maximum flow should not be treated as a guaranteed flow at every temperature rise. Check the unit’s electrical requirements and have the installation assessed before purchase.
ThermoMate 4.5kW Under-Sink Heater
Best for: A sink or other point-of-use location where a compact 120V heater can serve a modest hot-water demand. The listed flow range is 0.26 to 1.0 GPM, and the temperature range is 85°F to 131°F.
This ThermoMate model uses self-modulating power to regulate energy use according to demand. Its listing says it provides hot water on demand and can remain on standby. The cast aluminum heating element is separated from the water line, and the water tube is made of SUS 304 stainless steel.
Why it was selected: Its under-sink format and stated low flow range make it a more targeted option than a high-capacity heater for a single faucet. It is also a clear choice for shoppers who want a 120V point-of-use model.
Limitation: Installation requires hardwiring, a 40A double-pole circuit breaker, at least 8 AWG wire, and 1/2-inch NPT water connections, according to the listing. Confirm that the electrical circuit and plumbing match before buying.
ThermoMate 8kW Point-of-Use Heater
Best for: Point-of-use applications that need more capacity than a small under-sink unit, provided the electrical supply and expected flow are appropriate. The product lists up to 1.56 GPM at a 35°F temperature rise and a maximum temperature of 131°F.
The 8kW heater uses self-modulating technology that the listing says monitors water temperature and flow to adjust energy use. It has a compact, wall-mounted design, side water connections, and a cast aluminum heating chamber. The listing describes the heating element as separated from the water lines and notes a 24-hour standby feature.
Why it was selected: It sits between the lower-output sink models and the larger 18kW and 27kW units. That makes it a comparison option for buyers who want a point-of-use heater with a stated flow figure and temperature-rise condition.
Limitation: The flow figure is specifically given at a 35°F rise; a larger temperature rise can reduce available flow. The listing requires a double-pole 40A breaker, so check the installation requirements with a qualified electrician.
Mini 3kW 120V Plug-In Heater
Best for: Low-flow use at one point of use where a 120V plug-in model is preferred. The product includes an LED digital display and a control knob for power and temperature adjustment, with a listed adjustment range of 30°C to 55°C.
The listing describes protection features that include overheat, leakage, dry-heating, and over-pressure protection, as well as IPX4 waterproofing. It also lists automatic fault diagnosis and temperature memory. These stated features make it a practical model to compare when the priority is a small, single-point heater with straightforward controls.
Why it was selected: It offers a different installation and capacity profile from the hardwired 240V options in this list. The product information directly identifies it as a low-flow unit, helping buyers rule it in or out based on intended use.
Limitation: The listing says it is not suitable for showers or several taps running at once. Output temperature is affected by heater wattage, inlet flow, and incoming water temperature, so it should not be selected for high-demand applications.
ThermoMate 27kW Electric Tankless Heater
Best for: Households considering a higher-capacity electric heater for several fixtures. The listing gives a maximum flow of up to 6.6 GPM and says it may serve three to four fixtures, depending on incoming water temperature. It gives examples such as two showers and a bathroom sink.
The 240V unit uses a cast aluminum heating block with iron-chromium-aluminum alloy heating elements. The product information describes a water-and-electricity separation structure intended to help prevent scale buildup and corrosion. Its stated standard water connections are 3/4-inch NPT.
Why it was selected: Its listed maximum flow is the highest among these five products, so it is the option to compare first when several fixtures may need hot water at once. Actual performance depends on the required temperature rise and simultaneous demand.
Limitation: The listing calls for at least 150 amps of total main-panel capacity, three separate 40A double-pole breakers, and three sets of 8 AWG copper wire. Buyers should verify panel capacity and installation feasibility before choosing it.
Buying Guide: How to Choose a Self-Modulating Tankless Heater
Self-modulation describes how a heater changes its power output as water flow and temperature conditions change. It does not mean that every model can supply every fixture or maintain the same flow in all climates. Match the heater to the number of fixtures used at once, the required hot-water temperature, and the temperature of incoming water.
What does self-modulating mean?
A self-modulating electric heater adjusts its heating input in response to demand, rather than operating as if maximum output were needed at all times. The products in this guide describe their technology as regulating power according to water flow, temperature, or hot-water demand. Performance still depends on the heater’s capacity and the water temperature rise required.
Compare the model’s stated flow at a stated temperature rise where available. For example, the ThermoMate 8kW listing gives 1.56 GPM at a 35°F rise. That figure is more useful for comparison than a flow number without a temperature condition, but it is not a promise of identical output in every installation.
How much flow do you need?
Estimate which fixtures may run at the same time, then use their combined hot-water demand as a starting point. A heater intended for one sink is not a substitute for a model sized for multiple showers. The 3kW mini heater is explicitly described as unsuitable for showers or multiple taps at once, while the 27kW model is presented for several fixtures subject to incoming water temperature.
Maximum flow claims are not interchangeable. A flow rating may be tied to a particular temperature rise, and a colder inlet temperature requires more heating to reach the same outlet temperature. Ask an installer to help estimate the required flow and rise for the home’s location and intended use.
Should you choose a point-of-use or larger unit?
A point-of-use heater serves a specific sink or nearby fixture. It can be useful when the demand is limited to that location, and its smaller listed capacity may fit a dedicated application. The ThermoMate 4.5kW and mini 3kW models are examples of this category; the 8kW option offers a higher stated point-of-use flow.
A larger heater may suit households seeking hot water at multiple fixtures, but the electrical service requirements can be substantial. The 18kW Rheem and 27kW ThermoMate have higher listed flow capacities than the small point-of-use products. Verify the manufacturer’s installation requirements and the home’s available electrical capacity before purchase.
What electrical requirements should you check?
Do not assume that a 120V or 240V label alone tells you whether a unit can be installed on an existing circuit. Check the specified voltage, breaker size, number of circuits, wire gauge, and whether the unit is plug-in or hardwired. For example, the ThermoMate 4.5kW listing specifies hardwiring and a 40A double-pole breaker; the 27kW listing specifies three breaker circuits and a minimum panel capacity.
Have an electrician assess the panel and wiring when the manufacturer’s requirements are unclear or exceed the capacity of existing circuits. Electrical installation needs can determine whether a particular model is practical, even when its flow rating appears suitable.
Which controls and construction details matter?
Look for controls that suit the way the heater will be used. The Rheem has an external digital thermostat and LED display. The mini 3kW product uses a knob and LED display. Compare the listed temperature range and control method rather than assuming all units offer the same adjustment options.
Also review the listed heating materials and water connections. The products here describe different designs, including copper immersion elements, cast aluminum heating chambers, stainless steel water tubing, and 1/2-inch or 3/4-inch NPT connections. Confirm plumbing connection size and installation layout before ordering.
How should efficiency claims be interpreted?
Self-modulating operation is intended to match heating output to demand, but energy use depends on the household’s hot-water habits, incoming water temperature, set temperature, and system installation. One product listing states an efficiency figure, and another makes a cost-saving claim; these are manufacturer-provided claims, not guarantees of a particular household result.
Compare products using the same criteria: suitable flow, temperature rise, electrical requirements, and intended fixtures. A smaller unit may be a better fit for one sink, while a higher-capacity model may be necessary for concurrent household use. Choosing based only on an efficiency percentage or maximum flow can lead to a poor match.
Frequently Asked Questions
Is a self-modulating tankless water heater electric or gas?
Self-modulation is a control approach, not a fuel type. All five products in this guide are electric models. If you are shopping for a gas heater, check its fuel type and installation requirements separately.
Can a point-of-use tankless heater run a shower?
Not necessarily. The mini 3kW model is explicitly described as unsuitable for showers, and the 4.5kW and 8kW products are presented as point-of-use options. Check the model’s flow at the required temperature rise before considering it for a shower.
Does a higher maximum GPM guarantee more hot water?
No. Maximum flow is affected by the temperature rise needed, incoming water temperature, and simultaneous demand. Compare flow figures with their stated temperature conditions and confirm that the unit can support the fixtures you plan to use together.
Can I install a 27kW electric tankless heater on any home panel?
No. The listed 27kW model requires at least 150 amps of total main-panel capacity, three 40A double-pole breakers, and three sets of 8 AWG copper wire. An electrician should verify that the home can meet those requirements.
What does a temperature-rise figure mean?
Temperature rise is the difference between the incoming water temperature and the heater’s outlet temperature. A greater rise requires more heating, which can reduce the flow a unit can deliver. Consider local incoming-water conditions when sizing a heater.
Is a tankless heater always more efficient than a tank heater?
Not automatically in every household or installation. Self-modulating heaters adjust output to demand, but actual energy use depends on usage patterns, inlet temperature, set temperature, and equipment sizing. Treat product efficiency and savings figures as listing claims rather than guaranteed results.