Best Electric Tankless Water Heaters

Best Electric Tankless Water Heaters

The best electric tankless water heater is the one whose kilowatt rating matches your peak flow rate, incoming water temperature, available electrical service, installation space, and acceptable operating cost—not simply the model with the highest advertised gallons per minute.

Quick recommendations by household situation

These are starting points, not guarantees. A heater rated for 18 kW may be adequate in a warm region but undersized for the same shower in a northern winter. Before choosing a model, calculate the required temperature rise and confirm the electrical installation with a qualified electrician.

Rheem 18kW 240V Tankless Electric Water Heater, Gray

Included for 'Best Electric Tankless Water Heaters' because the listing specifies Gray, details this guide uses to compare options.

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Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray

Included for 'Best Electric Tankless Water Heaters' because the listing specifies Gray, details this guide uses to compare options.

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18kW Tankless Water Heater Electric, NOVIEEX 240V 3s Instant Hot Water Heater, Flow Rate Max 4.9 GPM, Supply for 2-3 Points, Indoor Installation, Ideal for Apartment, Garage, Salon, Bathroom, Kitchen

Included for 'Best Electric Tankless Water Heaters' because the listing specifies NOVIEEX 240V 3s Instant Hot Water Heater and Flow Rate Max 4.9 GPM, details this guide uses to compare options.

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How to size an electric tankless water heater

Electric tankless units heat water only while it flows. Their output is limited by the relationship between flow, temperature rise, and electrical power:

Required kW ≈ flow rate in GPM × temperature rise in °F × 0.1465

Temperature rise is your desired outlet temperature minus the incoming water temperature. For example, suppose a shower uses 2.0 GPM, the desired water temperature is 105°F, and winter groundwater is 45°F. The required rise is 60°F:

2.0 × 60 × 0.1465 = 17.6 kW

An 18 kW heater is technically close to the requirement, but it leaves very little margin for colder water, a higher-flow showerhead, or another fixture. A 24 kW unit is the more practical choice if the electrical service can support it.

Repeat the calculation for every fixture likely to operate at the same time. A typical shower uses 1.5–2.5 GPM, a bathroom sink often uses 0.5–1.0 GPM, and a kitchen faucet may use 1.0–2.2 GPM. A bathtub filler can exceed 4 GPM, although tub filling usually tolerates a lower temperature.

Worked simultaneous-use example

Two 2.0-GPM showers running together equal 4.0 GPM. At a 55°F temperature rise:

4.0 × 55 × 0.1465 = 32.2 kW

That demand is beyond many residential electrical services. A 36 kW heater may cover it in favorable conditions, but only if the home has sufficient service capacity and the fixtures do not exceed the assumed flow. Otherwise, use lower-flow showerheads, stagger usage, install two smaller units in suitable zones, or retain a storage water heater for peak demand.

Electrical service is the main compatibility check

High-output electric tankless heaters draw substantial current. At 240 volts, current is approximately:

Amps = watts ÷ volts

These figures describe heater current, not the breaker size or conductor size. Actual circuit requirements depend on the model, local electrical code, continuous-load rules, wiring distance, ambient conditions, and the rest of the home’s loads. Do not connect a 24 kW or 36 kW heater to an existing water-heater circuit without a professional assessment.

A 100 A electrical service can be a serious limitation even if the main breaker appears large enough. The electrician must account for the range, electric dryer, heat pump, air conditioner, electric vehicle charger, well pump, and other simultaneous loads. A service upgrade can eliminate the supposed savings of a tankless installation.

Best models by specification class

Best compact choice for modest demand: EcoSmart ECO 11 or Rheem RTEX-13

These lower-output units suit a single point of use, a small apartment, or a warm-climate home with one bathroom. They are not sensible replacements for a whole-house heater in a cold climate. At 2.0 GPM and a 60°F rise, the calculation requires about 17.6 kW, so an 11–13 kW unit would need either lower flow or warmer incoming water.

Best middle ground: EcoSmart ECO 18, Rheem RTEX-18, or Stiebel Eltron Tempra 20 Plus

The 18–20 kW class is a reasonable whole-house starting point for one bathroom, particularly where groundwater is moderately warm. It can usually support one normal shower while handling a low-flow sink, but simultaneous high-flow fixtures may exceed its capacity. Stiebel Eltron’s electronically controlled Tempra Plus models are designed to modulate output as demand changes, while the Rheem and EcoSmart lines offer widely available residential sizing options.

Best for larger single-family demand: Rheem RTEX-24, EcoSmart ECO 27, or Tempra 24/29 Plus

The 24–29 kW class provides more useful winter performance than an 18 kW unit, but it also creates a much more demanding electrical installation. Choose this range when the calculated peak demand is around 20–25 kW and the service calculation supports it. A 27 kW heater should not be selected merely because it has a higher maximum GPM number; its real output still falls as the temperature rise increases.

Maximum residential output: EcoSmart ECO 36, Rheem RTEX-36, or Tempra 36 Plus

These units are aimed at large homes with substantial electrical capacity. They may support multiple fixtures in a warm climate, but a 36 kW heater can draw roughly 150 amps at 240 V. In many homes, the better design is a smaller heater serving the main bathroom, a second unit for another zone, or a hybrid system that combines storage capacity with lower instantaneous electrical demand.

Operating cost: efficient does not always mean cheaper

Electric tankless heaters are nearly 100% efficient at the point of use, because they do not maintain a hot storage tank. They can reduce standby losses, especially in homes with low or irregular hot-water use. However, electricity may cost more per unit of delivered heat than natural gas, and a tankless heater does not reduce the energy required to heat the same gallons of water to the same temperature.

Use this rough energy estimate:

kWh per day ≈ gallons of hot water × temperature rise × 0.00244

If a household uses 60 gallons with a 60°F temperature rise, the energy is about 8.8 kWh per day. At an electricity rate of around $0 per kWh, that is approximately $1 per day, or about $640 per year before fixed charges. Actual usage, incoming temperature, shower duration, and utility rates will change the result.

Compare that annual cost with the existing system before buying. Installation can include heavy-gauge wiring, multiple breakers, a larger panel, plumbing changes, isolation valves, and possible service work. A unit costing roughly $400–$1,000 can become a several-thousand-dollar project when the electrical infrastructure is inadequate.

Installation details that affect performance

• Water flow: The heater needs enough flow to activate, but excessive flow reduces outlet temperature. Clean inlet filters and avoid assuming a high advertised GPM applies at your winter temperature rise.
• Pipe sizing: Use appropriate water-supply pipe and keep the hot-water run short where practical. Long waits at distant fixtures are a plumbing-layout issue, not a heater-capacity issue.
• Electrical conductors: Follow the exact manual and local code. High-current units commonly require multiple dedicated circuits.
• Service access: Leave the manufacturer’s required clearance so filters, heating elements, sensors, and terminals can be inspected.
• Water quality: Hard water can cause scale on heating elements and sensors. Follow the manufacturer’s flushing schedule; annual descaling is a common starting point in hard-water regions.
• Temperature control: Set the heater near the temperature you actually need instead of mixing large amounts of cold water. This can improve usable capacity and reduce scalding risk.

Decision guide before you buy

Bottom line

For many homes, an 18–24 kW electric tankless water heater is the practical range, with models such as the EcoSmart ECO 18, Rheem RTEX-18, Rheem RTEX-24, and Stiebel Eltron Tempra Plus series offering suitable specification classes. Smaller 11–13 kW units make sense for limited demand, while 27–36 kW models require careful service planning. Calculate peak flow and winter temperature rise first, then verify amperage, wiring, panel capacity, water quality, and total installed cost. That process will identify the best electric tankless water heater for your home more reliably than a headline GPM figure.