An electric tankless water heater is a good fit only when the required flow and temperature rise match the home’s electrical capacity. Small point-of-use units can serve one hand basin or low-flow fixture. Whole-home models may demand several high-current circuits and an electrical-service upgrade, especially in cold climates.
That makes sizing more important than brand rankings. This guide explains the checks to complete before comparing models or paying for installation.
The short answer
| Situation | Likely fit | Main caution |
|---|---|---|
| One hand basin or remote sink | Small point-of-use heater | Confirm minimum flow and circuit requirements |
| One efficient shower | Possible with correct sizing | Winter inlet temperature determines capacity |
| Several simultaneous fixtures | Large whole-home unit or multiple units | Very high electrical demand |
| High household demand with limited panel capacity | Often a poor fit | Consider storage or heat-pump water heating |
How electric tankless heaters work
A flow sensor activates electric resistance elements when a tap opens. Water is heated as it passes through, so there is no tank storing hot water. The unit must provide all required heat during those few seconds of flow. If demand exceeds capacity, outlet temperature falls or the controller restricts flow.
Tankless does not mean limitless at any flow rate. It means the heater can continue operating while demand remains within its rated temperature-rise curve.
Size by flow and temperature rise
First estimate the fixtures that may run together. Add their measured or labelled flow rates. Then subtract the coldest expected inlet-water temperature from the desired outlet temperature.
Required heating power in kilowatts is approximately: litres per minute × temperature rise in °C × 0.07. For example, heating 6 L/min by 30°C requires about 12.6 kW before allowing for real-world limits. In US units, use the manufacturer’s gallons-per-minute chart for the required °F rise.
The U.S. Department of Energy’s technical guide to transitioning to a tankless water heater emphasises flow, temperature rise and electrical supply. Use the model’s certified performance data, not its maximum flow headline at an unspecified inlet temperature.
Check the electrical service before shopping
- Required voltage, phase and frequency must match the property.
- Large units may require multiple dedicated breakers and heavy conductors.
- The switchboard or service entrance must have enough spare capacity.
- Disconnects, grounding, residual-current protection and cable sizing must meet local rules.
- Installation usually requires both a licensed electrician and compliant plumbing work.
Do not buy a whole-home unit on the assumption that an existing storage-heater circuit will be sufficient. The instantaneous load can be much higher. Ask for an electrical load assessment and an installation quote before committing to a model.
Efficiency is not the same as low running cost
Electric resistance heating converts most incoming electricity into heat at the appliance, and tankless designs avoid tank standby losses. Running cost still depends on electricity price, hot-water use and the alternative system. A heat-pump water heater can move heat rather than create it through resistance and may use substantially less electricity over time. The U.S. Department of Energy’s guidance on energy-efficient residential water heaters highlights the efficiency advantage of heat-pump models where their installation requirements can be met.
Compare annual energy estimates, tariffs, installation costs and expected service life. A cheaper unit that triggers a switchboard upgrade may cost more overall than a well-sited storage or heat-pump system.
Features worth paying attention to
- Low activation flow: important for handwashing and low-flow fixtures.
- Modulating control: adjusts output as flow and inlet temperature change.
- Temperature limit: helps manage scald risk but does not replace code-required tempering.
- Service access: isolation valves and accessible strainers make maintenance easier.
- Scale instructions: hard water may require periodic flushing or pretreatment.
- Independent certification: check electrical, plumbing and efficiency marks required in your country.
- Warranty terms: confirm whether professional installation and maintenance records are conditions.
Common reasons for disappointing performance
Undersizing is the biggest problem. Other causes include a winter inlet temperature lower than the sales example, several fixtures running together, voltage drop, restricted strainers, scale on heating elements and flow below the activation threshold.
Long pipe runs still waste water while you wait for hot water to arrive. A point-of-use unit can solve that problem more directly than a central heater. If you are comparing fuels, our gas tankless water-heater guide covers different venting and combustion considerations.
Safety and maintenance
High-power water heaters combine water, electricity and scalding temperatures. Installation, circuit protection and commissioning should follow local law and the exact manufacturer’s instructions. Never energise a unit without the required water flow, and isolate power before cleaning a strainer or servicing plumbing.
Periodically inspect for leaks, clean inlet screens and follow the specified descaling procedure. When water supply is interrupted, follow the heater maker’s shutdown advice; the principles in our guide to whether you should turn off a water heater when the water is off help explain why dry operation must be avoided.
A better buying process
- Measure fixture flow and identify simultaneous demand.
- Find the coldest realistic inlet-water temperature.
- Calculate the required temperature rise and power.
- Have an electrician assess service and switchboard capacity.
- Compare installed cost with storage and heat-pump alternatives.
- Select a compliant unit with verified performance at your design conditions.
- Plan isolation valves, maintenance access and safe temperature control.
The best electric tankless water heater is therefore not a universal model. It is the smallest compliant unit that can meet the property’s real peak demand without overloading the electrical system.
High-productivity water heaters are the best ones to have as they cause less air pollution, both indoors and outdoors. When you use a water heater system, you consume less energy and less water that means you will save the cost of huge electricity, gas and water bills. If you experience a spike in your gas or water bill after having a water system in your house, call a professional technician who has experience providing outstanding residential and commercial water heater services.