Electric Tankless Water Heaters: Do They Work?
Electric tankless water heaters work well in the right house: a small home in a warm climate, a single bathroom, or one sink far from the main heater. In a cold-water region, or a house with an older panel and two showers running at once, a whole-house electric unit often can't keep up, and the wiring it needs can cost more than the heater.
The deciding numbers are your incoming water temperature, how many gallons per minute you run at the same time, and how much amperage your electrical panel has left. Get those three right and an electric on-demand heater does its job. Guess at them and you'll have lukewarm showers every January.
How an electric instant water heater works
There's no storage tank. When you open a hot tap, a flow sensor sees water moving and switches on heating elements inside a small chamber. The water heats as it passes through, and the elements shut off when the tap closes. That's why people call them instantaneous, on-demand or instant water heaters. They are all the same thing.
The physics is simple and unforgiving. Eemax, a Rheem brand, gives the formula as temperature rise = kW × (6.83 ÷ GPM). More kilowatts means more heat. More flow means each gallon gets less of it. So an electric tankless unit trades flow for temperature: slow the flow and the water gets hotter, open another shower and every fixture cools down.
The Department of Energy says to assume incoming water at 50°F unless you know otherwise, and that most uses call for 120°F. That's a 70°F rise. In the Deep South the ground water is warmer and the job gets much easier. In Minnesota in February it gets much harder.
What the big models really deliver
Here are Stiebel Eltron's own ratings for its Tempra whole-house line at 240 volts, at a 3.0 gpm flow, which is roughly one full shower plus a sink:
| Model | Power | Breakers needed | Temperature rise at three gpm |
|---|---|---|---|
| Tempra 12 | 12 kW | One 50 A double-pole | 27°F |
| Tempra 15 | 14.4 kW | Two 30 A double-pole | 33°F |
| Tempra 20 | 19.2 kW | Two 40 A double-pole | 44°F |
| Tempra 24 | 24 kW | Two 50 A double-pole | 54°F |
| Tempra 29 | 28.8 kW | Three 40 A double-pole | 66°F |
| Tempra 36 | 36 kW | Three 50 A double-pole | 82°F |
Read that last row carefully. The biggest unit in the line gives an 82°F rise at 3.0 gpm, but only a 55°F rise at 4.5 gpm. With 50°F water coming in, that's a warm shower, not a hot one, once a second shower opens. And it takes 150 amps of breakers to get there.
Voltage matters too. On 208 volts the Tempra 36 drops from 36 kW to 27 kW. Stiebel's sizing guide says to go up one model size for a 208-volt install. Product listings use different voltage labels for the same class of heater. What matters is the voltage your home actually measures.
Electric tankless vs the other options
| Option | What it handles well | Watch out for |
|---|---|---|
| Point-of-use electric tankless | One sink or a remote bathroom | Small units, small flow; Stiebel's smallest whole-house model alone needs a 50 A breaker |
| Whole-house electric tankless | One or two fixtures at a time in mild climates | Can need three 50 A breakers and a panel upgrade |
| Gas tankless | A 70°F rise at 5 gpm is typical for gas units | Needs gas line sizing and venting |
| Electric tank | Several fixtures at once, simple wiring | Standby heat loss; tanks last 8 to 12 years |
Our view: if you have gas at the house, a gas tankless or a good tank heater will almost always serve a whole family better than whole-house electric tankless. Electric tankless earns its place in three spots. One is under a sink at the far end of a long pipe run. Another is an addition or garage apartment with its own small load. The third is a warm-climate home with plenty of spare panel capacity and no gas.
If you're weighing a heat pump water heater instead, it uses electricity far more efficiently than resistance heat but needs a tank and room around it. See heat pump water heaters. For a straight fuel comparison, read gas vs electric water heaters.
Where electric tankless makes sense
Point of use. A small unit under the kitchen or bathroom sink cuts the wait for hot water and doesn't care what the main heater is doing. Our point-of-use water heater guide covers these, including the small models sold for a single sink. On Stiebel's chart, even the smallest whole-house model, the 12 kW Tempra 12, needs a 50-amp double-pole breaker.
Warm climates, small households. If your ground water stays warm most of the year and you rarely run two showers at once, a large electric unit can handle the house.
Homes without gas. If the alternative is a standard electric tank, tankless saves space. Whether it saves money depends on how much hot water you use. The DOE estimates demand heaters are 24% to 34% more efficient for homes using 41 gallons or less of hot water a day, and 8% to 14% more efficient for homes using around 86 gallons.
Where it doesn't fit: cold-water regions with a family that showers back to back, homes on smaller or full panels, and anyone expecting a high-flow whole-house electric heater to behave like a gas unit. The DOE notes that gas tankless models generally put out more flow than electric ones.
Outdoor, portable and solar setups
Exterior electric tankless installs exist, but freeze protection is the problem. An outdoor unit has to stay powered to protect itself, and a winter outage can crack the heat exchanger. Check the manual for the model's outdoor rating before you plan one.
Portable electric tankless heaters are mostly for RVs, cabins and job sites, and they share the same limit as every electric unit: flow and kilowatts. A solar setup usually pairs solar collectors with a storage tank, and an electric tankless unit can serve as the booster after it. That's a system design job, not a box you buy off the shelf.
Install notes before you buy
- Get a load calculation from a licensed electrician first. If the panel can't carry it, HomeGuide puts panel replacement at $850 to $1,700.
- Budget for the circuits. HomeGuide lists a new dedicated circuit at $250 to $900, and whole-house units need two or three of them.
- Stiebel requires copper conductors rated 75°C or higher. Aluminum feeders from an old tank install don't qualify.
- HomeAdvisor puts typical electric tankless installation at $450 to $1,500. Wiring and panel work come on top of that.
- Check rebates with your electric utility and your state energy office before you buy. Programs change often, and most require a listed model and a licensed installer.
In hard-water areas, scale builds up on the elements and in the heat chamber. Plan on periodic flushing; our tankless flush guide shows how.
FAQ
Are tankless water heaters gas or electric?
Both exist. Gas units (natural gas or propane) put out more hot water per minute, and the DOE notes gas models generally have higher flow rates than electric ones. Electric units are simpler to vent, since they don't need a flue, but they draw heavy current.
Are electric tankless water heaters any good?
They're good at small jobs and at whole-house duty in warm climates with modest demand. In cold regions, even the largest Stiebel unit gives only a 55°F rise at 4.5 gpm, which won't make two showers hot at once. Size them honestly and they work; oversell them and they disappoint.
Do gas tankless water heaters need electricity?
Yes. Gas tankless units use household power for the controls, fan and electronic ignition. In a power outage, a gas tankless heater stops making hot water unless it's on a backup power source.
What is the downside of an electric tankless water heater?
The electrical demand. A whole-house model can need two or three large breakers, which may force a panel upgrade. Flow drops in winter as incoming water gets colder, and hard water scales up the elements.
Is an instantaneous water heater the same as a tankless water heater?
Yes. Instantaneous, on-demand, instant and tankless all describe a heater that warms water as it flows, with no storage tank.
Are electric instant hot water heaters worth it?
For one remote sink or a small household in a warm area, often yes. For a family of four in a cold climate with no spare panel capacity, usually no: a heat pump or standard tank costs less to install and won't run out of steam mid-shower. Compare installed costs on our tankless water heater cost page before you decide.
Sources
- Stiebel EltronTempra Trend and Tempra Plus Submittal Sheet (opens in a new tab)
- Eemax (Rheem)Eemax Sizing Guide (opens in a new tab)
- U.S. Department of Energy, Energy SaverSizing a New Water Heater (opens in a new tab)
- Stiebel EltronTempra Trend and Tempra Plus Sizing Guide (opens in a new tab)
- HomeGuideWater Heater Installation and Replacement Cost (opens in a new tab)
- U.S. Department of Energy, Energy SaverTankless or Demand-Type Water Heaters (opens in a new tab)
- HomeAdvisorHow Much Does a Tankless Water Heater Cost? (opens in a new tab)