How much electricity does a tankless electric water heater use?

Water heaters use lots of electricity. While a tankless system is far more efficient than any traditional tank boiler, the US Department of Energy estimates that modern heaters still use almost 17% of a home’s total energy consumption.

That makes tankless water heaters the most energy-hungry home appliance by some margin. So it’s important to know how much power a tankless electric water heater uses, and whether you should think about a natural gas tankless water heater or propane tankless water heater instead.

Here’s an explainer on electricity usage and tankless water heaters.

How much electricity does a tankless water heater use?

Let’s start with some ballpark figures. A family of four in a mid-size home should need an electric tankless water heater with a minimum heating capacity of 18 kW.

Tankless water heaters only need to be active for a short amount of time to produce hot water. They also rarely run at full capacity. So while this heater might draw 18 kW for the first 30 seconds of usage, it will require less energy as it operates.

A family of four tends to use somewhere around 50 gallons of hot water per day. While traditional tank heaters may need to run for several hours to meet this demand, tankless heaters are far more efficient. As a result, this family is likely to only need a single kilowatt-hour of electricity for their heater per day.

For the sake of simplicity, let’s say that the heater runs at half capacity on average. If the average cost of electricity across the US is roughly 15 cents per kWh, then:

  • They can expect to spend roughly 9 kWh or $1.35 of electricity per day.
  • On a monthly basis, that’s 270 kWh, or $40 dollars.
  • Per year, that’s 3285 kWh, or $493 dollars.

Of course, exact electricity usage depends upon how many hot water outlets are in your home, your local climate, as well as your hot water consumption habits. The price of electricity also varies by several cents per kWh across states:

If you live in an apartment or with only one other person, expect to use considerably less electricity than the estimate above.

If you live in a state with a mild climate and/or lower than average electricity costs, expect to use moderately less electricity than the estimate above.

If you live in a state with a cold climate and/or higher than average electricity costs, expect to use slightly more electricity than the estimate above.

One thing that makes calculations easier is that most electric heaters are highly efficient, deploying >98% of energy towards heating water. So there’s no need to factor in wasted energy.

How many kWh does a tankless water heater use per month?

Let’s dive a little deeper into those kilowatt-hours. How many kWh of electricity do tankless water heaters need to handle showers, baths, and other regular household tasks?

If we stick with our 18 kW heater and guesstimate required water temperatures for common household tasks, then we can say:

Activity / duration Water temperature Heater uses
Non-power shower (10 mins) Hot (90% capacity) 2.7 kWh
Hand-washing dishes (5 mins) Medium (75% capacity) 1.2 kWh
Bathing the dog (10 mins) Lukewarm (50% capacity) 1.5 kWh

Factoring in flow rates and outside temperature

Remember, if you want to run multiple showers and other hot water appliances at the same time without restricting water flow, you’ll need a heater with a sufficient capacity.

An 18 kW heater and a 36 kW heater can both produce water temperatures up to 140 degrees, which is the maximum recommended set temperature. However, the 36 kW heater will be able to produce roughly double the amount of water at any single time.

The temperature of incoming water also has a large effect on how much power a tankless heater needs to deploy. If there’s a large difference between the temperature set on the control panel and the water entering the unit, then a heater will need to operate at full heating capacity for longer.

In very cold climates, it’s advisable to purchase a heater with a higher kW capacity than instruction manuals recommend for your needs.

Why do electric tankless heaters need such high kW?

It can seem counterintuitive that supposedly efficient products come with such high kW requirements. But electric tankless heaters achieve their efficiency by deploying a large amount of energy to water in a very short space of time.

For example, large electric water heaters can use up to 36kW when heating water. Compared to the average dishwasher, which consumes around 1.8kW, the difference is stark. But remember that tankless heaters only need to deploy power for a matter of seconds (usually under 10), while most hot water appliances in the home, including old-style tank heaters, run for hours at a time.

So while they might spend lots of electricity during a short period after you turn on a shower or faucet, tankless heaters do their job quickly and can then sit in a rest mode for the remainder of the day. This leads to around 40% reductions in overall energy costs compared to tank heaters.

Key points

  • Most American families use around 50 gallons of hot water per day. To meet this demand, an electric tankless water heater will use 9–12 kWh of electricity.
  • An electric tankless water heater needs about 3 kWh of electricity to heat a 10-minute shower.
  • Electric tankless water heaters draw large amounts of electricity (up to 36 kW) over short periods of time. This boosts efficiency but requires robust wiring to avoid power surges.

How do electric tankless heaters compare to gas tankless heaters?

Of course, electric tankless heaters aren’t the only game in town. Gas and propane tankless water heaters provide many of the same benefits in a different fuel format.

How much money can I save by using gas vs electricity?

This is a difficult question to answer because it depends upon specific features of your home and location.

Both gas and electric tankless heaters need large power supplies that often mean making adaptions to your wiring or piping. Most natural gas heaters need 3/4 inch gas pipes, while propane heaters require large tanks installed outside, and electric heaters need multiple large circuit breakers.

Upgrading gas or electricity hardware can be expensive, so if your home is already set up for any of these applications, it’s not cost-efficient to switch to another fuel.

The same generally goes for switching fuel in general. Changing from a traditional gas heater to an electric tankless or vice versa will cause extra expenses that can take many years to offset. 

More practically, you can make immediate savings on an electric heater over gas thanks to a significant difference in base prices. On average, an electric unit can be several hundred dollars cheaper than an equivalent gas heater.

Most parts of the country enjoy cheaper gas rates than electricity, so when it comes to month-to-month costs, gas heaters tend to win out. However, gas heaters also require more frequent servicing and can encounter more complex issues and repairs.

Add all these factors to the fact that fuel prices continue to become more volatile, and it becomes hard to make general statements about the cost-efficiency of one heater type over the other.

Many experts claim that the costs of each fuel type tend to equal out over the lifespan of a heater, which the department of energy puts at around two decades. If you’re installing a heater in a new home, we recommend a tankless electric unit.

How much electricity do gas tankless heaters use?

Both natural gas and propane heaters still need to be connected to a 120V outlet. This is because they contain flow sensors and microprocessors, and may also have electronic ignitions.

However, the amount of electricity used by most gas heaters is very small, comparable to any other standard electronic appliance such as a TV or lamp. If a gas heater is fitted with extra features like anti-freeze protection, then it will draw substantially more.

Check out our longer comparison article for a wider look at the similarities and differences between gas and electric tankless water heaters—especially if you’re trying to decide between the two.