How much does it cost to run a tankless water heater?
About $47.76 a month on the typical usage below, at the U.S. average $0.1816/kWh. Set your own rate and usage and the figure updates instantly.
Electric tankless units pull a huge 12,000–27,000W but only while hot water is actually flowing.
Running a tankless water heater at 18,000W running 2% of 24 h costs about
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An electric tankless water heater is the opposite of a space heater in one important way: the wattage looks terrifying, but the runtime is tiny. To heat water instantly, without a tank, it has to slam 18,000 watts or more into the incoming water the instant you turn on the tap — that's more power than a whole central AC system. But it only pulls that power while water is actually flowing, typically adding up to well under an hour a day even for a busy household.
That's why a tool that draws as much as a dozen space heaters combined can still land a reasonable line on your bill: multiply 18,000 watts by 0.5 hours instead of 8, and the math changes completely. The real cost driver isn't the unit's peak wattage — it's shower length and how many fixtures run hot water at once, since running two showers and a dishwasher simultaneously can force the heater toward the top of its 12,000–27,000W range to keep up with demand.
What drives the cost of running a tankless water heater
- Total hot-water runtime per day, not the heater's peak wattage — this unit only draws its rated power while water is actively flowing through it
- How many fixtures demand hot water at once — simultaneous use (shower + dishwasher, two bathrooms at once) pushes the unit up toward the 27,000W high end of its range instead of the ~18,000W typical draw
- The temperature rise needed — colder incoming groundwater (winter, northern climates) makes the unit work harder and pull more watts to hit the same output temperature
How to cut it
- Space out overlapping hot-water draws (shower + dishwasher + washing machine) so the unit doesn't ramp to its highest wattage tier to keep up with simultaneous demand
- Lower the target output temperature a few degrees on the unit's thermostat — less temperature rise per gallon means less wattage needed to hit it
- Install low-flow showerheads and faucet aerators; tankless units work on a flow-rate trigger, so less water moving means less time (and often less power) spent heating
- Fix drips and leaks on hot-water lines — with electric tankless, every drop that triggers the heating element costs money even if no one's using it
Data basis
- Electricity price
- U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.B, June 2026 year to date, released 2026-08-26. Retrieved 2026-09-11. Average residential revenue per kWh. A benchmark average, not a utility tariff — it excludes the fixed charges, tiers, time-of-use windows, taxes and fuel adjustments on a real bill.
- Appliance energy
- A representative running power for a tankless water heater, compiled from published manufacturer specifications and public appliance-energy guidance, and presented as a typical figure with the usual range shown above. It is not a measurement of your unit, and we do not claim a single primary source for it — the nameplate on your appliance, or an hour with a plug-in energy meter, beats any typical figure we can publish.
- Usage model
- Cycling load — 18,000W running 2% of 24 h
- International prices
- rounded typical household tariffs for each market, in that market's own currency unless the row says US$ equivalent. These are planning benchmarks with no single primary source; enter the per-kWh price from your own bill for an accurate figure.
- Last reviewed
- 20 August 2026. Next scheduled review 20 February 2027.
- Corrections
- Think a figure is wrong? Tell us at corrections@watt-dollar.com and see our corrections policy.
Common questions
How much does it cost to run a tankless water heater per month?
On the assumption shown above — 18,000W running 2% of 24 h — a tankless water heater costs about $47.76 a month, or $573 a year, at $0.1816/kWh. That price is the U.S. average; set your own rate and usage above and the figure updates.
How is that worked out?
Running power x hours connected x the share of that time the motor actually runs. At 18,000W running 2% of 24 h that is 8.64 kWh a day, which at $0.1816/kWh costs $1.57 a day.
How can I cut the cost of running a tankless water heater?
Space out overlapping hot-water draws (shower + dishwasher + washing machine) so the unit doesn't ramp to its highest wattage tier to keep up with simultaneous demand As a worked example: getting the run share down from 2% to 1.6% takes it from $573 to $458 a year, about $115 less.
Why does a tankless water heater use so much more power than a tank heater if it's supposed to be more efficient?
Because of how the two are used, not because tankless pulls less power — it actually pulls far more. A gas or electric storage tank heater runs for long stretches to keep 40-50 gallons hot around the clock, even when no one's home. A tankless unit only fires when a tap is open, often less than 30-60 minutes a day combined, so the huge wattage rating is misleading if you're comparing it to a tank heater's lower but much longer runtime.
Does it cost more per month for a family of four vs. a couple, or does usage stay about the same?
Two people showering back-to-back barely changes the runtime total, since draws are sequential and each shower still only pulls power while water flows. Two people showering at the same time is different: the unit has to raise output to cover both simultaneously, which is what pushes it from its typical ~18,000W draw toward the 27,000W top of its range — and in some cases can trigger a secondary heating element or reduce flow/temperature if the unit is undersized.