How much does it cost to run a tankless water heater?
We've pre-filled a typical tankless water heater below. Set your electricity rate and adjust the hours to match how you use yours — the cost updates instantly.
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
Common questions
How much does it cost to run a tankless water heater per month?
At a typical 18,000W and about 0.5 hours a day, a tankless water heater costs roughly $46 a month at $0.17/kWh. Set your own rate and hours above for an exact figure.
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
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.
Compare other appliances
Put an honest running-cost calculator in front of ready-to-buy shoppers
WattDollar reaches people at the exact moment they're deciding whether an appliance, heat pump or EV charger is worth it — and what a more efficient model would save. A warm, high-intent audience, with no lead form in the way.
- Embeddable & co-brandable — drop the calculator on your product or store page so shoppers see real running costs without leaving.
- Warm, high-intent traffic — visitors arrive already shopping by appliance type; the results screen is the natural place to recommend an efficient model.
- Affiliate & revenue-share friendly — one editable config points the "shop a more efficient model" link at your store, Amazon tag or charger lineup.