Does a Tankless Water Heater Need Electricity: Gas and Electric Models

A tankless water heater has no tank to keep warm, so people assume it needs nothing but gas. It needs both.

Short answer: yes, every tankless water heater needs electricity. A gas tankless unit uses a 120 V supply for its control board, electronic ignition, combustion fan, flow sensor and freeze-protection heaters. The gas valve won't open without it, so a power outage means cold water immediately. An electric tankless unit is a much bigger electrical load: whole-house models draw roughly 18 to 36 kW at 240 V across several dedicated circuits, which is a service-capacity question before it is a hot-water question.

Choosing between tankless and a tank is covered in tankless vs tank water heater. Gas tank heaters, some of which don't need power, are covered in does a gas water heater need electricity. This page is only about tankless.

What a gas tankless water heater uses electricity for

Every item on that list runs on the 120 V supply. There is no pilot flame and no thermocouple to keep a valve open the way a standing-pilot tank does. Non-condensing and condensing units both work this way; the condensing models add a second heat exchanger and a condensate drain but the electrical dependence is the same.

How it connects

Most residential gas tankless units are supplied at 120 V, 60 Hz. Some ship with a power cord for a nearby receptacle; others are hardwired to a junction box. The installation manual says which, and NEC 110.3(B) makes the manual binding. Where the heater is hardwired, it should be on a circuit the manual permits, with the disconnecting means the manual and NEC Article 422 Part III require. Rinnai's FAQ states its tankless heaters need 120 V AC, 60 Hz and less than 3 A, which is why a shared circuit is often acceptable; follow the manual for your unit.

How much power it draws

Manufacturers list the figures on each model's specification sheet. Two examples:

So the running draw is tens of watts to a couple hundred, standby is a few watts, and freeze protection can be the largest load on the unit. On an annual electric bill the heater itself is a rounding error. The reason to care about the number is backup sizing.

No power, no hot water

A gas tankless unit has no reserve. A tank heater that loses its burner still holds forty or fifty gallons of hot water for hours; a tankless unit holds a few cups in the heat exchanger. The moment the control board loses power, the next tap you open runs cold.

That makes a tankless unit the one gas water heater that always needs backup power if hot water matters during an outage.

Running it on backup power

The load is small, so almost any backup source can carry it. The constraint is power quality rather than size.

• Clean output. The control board is electronics. Manufacturers commonly call for stable voltage and frequency when running from a generator, and some void coverage for damage from poor power. An inverter generator or a pure sine wave power station is the safe choice; see sensitive electronics and generators.
• Cord-connected units can be unplugged from the wall and plugged into a power station or a heavy-duty outdoor cord from a generator placed outside.
• Hardwired units need a transfer switch or interlock installed by an electrician; see generator interlock kit vs transfer switch. Don't convert a hardwired connection to a cord unless the manual permits it.
• UPS. A computer UPS can ride through short outages. Check that its output is pure sine wave and that its watt rating covers the freeze-protection draw, not just the running draw.

Sizing a power station is simple arithmetic: watt-hours of storage divided by the heater's average draw. At the Rinnai figure above, 83 W during a shower and 2 W between showers, a station rated around 2 kWh would run the heater for a day or more of normal use; the exact runtime depends on how much hot water is drawn and on the station's inverter losses. In freezing weather, budget for the anti-frost draw running continuously.

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Turn off any eco or auto-shutoff timer on the station so the outlet doesn't power down while the heater idles at 2 W. Remember that the water supply matters too: on a private well, the pump is a far larger load than the heater; see sizing a generator for a well pump. If the furnace is the real concern during the same outage, see running a furnace on a generator.

Freeze protection in a winter outage

The one scenario where an unpowered tankless unit is at risk is freezing weather. The freeze-protection heaters are electric. Rinnai's builder specification notes that the R94LSi has solenoid valves that drop out to help drain the unit if power is lost, and manufacturers publish a manual draining procedure for extended outages. If the unit is outdoors or in an unheated space, follow that procedure as soon as a long cold outage starts, or keep the unit on backup power. A frozen heat exchanger is not a warranty repair.

Electric tankless: the other direction

An electric tankless heater needs no gas and no vent, and it also needs no 120 V control circuit, because the whole unit runs on 240 V. The issue is scale. Heating water on demand with resistance elements takes a lot of power at once:

Each element bank gets its own double-pole breaker and its own conductors, sized from the nameplate per NEC Article 422 and the ampacity tables in 310.16, with copper gauges that are often 8 AWG or larger. On a 100 A service, a 27 kW heater exceeds the entire service by itself. Even on 200 A, adding 112 A of water heating to a home with electric range, dryer, air conditioning or an EV charger can push the calculated load past the service rating. Run the home electrical load calculator before pricing an electric tankless unit, and read how to do a home electrical load calculation for what the inspector will expect.

Small point-of-use electric tankless units, a few kW for a single sink, are a different product and usually fit on one circuit.

Which is right for an outage-prone home

• Gas tankless needs only a small, clean 120 V source to keep working, so it pairs well with a power station or inverter generator.
• Electric tankless cannot realistically be backed up; even a large standby generator struggles with a 27 kW resistive load on top of everything else. In an outage it is off.
• A standing-pilot gas tank needs nothing, which is why some homeowners in outage-prone areas keep one; see does a gas water heater need electricity.

If the unit is due for replacement, the water heater replacement cost calculator compares the options.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC 110.3(B). Listed equipment installed per its instructions, including supply voltage and circuit requirements.
• NEC Article 422. Appliance branch circuits (422.10), disconnecting means (Part III) and cord connection (422.16).
• NEC 310.16. Conductor ampacity for the electric tankless circuits.
• NEC Article 702. Transfer equipment for optional standby power.
• NFPA 54 / IFGC. Venting and gas supply for gas tankless units.
• Rinnai, Builder's Specification, R94LSi (REU-VB2735FFUD-US): electrical consumption and freeze-protection notes.
• Navien, NPE-240A2 product specifications: electrical supply and power consumption.
• Rinnai, Residential FAQ: electrical supply requirements.

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Does a gas water heater need electricity
• Tankless vs tank water heater
• How to size a portable power station
• Water heater not heating
• Price it: water heater replacement cost calculator

More in our heating, cooling and water heating guides.