“Will a generator run my tankless water heater” sounds like it should have one answer, but it splits into three very different projects depending on what’s actually installed. A gas unit barely registers as an electrical load. A small electric unit is a reasonable ask for a portable generator. A whole-home electric unit can out-demand generators that are marketed as whole-house-capable. This guide breaks down the real wattage and amperage for each so the generator gets sized to the heater it’s actually backing up, not a guess.
Why gas and electric tankless heaters need completely different backup power
| Heater type | What draws power | Typical draw | Generator that covers it |
|---|---|---|---|
| Gas / propane tankless | Ignition, control board, sensors, blower | ~60-150W (300-400W startup surge) | Small 2,000W inverter generator |
| Point-of-use electric tankless | Resistive heating element | ~12-30A at 120V (1.5-7kW) | Mid-size 4,500-5,500W dual-fuel portable |
| Whole-home electric tankless (24kW) | Resistive heating elements | ~54A at 240V | Large portable or dedicated standby generator |
| Whole-home electric tankless (36kW) | Resistive heating elements | ~150A at 240V | Often exceeds a single residential standby generator |
The split comes down to what electricity is actually doing in each system. A gas tankless heater burns gas to make heat — electricity only runs the “brains”: the igniter that lights the burner, the control board that manages temperature and gas flow, the sensors that watch for problems, and the blower that vents exhaust. An electric tankless heater has no combustion at all; every watt of heat comes directly off the electrical circuit, which is why its draw scales so much higher, a point our electrical requirements guide covers in more depth for panel and breaker sizing.
By the numbers
- 60-100 watts — what a gas tankless water heater draws while running, with startup briefly reaching 100-150 watts and surges up to 300-400 watts for a few seconds during ignition. — A.O. Smith / hotwater.com
- 150 amps — the peak draw of a 36kW whole-home electric tankless unit at 240V, already established on this site’s electrical requirements guide and solar power guide as the number that drives panel, wiring, and now generator sizing.
- 19,500-24,000 watts — the rated output of Generac’s 22kW standby generator, the top-selling whole-home backup model, delivering 22,000W on propane but only 19,500W on natural gas — both below a single 36kW electric tankless unit’s full draw. — Generac
- 12-30 amps — the typical draw range for point-of-use electric tankless units (1.5-7kW), light enough to run on a mid-size portable generator alongside a few other household circuits.
Gas tankless: almost any generator will do
Because a gas tankless unit’s electrical draw is limited to ignition, controls, sensors, and a blower motor, backup power for it is one of the easiest generator sizing decisions in a home. A.O. Smith’s own documentation puts running draw at 60-100 watts and startup surges no higher than 300-400 watts — well within what a small, portable inverter generator handles without strain, and far quieter and cheaper than sizing for anything close to a whole-home load.
Champion 2000-Watt Ultralight Portable Inverter Generator
- Rated running wattage is more than 10x what a gas tankless heater's control board and igniter actually draw, leaving headroom for a sump pump, fridge, or a few lights on the same outage.
- Inverter output keeps under 3% total harmonic distortion, clean enough for a tankless unit's electronic control board — not every generator type is.
- Ultralight build makes it realistic to keep in a garage specifically for a hot-water outage, rather than only pulling out a full whole-house unit.
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Point-of-use electric tankless: a mid-size portable generator covers it
A single point-of-use electric unit — the kind installed under one sink or feeding one shower, typically 1.5-7kW on a 120V or 240V circuit — draws far less than a whole-home unit, generally in the 12-30 amp range. That’s within reach of a mid-size dual-fuel portable generator, with enough headroom left to run a refrigerator or a few lights on the same outage, which is not the case for a whole-home unit.
| Generator | Running watts | 120V amps | Covers |
|---|---|---|---|
| DuroMax XP5500HX (dual fuel) | 4,500W | ~37.5A @ 120V / ~18.75A @ 240V | Point-of-use electric units up to ~3.5-4kW, plus a few other circuits |
| Champion 2000W inverter | 1,700W | ~14A @ 120V | Gas units only — undersized for most electric tankless heaters |
DuroMax XP5500HX Dual Fuel Portable Generator
- The 120/240V 30A outlet is what makes this usable for a point-of-use electric tankless unit — a straight 120V-only generator can't feed a 240V single-point heater at all.
- Dual-fuel flexibility (gasoline or propane) matters for outage prep specifically, since gasoline can be hard to source during a wider regional power event but propane tanks store indefinitely.
- 4,500 running watts leaves meaningful headroom above a typical 1.5-3.5kW point-of-use heater's draw for a fridge or a couple of lights on the same panel.
Whole-home electric tankless: why even a home standby generator struggles
This is where the math stops working the way people expect. A whole-home electric tankless unit doesn’t sip power the way a gas unit’s control board does — it draws its full rated kW continuously while running, because every bit of the heating happens on the circuit. A 24kW unit pulls about 54 amps at 240V, and a 36kW unit can pull up to 150 amps, numbers already established on this site’s electrical requirements guide. Generac’s 22kW model — the best-selling whole-home standby generator in the country — delivers 22,000 watts on propane but only 19,500 watts on natural gas, according to Generac’s own specifications. Both figures sit below what a single 36kW electric tankless unit demands on its own, before a refrigerator, well pump, or HVAC system draws a single additional watt.
In practice, this is why whole-home electric tankless installs on generator backup almost always pair with a managed or “smart” transfer switch rather than a simple whole-house switch: the panel sheds other major loads automatically whenever the water heater fires, so the generator’s total output never has to cover everything at once. A generator sized to the water heater’s full 150A draw and the rest of the home simultaneously would mean stepping up to a generator larger than most homes install for anything else in the house.
| Setup | Generator draw from the heater alone | Realistic backup plan |
|---|---|---|
| Gas tankless, any size | 60-150W | Small portable inverter generator |
| Point-of-use electric (1.5-7kW) | 12-30A @ 120/240V | Mid-size dual-fuel portable generator |
| Whole-home electric, 18-24kW | ~33-54A @ 240V | Large portable or entry standby generator, likely with some load shedding |
| Whole-home electric, 27-36kW | ~75-150A @ 240V | Standby generator sized above the heater's draw, with a managed transfer switch |
If a whole-home electric tankless heater is already installed and outages are a real concern in a given climate, switching to a gas or propane unit is usually the far cheaper backup-power fix compared to upsizing a standby generator to cover it — see our electric vs. gas tankless comparison for the full tradeoff beyond just outage resilience.
The bottom line
A gas tankless water heater is one of the easiest appliances in a house to back up — its electrical draw is limited to ignition, controls, and a blower, so a small inverter generator in the low hundreds of dollars covers it with room to spare. A point-of-use electric tankless unit steps up to a mid-size dual-fuel portable generator, still a modest purchase. A whole-home electric tankless unit is the outlier: its continuous 33-150A draw, scaling with kW rating, can approach or exceed what even the best-selling residential standby generators are rated to deliver, which is why those installs typically need a generator sized specifically for the heater plus a load-shedding transfer switch, not a generic whole-house unit picked off a sizing chart. Anyone choosing between fuel types with outage resilience as a real factor should weigh that gap before buying, not after the first outage without hot water.