Best Generator for Running an Electric Water Heater

Hot water is one of the comforts people miss most in a long outage. It’s also one of the hardest to restore with a portable generator. A standard electric tank water heater uses 240-volt elements rated around 4,500 watts, and heating a cold 50-gallon tank can keep that element on for well over an hour. It can be done, but only with the right generator, a proper transfer switch, and some discipline about when hot water gets made. This guide covers the numbers and the smartest approaches for tank, heat-pump, tankless, and gas units.

Water heater types and their electrical demand

Reheat time is based on the energy needed to raise 50 gallons by 65°F, roughly 7.9 kWh, and ignores standby loss. Your nameplate lists the actual element wattage.

Generator picks by water heater

Standard electric tank: 7,500-9,500W with 240V

Because most tanks only ever run one element at a time, a 4,500W load is the design figure. A 7,500W generator with an L14-30R outlet feeds it through a transfer switch while still covering the refrigerator and lights, as long as you don’t run the microwave or well pump at the same time. At roughly half load, expect around 0.6-0.8 gallons of gasoline per hour, which means one full reheat costs about 1-1.5 gallons.

Heat-pump water heater: 3,500-5,500W with 240V

Switch the heat-pump water heater to heat-pump-only mode during an outage and its backup elements stay off. The compressor then draws around 400-550W. Many units are 240V, so the generator still needs a 240V outlet, or the unit must be one of the newer 120V plug-in models. This is by far the most fuel-efficient electric option.

Gas water heater: 1,000-2,000W inverter

A standing-pilot gas heater needs no electricity at all. Power-vent and electronic-ignition models need only a few hundred watts for the blower and controls, and they’re sensitive to power quality, so an inverter is the right match.

Scheduling hot water like an off-grid homeowner

You don’t need hot water on demand during an outage. You need a tank of it once or twice a day. Turn the water heater circuit off at the transfer switch most of the day. Then pick a window, such as a morning generator run, when you switch off the dryer, oven, and other heavy loads and give the water heater about 90 minutes. A well-insulated tank loses only a few degrees an hour, so evening showers are still warm. Dropping the thermostat from 140°F to 120°F also cuts reheat energy by roughly a quarter.

The connection has to be done right

Electric water heaters are hardwired to a dedicated 240V breaker, typically 30A. The only safe way to feed one from a generator is through a manual transfer switch or a panel interlock with a generator inlet, installed by a licensed electrician. Never try to backfeed the panel through a dryer or range outlet. Current flows out to the utility transformer and can kill lineworkers repairing the grid. Also note: if the tank has been drained or partly emptied, energizing the elements while dry burns them out within seconds. Confirm the tank is full before switching the circuit on.

Carbon monoxide and utility rooms

Water heaters often share a garage or utility closet, and that’s no place for a running generator. Keep the generator outside, at least 20 feet from the house, with the exhaust pointed away from doors, windows, and the water heater’s own flue or intake if it’s gas-fired. Install battery CO alarms on every level. Gas water heaters with atmospheric venting are an additional CO source, so keeping the flue clear matters during storms.

FAQ

Can a 5,000-watt generator run an electric water heater?

Only if it has a 240V outlet, and only with little else running, since a 4,500W element leaves about 500W for everything else. A 7,500W unit gives you room to keep the refrigerator and lights on during the reheat.

Can I run a tankless electric water heater on a portable generator?

Whole-house electric tankless units draw 18-36kW, beyond almost every portable generator. Point-of-use tankless units (3-7kW) at a single sink are more feasible on a large 240V generator.

Bottom line

A standard electric tank needs a 7,500W+ generator with 240V output, a licensed electrician’s transfer switch, and a scheduled daily reheat window. Heat-pump models in heat-pump-only mode and gas units with electronic ignition are far easier to power. Keep the generator outside and the CO alarms working.