What size generator do I need for an electric water heater?

Hot showers in an outage are possible, but an electric water heater is one of the biggest loads in a home. It changes the kind of generator you need, not just the size.

How many watts does an electric water heater use?

A tank water heater heats with electric elements, like the coils in a toaster. There's no motor, so there's no start-up surge. Running and starting watts are the same.

Source	Watts
Champion chart	3,000–4,500 W
LCEC worksheet	4,500 W
Our calculator	4,500 W

Most standard tanks have two elements, an upper and a lower. Usually they take turns rather than heating together, so the label's element rating is what the heater pulls. Check your own label, because some tanks use 5,500 W elements, and a few heat with both at once.

Why it needs 240 V

Most standard electric tank water heaters in the US run on 240 V. A generator's normal 120 V outlets can't power it. You need:

1. A generator with a 240 V outlet. The common one is the 4-prong L14-30, rated 30 A at 120/240 V. That's 7,200 watts. Bigger generators may have a 50 A 14-50 outlet.
2. A transfer switch or interlock installed by a licensed electrician. This is how the generator reaches the water heater's hard-wired circuit.
3. The right cord and inlet box. See 30 amp vs 50 amp generator cords.

Never backfeed the house through a dryer outlet or a wall outlet. It can electrocute a utility worker, damage the generator and start a fire. See interlock vs transfer switch.

A worked example: water heater, fridge and lights

• Running: 4,500 (water heater) + 400 (fridge) + 100 (lights) = 5,000 W
• Biggest surge: the fridge, 1,200 − 400 = 800 W (the water heater has none)
• Starting: 5,000 + 800 = 5,800 W
• With 20% headroom: about 6,000 running and 7,000 starting watts

That's a 6,000–7,500 W generator with a 240 V outlet. The calculator above is set up with this list, and it will flag the 240 V need.

Will a 5,500 watt generator run a hot water heater?

It can run a 4,500 W water heater by itself, but with only about 1,000 W to spare. The fridge fits alongside it. A well pump or window AC starting at the same time won't. A 7,500 W generator gives you room for the heater plus a few essentials.

Generator (running W)	Water heater (4,500 W)
3,500 W	No
5,000 W	Alone only, near its limit
5,500 W	Yes, with a fridge and lights, but little headroom
7,500 W	Yes, with room for a sump pump or furnace fan
10,000 W	Yes, with most essentials

Will a 7,500 or 10,000 watt generator run a water heater and more?

• 7,500 W: water heater (4,500) + fridge (400) + 1/2 HP sump pump (1,000) + lights (100) = 6,000 running watts. Starting is 6,000 + 1,150 for the pump = 7,150 W. That fits, with the heater running.
• 10,000 W: add a furnace fan (700) and a 1/2 HP well pump (1,000) and you're at 7,700 running watts. The well pump's surge brings starting to about 9,700 W. That fits, but only just, so let the water heater rest when the well pump is working hard.

Our calculator uses 3,000 starting watts for the well pump, from the pump motor maker's minimum generator size. See generator for a well pump.

Take turns to use a smaller generator

You don't need hot water all the time. A simple routine helps:

1. Turn the water heater on for an hour or two with only the fridge running.
2. Turn it off at its breaker or the transfer switch.
3. Then run everything else: sump pump, furnace fan, microwave, well pump.

A full tank stays hot for many hours if nobody uses it. This lets a 5,500 W generator cover a lot of ground.

Gas water heater instead?

Many gas tank water heaters with a standing pilot need no electricity at all. Power-vent and tankless gas models do need power, but far less than an electric tank. Check for a plug or power cord on yours.

Check your own water heater

1. Find the label on the side of the tank, often behind a small cover.
2. Look for the element watts and the voltage, for example "4,500 W at 240 V."
3. Look for "non-simultaneous." That means only one element heats at a time.
4. Check the breaker: a double breaker (two handles tied together) means 240 V.

Turn the power off at the breaker before you remove any cover on the tank.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.