WeekendCalc

What Size Generator to Run an electric water heater?

An electric water heater needs about 4,000 running watts and 4,000 watts to start, so on its own you should shop for a generator rated for about 5,000 running watts, which includes 20% headroom.

Your project

W
W

The surge above running watts for the biggest motor, from its label.

Results

Generator size to shop for
5,000 running watts
With about 20% headroom
Total running watts
4,000 W
Peak watts at startup
4,000 W
Running load at 120 V
33.3 amps
Appliances selected
1 items
  • Start the biggest motor first, and add loads one at a time. Wattages are typical, so check each appliance label (a W or A rating) and use that if it differs. An inverter generator runs electronics more safely than a conventional one.

Generator size for an electric water heater

These use typical wattages (running and starting), plus 20% headroom on the running load. Your appliance's label is the best guide.

What you runRunningPeak at startupGenerator to shop for
Electric water heater alone4,000 W4,000 W5,000 running watts
+ refrigerator, lights, laptop and router4,950 W6,450 W6,500 running watts
+ the essentials (fridge, sump pump, furnace blower, lights, laptop)6,550 W8,100 W9,000 running watts

Motors need a surge when they start, which is why the starting watts are higher than the running watts. Build your list in the generator size calculator, and see the generator sizing guide for safety (never run a generator indoors) and how to connect it properly.

Want to change anything else? The calculator above is the full Generator Size Calculator: edit any number and the results update instantly.

Tools & materials for this project

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Portable generator (5,000 W)Inverter generator (3,500 W)Generator transfer switch kitCarbon monoxide alarmFuel stabilizer

Frequently asked questions

What size generator do I need to run an electric water heater?

An electric water heater needs about 4,000 running watts and 4,000 watts at startup, so shop for a generator rated for about 5,000 running watts if it is the only thing you run. Add the watts of anything else you want on at the same time.

Can a 3,500 watt generator run an electric water heater?

Not on its own. An electric water heater alone needs a generator rated for about 5,000 running watts, and with a fridge, lights, laptop and router about 6,500 running watts.

Can a 2,000 watt generator run an electric water heater?

Not on its own. With about 4,000 W at startup, check the generator's peak (starting) rating as well as its running rating.

Related sizes and guides

Generator for a window air conditioner3,000 running wattsGenerator for a central air conditioner9,000 running wattsGenerator for a electric dryer6,500 running wattsWhat size generator do I need?GuidePower Station Size CalculatorThe quiet battery alternative

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.