Can A Generator Run A Hot Water Heater?

When the temperature drops below freezing, we need our water heater up and running even during a blackout. A generator provides a reliable source of power. But you can count on it to run a water heater throughout a power outage? We’ve done some research, and here's what we've found. 

A generator can run a water heater considering that it matches or has more output wattage required by the water heater; otherwise, it will result in a power surge which could damage both the generator and water heater.  

Unlike electricity, a generator can only supply a limited amount of power. Read until the end to learn how long it can power a water heater and the safe ways to connect your water heater. 

A gasoline powered portable generator at home, Can A Generator Run A Hot Water Heater?

How Many Hours Can A Generator Run A Water Heater?

A regular water heater runs for at least three hours a day only. But you can run it on a generator longer or as long as you need it. However, you must also keep in mind the standard runtime of your generator to prevent overheating.

What Size Generator Do I Need For My Water Heater?

Man firing up gas powered portable inverter generator

If you have a 5-kilowatt water heater, you need a 6-kilowatt generator. The larger the generator's capacity, the better- it can keep up with the electrical rate of the water heater.

The idea is to get a generator that exceeds the total output wattage of your home appliances, or you’d risk blowing off electrical wiring.

Homeowners with a water heater that is difficult to unplug from the house wiring need a generator that can power up the entire household as you cannot simply connect the water heater to the generator.

If your generator has a limited amount of power, avoid using other home appliances while the water heater is running.

Will A 5000-watt Generator Run A 50-gallon Water Heater?

A 50-gallon water heater uses 4,500W, so a 5,000W generator is enough to power it. A water heater this size requires 4,500W to ignite and 3,500W to heat water for a few hours. Basically, it can consume the total watts, so use it with caution. 

Can A Generator Run A Tankless Water Heater?

Electric water heater on the white brick wall in bathroom

As long as the generator has enough power to run it, yes it can. Some other ways to run a tankless water heater include a battery backup. It’s a small battery you connect your tankless water heater to get power and run.

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But you have to know how many watts your water eater uses and how many watts the battery backup can provide. Typically, a battery backup uses 360W, which is enough to power a 76W water heater.

How To Connect A Water Heater To Your Generator?

Gasoline electric generator connected to house to provide power

Here are step-by-step instructions to safely connect your hot water heater to a generator:

1. Uncover the immersion heater on the tank, then check the bottom plate, and note the power required for each heater in watts. Also, take note of the voltage designed for them. Most immersion heaters use 230 volts which can consume a few kilowatts.

To calculate the current amps, divide the kilowatt power in watts by the voltage. Multiply the kilowatt by 1,000 to obtain the watts, then divide it by 230 volts. Suppose your water heater uses 3.0 kilowatt; the result is 13.0 amps.

2. To check the volts, look at the generator front panel, most of them come with either 230-volt or 115-volt outlets. Check if the 230-volt outlet uses a standard plug-in or a special one. Refer to the generator’s manual for the rated power in kilowatt.

3. Install a jbox or junction box. If your water heater has two heating elements, install two circuit breakers near the tank. (Ask for assistance, if necessary.) Measure the length of the wire needed to connect the generator to the jbox and the cable that will connect the junction box to the immersion heater located on top of the water tank.

You may have to purchase the necessary electrical tools to set up the wirings. Get the 3-wire cable suitable for the current and plug that will go to the generator.

4. Once the junction box and circuit breakers are installed near the water tank, you can now plug in the wire from the junction to the immersion heater. Secure them firmly. Cover the heating elements to avoid exposing the cables, then connect the wire from the generator to the jbox.

5. When done, switch off the circuit breaker, then turn the generator on. Next, turn on the first circuit breaker on the junction box. The generator should start powering up the first heating element of the water heater. You can now turn on the second heating element. Just ensure that the generator isn’t making a strange noise and is working normally.  

Important Safety Tips When Running A Water Heater On Your Generator

  • A 3,000- to 4,000-watt generator is enough to power some lights, the television, and even the refrigerator. But it doesn’t necessarily mean that you can run them along with your water heater. Remember that you shouldn’t use other appliances while the water heater is running.  
  • Do not modify the writing inside the water heater.
  • Limit your electrical current to 30 amps, 4,500W for a residential water heater. 
  • Stock extra fuel for your generator if there’s a storm, 
  • Don't store gas inside the generator. If there's gas left after using it, drain and clean it. Gas can cause your generator to break down. 
  • Ensure that your generator has sufficient gas while operating. If the gas reads low, switch it off, then let it cool for five to ten minutes before refilling it. 
  • Avoid refueling a hot generator as a small gas spill can cause a fire. 
  • Don't attempt to connect the generator to your house's electrical wiring unless you're an experienced electrician.

Is It Safe To Run A Water Heater On A Generator?

Yes. As long as the rating of the generator matches or is greater than the rating of your water heater, you won’t have any problem with it. A small generator that is forced to provide excessive power will eventually overheat and shut down.

How Long Can A Generator Run Continuously?

It depends on the fuel source and the size of your generator. But gas-powered generators can usually run for 6-18 hours, which is the standard runtime. Most portable generators also require maintenance after every 100 hours of service.

On the other hand, standby heaters used to power an entire residential area and commercial buildings can provide non-stop power for two to three weeks.

Can You Run A Generator Overnight?

Generator running a night to generate power supply

Running the generator overnight is okay, considering that it has enough fuel to power it. Keep in mind that it shouldn’t run out of gas or risk power surges which is the fastest to kill your appliances.

However, it is not really recommended unless there’s an emergency. Leaving the generator running all night can cause excess heat to build up which can permanently damage your generator. Also, some generator noise violates sound ordinances.

What Happens When A Generator Is Overloaded?

When a generator overloads, it will result in an unusual voltage spike and intermittent power. It can also overheat, and too much heat can burn some of its components and shorten its lifespan. 

How To Know If Your Generator Is Overloaded?

The circuit breaker is one way to tell if the generator is overloaded. Most generators come with it. This turns off automatically when the generator gets overwhelmed.

Another is the sound. A normal generator sounds like a vacuum or dishwasher. A strange noise coming from it is an indication it provides more power than it can handle. Unplug some appliances or turn off the generator. 

How To Prevent Generators From Overloading?

It is important to run a generator within its capacity only. Always check the generator wattage that is printed on the device.

Remember, if you have a 5,000W generator and a 4,500W water heater, don’t run other appliances. Maybe heat the water first, then use the other appliances afterward.

Lastly, check if the generator has loose components before running it. Using your device properly can prevent electrical surges and extend its lifespan.


A generator is a great alternative source of power. It can run a water heater, given that it has greater wattage than the outage demanded by the water heater. Use it within its capacity and avoid using other home appliances while the water heater is running to prevent power surges.  

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