How to Run Water Heater on Generator: DIY Hacks with Pics


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While power outages might not be common where you live, extreme weather like snowstorms and hurricanes can catch you unawares, so it’s best to be prepared. Many people are familiar with running lights and small electrical appliances like toasters, hair dryers or coffee makers on a generator. But when it comes to the biggies — air conditioner, dishwasher, clothes dryer or water heater, things aren’t as simple.

So, can I run my electric water heater on a generator?

A generator’s running power rating (in watts) determines if it can be used to run an electric water heater for short periods like 1 hour. A standard 40-gallon storage-type water heater typically runs on a 240V circuit, drawing 4500W. Thus, generators rated 5000W or more can power such water heaters.

But is it safe to run a water heater on a generator? How much $$$ would it cost? Could you easily hook up your water heater to the generator? And last but not least important, how much hot water can you expect from a generator-powered water heater?

For answers to all these questions, dive in!

Things to Consider in Connecting a Water Heater to a Generator

You now know that you can hook up your water heater to the generator. But before you get your hands dirty, there are a few important factors to keep in mind:

1. Water Heater vs. Generator Power Rating

As outlined in the introduction, the most important factor is whether your generator has enough juice to power your water heater.

A standard (30-, 40- or 50-gallon) electric water heater needs anywhere from 3500W to 5500W of power. To power such a unit, a generator needs at least 500W in excess of the water heater’s power rating, which is about 4000W to 6000W.*

PortableGenerator-PowerRtg-Starting&Running_labld.png

*Note: Generators typically have a starting as well as a running power rating. The former, usually higher rating refers to the number of watts the generator can produce briefly, while an appliance with a motor is starting up. The second is the generator’s running power rating, which refers to the maximum watts the generator can deliver during operations (as shown in the image above).

Since water heaters have no motor, you just need to check that the generator’s running power rating exceeds the water heater power rating.

But what if you have a smaller generator, say, 2500W?

A smaller generator with a power rating as low as 2500W can be used to run a single-element storage water heater with a lower voltage rating such as 110V or 120V and a power rating of 1500W or 2000W. These are popular power configurations for compact, under-the-sink or wall-mounted models.

2. Generator Fuel Expense

The next most important concern is expense. How much would it cost to keep a water heater running on a generator? Well, for a standard, 40- or 50-gallon water heater, the $$$ would mount up pretty quickly.

The average 5000W generator uses 0.75 gallons of gasoline per hour. Running a water heater for about 4 hours a day would require 3 gallons’ worth of gasoline. At $3.20 a gallon of gas, that would cost almost $10 per day. 

It’s easy to see why folks aren’t too crazy about running their water heaters on their generators, even when there’s a power outage. In fact, if you’re having a professional electrician install a power transfer panel or a generator interlock switch* at home, they will recommend that you do not run appliances like your clothes dryer or water heater on the generator.

*Note: A power transfer panel and a generator interlock switch are both legally approved methods that provide home-owners a more permanent method of using their generator to power up household electric lights and appliances during a power outage.

Of course, when a power outage lasts for days, there comes a point where taking a hot shower can’t be postponed! So, is there a way to get the water heater to make hot water without blowing a wad of money on generator fuel? Read on to find out…

3. Lower is Slower – Cost vs. Recovery Rate

As you probably know, your standard, 40-gallon water heater has two heating elements. However, the water heater’s circuit is set up so that only one element ever works at any given time. This means, although it’s on a 240V circuit, your water heater is typically using only one, 120V leg at a time – luckily for you! 

Why should you care that your water heater runs only one 120V leg at a time?

Because this means that you can actually run it using only 120V of your generator’s 240V maximum output. And this means that the generator will consume, not half, but only a quarter (¼) of the fuel that it would normally require in order to power the electric water heater.

So what’s the catch, you ask?

You’re right, there is one. Which is, in order to heat water to the same temperature, your 240V water heater running on a 120V circuit is going to take four times as long! But, if you can get by with a shower that’s only lukewarm (or even tepid) instead of a hot one, it’s still a pretty good bet for getting hot (warm, anyway!) water during a power outage.

4. Safety Considerations

If you’re still asking how safe it is to run a water heater on a generator, the answer is that it’s quite safe, as long as you follow the proper procedure for connecting the appliance to the generator. 

This includes following the steps described below for hooking up the water heater to the generator, as well as taking the appropriate safety precautions, such as:

  • Making sure that the power supply to the water heater is turned off. (See how to do this in this handy post.)
  • Keeping all surfaces dry, including the water heater, generator, your tools, wiring materials and, of course, your hands!
  • Being careful to stand on wooden boards while working with the generator.
  • If setting up your water heater on your generator’s 120V circuit, ensuring that you follow the instructions in the generator manual for load balancing across electrical outlets.

Step by Step Guide to Connecting Water Heater to Generator

Now you’ve figured out that your water heater and your generator are a match. You’ve also reckoned whether you want to set up your water heater to draw full power (on a 240V circuit) or reduced power (on 120V). There’s only one thing left… doing the actual hooking up.

PortableGenerator-PowerOutlets-L14&L5_labld.png

There are two methods described below, one to connect the water heater to your generator’s 240V outlet, and another to connect your water heater to the generator’s 120V circuit (see the two different outlets in the image above).

Depending on your needs, follow the steps described in any one method. Be careful not to get methods #1 and #2 mixed up!

Method #1: Connecting Water Heater to Generator’s 240V Outlet

Note: Some generators (typically smaller ones) do not have a separate power outlet marked ‘240V’, but instead have an outlet marked ‘120/240V’. In these models, there’s usually a Voltage Selector Switch that the user must adjust to either the ‘120V only’ position or the ‘120/240V’ position. To run a water heater on a 240V circuit, this switch would need to be in the latter position.

Materials You Will Need:

  1. Sufficient length of 10/2 AWG cable (rated for 30 amps) to cover the distance from water heater to generator (which must be kept outdoors while operating).
  2. NEMA L14-30 plug (rated for 30 amps and 240V to 250V)
  3. Wire nuts (typically red, rated for 10-gauge AWG wire)

(See image below.)

PortableGenerator-WaterHeater-240V-HookUp_labld2.png

Step 1. Create a Customized Power Cable to Connect the Generator to the Water Heater:

  1. Follow the wiring schematic (as shown in the top-right panel of the image above) to connect the individual wires of the 10/2 cable to the L14-30 plug.
  2. Connect the white (/red) and black wires to the screws for either X or Y pins, and connect the bare, grounding wire to the screw for the ‘L’-shaped, grounding pin.
  3. Be sure to twist the wires around the screws to form secure connections before putting the L14-30 plug’s housing back on.
  4. Now, make sure that the electricity supply to your water heater is turned off (see how to do this in this post). 

Step 2. Connect the Customized Power Cable to the Water Heater:

  1. Locate the junction box at the top of your water heater.
  2. Replace the existing connections for water heater power with the wires from the customized power cable, using wire nuts to secure the connections. See how to make the connections in your water heater junction box in this helpful post.
  3. Using your generator’s ‘240V’ power outlet, plug in the customized power cable to connect it to the water heater. Make sure to use the twist lock to secure the cable, such that it can’t be yanked loose by accident.

Power on the generator, and enjoy hot water!

Method #2: Connecting Water Heater to Generator’s 120V Outlet

Note: This method can only be used if your generator has separate power outlets for 240V and 120V. Some models (typically smaller ones) have only one power outlet marked ‘120/240V’: in this case, you can either set the Voltage Selector Switch to the ‘120V only’ mode, and then choose the plug configuration specified in your generator manual while building your customized water heater power cable, or use the alternative method described below. 

An alternative way to reduce your generator’s fuel consumption while powering your water heater is to switch out the heating elements in your water heater for lower wattage elements (e.g., 1500W instead of 4500W). See how to do that in this step-by-step guide.

Materials You Will Need:

  1. Sufficient length of 12/2 AWG cable (rated for 20 amps) to cover the distance from water heater to generator (which must be kept outdoors while operating).
  2. NEMA L5-30 plug (rated for 30 amps and 120V to 125V).
  3. Wire nuts (typically red, rated for 10-gauge AWG wire)

(See image below.)

PortableGenerator-WaterHeater-120V-HookUp_labld2.png

Step 1. Create a Customized Power Cable for Connecting the Generator to the Water Heater:

  1. Follow the wiring schematic (as shown in the top-right panel in the image above) to connect the individual wires of the 12/2 cable to the L5-30 plug.
  2. Be sure to twist the wires around the screws to form secure connections before putting the L5-30 plug’s housing back on.
  3. Now, make sure that the electricity supply to your water heater is turned off (see how to do this in this post). 

Step 2. Connect the Customized Power Cable to the Water Heater:

  1. Locate the junction box at the top of your water heater.
  2. Replace the existing connections for water heater power, which at present come from the mains supply, with the wires at the other end of the customized power cable. See how to do this in this helpful post.
  3. Using your generator’s ‘120V’ power outlet (see image above), plug in the customized power cable to connect it to the water heater. Make sure to use the twist lock to secure the cable, such that it can’t be yanked loose by accident.

You’re done. Enjoy hot water!

You now have all the know-how you need to power up your water heater using your generator. One less thing to worry about for the next time there’s a longish power outage!

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Joonas

I like it when I'm able to fix everything that needs fixing around the house. In order to do that, I have to do a lot of research. This site will cover everything I learn and maybe help others do the same.

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