How To Fix It

How Many Amps Does Water Heater Use?

A water heater is probably one of the most important, yet least thought of, pieces of equipment in your home. When considering a water heater, one thing you must consider is the amperage the new unit will use.

The amperage is vital to ensure you do not overload your current home’s electrical system.

The standard amperage for an electric water heater is 18.8 Amps with 240Volts, 4500 Watt. A gas water heater in average uses less than 12 Amps with 115Volts (US and Canada). However, the exact amount of amperage a heater uses depends on numerous factors:

  • Heater Type
  • Fuel Type
  • Capacity
  • Energy Efficiency

Understanding the amount of amps a water heater needs is vital information for every homeowner. With all the choices available, deciding on a water heater can be a daunting task.

However, once you understand the various options described below, you will make an informed decision.

Heater Type: Tank or Tankless

Water heaters work one of two ways: with a tank that stores water that is continuously heated or with coils that heat water on demand. 

Tank units are also known as storage or container heaters. These units are the oldest and most traditional type of equipment.

Tankless devices are also known as on-demand heaters. These units are newer in design.

In general, storage water heaters require less amps than tankless heaters. However, they may require more power overall because they are constantly heating the water inside their tank.

Tankless heaters only run when there is a need and so their overall power draw may be less depending on the amount of actual use.

What Size Breaker for a 40-Gallon Electric Water Heater?

A 40-gallon tank is the most common size of residential container heaters. So, if you are installing this size unit in an older home, odds are you will be replacing a similarly sized unit. 

Have a Question? Ask HVAC Technician

Click here to use the chatbox to speak with one of our technicians.
No in-home service calls. No appointments.

If an old unit is installed, the installer will have to remove the old unit and replace it with the new one. No extra cost is usually required.

This size unit will usually require a 30-amp breaker, which is one of the most common size breakers used in residential homes. Because of this, most homes will not need any kind of electrical system upgrade when installing this type of device.

How Many Amps Does a Tankless Water Heater Use?

On average, an on-demand heater requires 120 amps to run, which is much higher than the traditional units that only need 30 amps to power them.

Many older homes have 100 to 200 amps available and will require an electrical system upgrade by a professional electrician.

This upgrade may include your electrician adding a subpanel of circuit breakers to your home to increase the total amount of amperage available.

Fuel Type: Electric or Gas

The type of fuel your water heater uses also affects the amount of power it needs to run effectively. A water heater will usually run on electricity or natural gas.

Electric Water Heaters

These types of units cost less than gas units, but they cost more to run them. This cost is because, overall, electricity is more expensive than natural gas. 

On average, an electric water heater will cost around $42 a month in energy use. 

However, this type of heater tends to be easier to install because it can simply be attached to your home’s electrical system instead of having a gas line run.

How Many Amps Does an Electric Water Heater Use?

An electric heater with a tank requires a 240-volt dedicated circuit with a 30-amp breaker. Because this kind of device requires a dedicated circuit, an electrical upgrade may be necessary for your home.

A tankless electric unit requires a 120-amp breaker. For many older homes, this type of device will require upgrading the electrical system. This upgrade is another added expense in up-front costs when it comes to on-demand water heaters.

Gas Water Heaters

Heaters that run on gas are usually 25 to 50% more expensive than units that run on electricity. However, because they cost less to run, they tend to make up for the price difference over time.

On average, a gas water heater will cost around $30 a month for an average house.

Because gas units must be attached to a gas line, there may be an added expense when installing such a device for the first time. This is especially true if you need to run a new gas line or upgrade your current natural gas system.

How Many Amps Does a Gas Water Heater Use?

A gas water heater with a tank will require 30 amps of electricity, which usually does not require any kind of modifications to the home’s electrical system–even for older homes with only a total 100- or 200-amp service.

A tankless unit that runs on gas will require a 120-amp breaker. Because of this type of breaker’s high amperage, many older homes will require modifications to the electrical system to accommodate this kind of device.

Should You Change Your Heater’s Fuel Type?

For many people, they may think that if they already have an electric water heater, it is impossible to install a gas unit or vice versa. This is not necessarily the case. 

If you are interested in changing the type of equipment you have, all you have to do is ask an expert. Many electricians and plumbers offer free consults. Find one in your area that does and simply ask them for an evaluation.

If you are considering converting to natural gas, you can call your local gas provider and find out if they offer service in your area. Some providers may even be able to give you an estimate of the costs right over the phone.

When contemplating changing your water heater’s type of fuel, take the following questions into consideration:

  • Is the fuel type available in your location?
  • How much will the installation cost?
  • Will the installation require a permit?
  • Will you need to upgrade your homes total amperage?
  • How much will your monthly bill be for this new fuel type?

Once you answer the above questions, you will know whether changing your heater’s fuel type is worth the time, effort, and money involved.

Though changing your heater’s fuel type may require more work and cost during installation, it may be more cost effective for you overall.


Water heaters are only useful when they provide you with hot water when you need it. Whether the unit has a tank or not, or runs on gas or electricity, if it is not the right size for your home’s water demands, it is not going to work for you.

The amount of electricity your heater draws also depends on the capacity of your unit.

For instance, storage water heaters have a range of 20- to 80-gallon size tanks. The larger the tank, the more power it will take to heat the water. 

A 40-gallon tank is usually sufficient for a 2- to 3-person household. It is recommended that you increase the size by 10 gallons for each additional person you have in your home.

The capacity of on-demand heaters is calculated by gallons per minute or flow rate. The higher the flow rate, the more power the unit will draw at one time, and the higher your electricity or gas utility bill.

Calculating the flow rate, you will need for your home requires you to add the flow rate demands of each of the fixtures you would want to run simultaneously. 

That is, if you want to be able to take a shower, run a load of laundry, and run the dishwasher at the same time, you must add the flow rates of each of these devices together. 

Then you would choose a tankless heater that has a higher flow rate than the number you calculated. You can figure out which size heater will work for you using the Tankless Water Heater Sizing Diagram.

When it comes to heater capacity, it is vital that you find a balance between your household’s needs and its electrical capability.

Choosing a unit that has the largest capacity available may sound appealing, but it may require more amps than your home is able to accommodate.

Energy Efficiency

Studies show that heating a home’s water accounts for around 18% of the home’s total energy use.

How efficiently your heater uses energy will also affect how many amps the unit uses. Usually, the more energy-efficient the heater is, the more amps it will need.

However, though a more energy efficient heater may require more amperage to power on, the unit will be used for less time and save you money in energy costs overall.

An older piece of equipment usually tends to be less energy efficient. If your unit is over ten years old, it is time to consider replacing it with a more efficient model.

Electric water heaters tend to have higher energy efficiency ratings than gas units.

However, because gas usually costs less than electricity, the two units tend to break even cost-wise. 

When looking for a new heater, look for the energy star logo. This logo means that the unit meets the energy star standards. These devices tend to be more energy-efficient and, in turn, more cost-efficient.


When you need it, having hot water is not just a luxury but a necessity in today’s day and age. As such, you need to find a unit that fits your budget–both in up-front costs and in regular use.

It is also essential that you consider the amount of overall energy your water heater will draw from your home to ensure your electrical system can handle the demand.

Leave a Comment