Informational Guide

How Many Amps Does A Water Heater Use? By Type & Size

How many amps does a water heater use? We answer this question by looking at how watts & amps are calculated and the contributing factors.

by Josh Mitchell

You need to know how much amperage is drawn by your water heater. If you don't know, you should read through the rest of this guide.  

We will teach you how much power to direct where and have an idea of the efficiency of your appliance. Here's the answer to "how many amps does a water heater use?".

The word amp is short for "ampere," the standard unit of measurement used to measure electricity. An amp is a single unit of an electrical current.

To determine the amperage draw, the number of watts is divided by the number of volts going through an outlet. You'll need to know what an amp is when determining the power needs for a water heater. 

1. Heater Type (Tank Or Tankless)

Water heaters can come with a tank or be tankless. A tank option stores heated water, while a tankless water heater heats water on demand in the pipes. The storage option requires fewer amps since they only run when they’re out of water. 

With a tankless heater, the system only uses amps when you use the hot water. This means the system provides varying levels of amperage. If you use a lot of water, then you'll use more daily amps. 

2. Capacity  

The larger your tank is, the more power it takes to heat the water- since it’s heating a lot more. Water storage tanks fall into a range of 20 to 80 gallons.

You don’t want to always go for the largest tank because of this. Your home might not have enough amps to run the unit effectively. Here is our guide on determining proper tankless water heater size.  

3. Wattage  

The wattage dramatically impacts the amount of amps your machine draws. The more watts it needs, the higher the amp level as well. The standard amperage for an electric water heater is: 

  • 18.8 amps 
  • 240 volts, 4500 watts 

In comparison, a gas water heater will use the following: 

  • Less than 12 amps 
  • 115 volts 

4. Energy Efficiency  

How efficiently your water heater runs will impact how many amps it uses. A more efficient heater needs more amps to turn on but runs for a significantly less amount of time-making it more cost-efficient overall. 

That means you want to check for higher amp levels. Older systems are usually less energy efficient. You may need to upgrade for better energy use. 

How Many Amps Does A Water Heater Use

How Watts & Amps Are Calculated?

When calculating the amperage, you need to know the amount of watts the water heater uses. Here’s the equation that you can use: 

P (W) = I (A) * V (V) 

So, you can use that formula to determine how many amps are in a watt. Let’s use 120v as an example. You want to plug that in where the volts go first. 

P (W) = 1 A * 120 V = 120 W 

The example shows us that one amp is 120 watts, in this equation. If you need to know what size breaker for a 30 gallon water heater, you’ll want to determine the amount of watts it uses. On average, these units use 25 amps 120v, which is around 3000 watts. 

Average Amperage Used by Residential Electric Water Heaters  

The standard amperage for an electric water heater is 18.8 amps with 240 volts and 4500 watts. A gas water heater uses a little bit less than that amount. 

Many people wonder, "Can I use a 40 amp breaker for a water heater?”. It’s recommended that you use a 30 amp double-pole breaker for this appliance that isn't connected with any other items. Most often, you're going to want to use a 30 amp.  

However, if your water heater requires more than 30 amps, you’re going to want to change out the breaker. A 30 amp circuit can only handle 30 amps worth of power. If it goes above that, the wires heat up, and damage can occur.

You want to make sure the amount of amps traveling through your system is doing so safely. A 40 amp breaker is better for higher usage water heaters. The system won’t trip until the electrical flow goes above 40. 

Hot Water Type  

Tank Size  


Breaker Size (Amps)  


30 Gallon 


30 amp 


40 Gallon 


30 amp 


50 Gallon 


30 amp 


80 Gallon 


30 amp 

Tankless Electric  



120 amp 

How Many Amps Do RV Water Heaters Use?  

An RV water heater doesn’t use much electricity. It’s very possible to run the system with a 2 amp DC battery. If it's electricity only, you'll likely use more amps- possibly around 15.

It's best to use gas since you can get away with only running the power to get the igniter lit. Once you’ve done that, the system can easily heat water with low effort. 

Our guide to the best tankless water heaters for RVs can be found here.  

How Many Amps Do RV Water Heaters Use

How Many Amps Do Tankless Water Heaters Use? 

Tankless water heaters require a lot more power to heat water than those that have a tank system in place. On average, they take at least 120 amps to operate, which uses most of a home's capacity.  

As a result, you may need to improve your electrical system in the home to make up for the greater demand of these tankless water heaters. 


Does a water heater need a dedicated circuit?  

Water heaters mostly need to have their own circuits. This is true for both tankless and tank water heaters, as they tend to draw a lot of power at one time to heat water- especially the tankless options. 

What wire do I use for a 30-amp breaker?  

You’re going to need a 10 gauge wire for a 30 amp breaker. The circuit breaker can only take one type of size, which protects your electrical system. 

Can I replace a 3500-watt water heater with a 4500 watt?  

Not without upgrading your electrical work. You can replace a lower watt in the same space- but can't do it the other way around. So while you could replace the 4500 with the 3500, you can’t replace the 3500 with a 4500 wattage model. 


In short, you want to make sure that you familiarize yourself with your water tank’s wattage and amp use. Doing so ensures you’re using the electrical systems efficiently and properly at home. 

Josh enjoys researching, testing and diving into home improvement & DIY products. He has a passion for tools, learning new skills and fixing the everyday problems that arise around the house.