Water Heater Wattage: How Many Watts Water Heater Uses?

In a lot of cases, it is useful to know exactly how many watts water heaters use. If you want to run a water heater using a generator or estimate electricity costs, you will need to know the water heater wattage.

How to check how many watts does your electric water heater draw?

Just check the label on the tank. You will find the specified wattage for your water heater, ranging from 1000 watts to over 10,000 watts if you have a very big hot water tank.

Note: If you find 2 wattages (4500W and 4500W, for example), this doesn’t mean water heaters require 9000 watts of electric input to run. These are water heaters with two heat exchangers (to increase efficiency expressed as UEF) but it is important to understand that both don’t come on at the same time. You will still only need a generator that can generate 4500 watts to run such a water heater.

Even if you don’t find the label, you can quite easily determine water heater wattage by boiler capacity like this:

A lot of water heaters actually use the same heater size: 4500 watt heater. This is best illustrated with roughly 3 categories of water heaters:

1. Smallest 10-gallon and 20-gallon heaters use anywhere between 1000 watt and 2000 watt heaters that run on a 120V circuit. A 1600 watt 120V heater is the standard heater for these smaller water tanks.
2. Majority of water heater sizes (30-gallon, 40-gallon, 50-gallon, 60-gallon, 70-gallon, and 80-gallon) use the standard 4500 watt heater that runs on a 240V circuit. That means that the most common water heater wattage is 4500 watts.
3. Biggest 90-gallon, 100-gallon and 100+ gallon heaters can use high-wattage heaters beyond 4500 watts. It is not uncommon to see a 10,000 watt heater for a 100-gallon water tank.

Here is the full table of how many watts do hot water boilers use when operating at 100% heating output:

Water Heater Wattage Chart

As you can see, most water heaters will require 4500 watts to run at 100% output.

Example: How many watts does a 50-gallon water heater use?

If you check the table above, you can see that the standard 50-gallon water heater uses 4500 watts of electricity.

Even smaller 30-gallon and 40-gallon heaters will use 4500 watt heaters. However, 20-gallon water heater will use anywhere from 1000 watts to 2000 watts of electricity.

This has an interesting effect:

Water in smaller 20 gallon water heater will warm up slower than in the bigger 30 gallon water tank. That’s because the difference in amount of water is 50% but the difference in the electric heating input is more than 100% (2000 watts vs 4500 watts).

It is also important to note that standard 4500 watt heaters run on an upgraded 240V circuit. The smallest water heaters (10-gallon and 20-gallon) usually require only the standard 120V circuit.

We hope this adequately illustrates how many watts do water heaters use. Based on the water heater wattage you can also size a generator for running a water heater in the event of a power outage.