How Much Electricity Does A Tankless Water Heater Use?

Electric tankless water heaters require a large amount of electrical power (when running). Surprisingly so, they are very energy efficient and economic. We will look into exactly how much electricity does a tankless water heater use (by using the power equation P = I * V and cost equation Cost = kWh * t (h)).

   Heating water is one of the most power-hungry processes. Tankless heaters heat up the water in seconds (5 seconds from cold to hot water). To achieve that, we need an incredible amount of electricity.

Does that mean that tankless heaters require a lot of electricity?

   Yes and no. When they are ‘on’, electrical tankless heaters require up to 27,000W of electricity. For comparison, an average washing machine will require a bit more than 1,000W to run (and it will run for 1-2h). However, electric tankless water heaters are not running all the time. All this energy is used to heat up water only when we use hot water.

   In fact, even according to the US Department Of Energy, tankless water heaters can significantly reduce monthly electricity bills:

“For homes that use 41 gallons or less of hot water daily, demand water heaters can be 24%–34% more energy efficient than conventional storage tank water heaters.” (DOE on energy efficiency of tankless water heaters)

   Most households use less than 41 gallons of water per day. 24% – 34% is quite a lot of energy savings. Electricity-wise, tankless water heaters are a smart financial investment (they also last for 10-15 years).

   But how can such a power-hungry appliance reduce the electricity bill?

   It all comes down to how many amps does a tankless water heater draw (since the voltage is always the same: 220/240V). Let’s calculate a bit to see how much electricity does an on-demand tankless hot water heater use:

   Calculating the maximum running costs of tankless water heaters is quite easy. The key information here is how many watts does a tankless water heater use. We’ll do one quick example here, with more to follow.

Example: You have a 20kW electric tankless heater. How much does it cost to run it?
Solution: 20 kW heater will spend 20 kWh worth of electricity if you run it for 1 hour. The average kWh price in the US is $0,1319. That means a 20kW tankless heater will spend, on average, 20 kWh x $0,1319 per kWh = $2,64 per running hour.

   That doesn’t sound a lot, right? Obviously, there are many factors that come into play like intake water temperature, hot water demands, and so on. For a 20kW tankless water heater, the $2,64 per hour is the maximum electricity usage (running at 100% heating output). You will likely run your heater 100% output for the first 15 seconds, and then the heating output percentage (and hence water heater costs) drops.

Now, the real question is this:

   How many hours per day do you run the tankless water heater? Here is how much it costs to run a 20kW electrical tankless heater from our example for different time periods:

   * 20 kW running at 100% output for 10 minutes (shower): $0,66.
   * 20 kW running at 100% output for 30 minutes: $1,32.
   * 20 kW running at 100% output for 1 hour: $2,64.
   * 20 kW running at 100% output for 2 hours: $5,28.
   * 20 kW running at 100% output for 5 hours: $13,20.

Be aware that we always predispose:
1) Price of electricity to be $0,1319 per kilowatt-hour.
2) Power of heater is 20kW.
In this case you should replace the electricity price of your market and the power your purchased.