How Many Amps Does A Tankless Water Heater Use? (42-167 Amps)

Electric tankless heaters use exclusively electricity to heat up water. As we know, heating up water is a very demanding task. That’s why electric tankless water heaters require a strong electricity current; ie. amps. How many amps do tankless water heaters use?

Tankless heaters can draw anywhere from 42 amps to 167 amps.

We can calculate exactly how many amps does a tankless water heater because we know two key factors:

  1. Overall power or wattage. Electric tankless water heaters can have a power input from 10,000W (10 kW) to 40,000W (40 kW).
  2. Voltage. To reduce the amp draw, all electric tankless water heaters operate on a 240-volt circuit.

Using the electric power equation (P = I×V), we can calculate the by expressing the I or electric current like this:

I (amps) = P (watts) / V (volts)

Example: Let’s say we have a 30 kW tankless water heater on a 240-volt circuit. How many amps does a 30 kW tankless water heater use? We use the electrical power equation in this way:

I (amps) = 30,000W / 240V = 125 amps

Such a heater draws 125 amps (you need several 20-amp or 30-amp breakers for installation).

We have calculated amp draws for tankless water heaters ranging from 10 kW to 40 kW and summarized them in the table here:

How Many Amps Do 10-40 kW Electric Tankless Water Heaters Draw?

In all these calculations, we presume that the heater is running on a 240-volt circuit.


Tankless Heater Size (in kW) Electrical Current (in Amps)
10 kW 42 amps
11 kW 46 amps
12 kW 50 amps
13 kW 54 amps
14 kW 58 amps
15 kW 63 amps
16 kW 67 amps
17 kW 71 amps
18 kW 75 amps
19 kW 79 amps
20 kW 83 amps
21 kW 88 amps
22 kW 92 amps
23 kW 96 amps
24 kW 100 amps
25 kW 104 amps
26 kW 108 amps
27 kW 113 amps
28 kW 117 amps
29 kW 121 amps
30 kW 125 amps
31 kW 129 amps
32 kW 133 amps
33 kW 138 amps
34 kW 142 amps
35 kW 146 amps
36 kW 150 amps
37 kW 154 amps
38 kW 158 amps
39 kW 163 amps
40 kW 167 amps

Using this amp draw table, you can accurately estimate how many amps does you tankless hot water heater use. You can then decide about how many amp breakers you’ll need for installation.

If you’re in the market for a tankless water heater, you can check our list of the best tankless units here.

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