How many amps does a tankless water heater use? (42-167 amps)

Electric tankless heaters use electricity exclusively to heat water. As we know, heating water is a very demanding job. Therefore electric tankless water heaters require a strong electric current; In other words. How many amps do amps tankless water heaters use?

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

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

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

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

I (amps) =p (Watt) / v (volt)

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 electric power equation like this:

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

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

We have calculated amp draw for tankless water heaters 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 of these calculations, we assume that the heater is operating on a 240-volt circuit.

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

Using this amp draw table, you can accurately estimate how many amps you use a tankless hot water heater. You can then decide how many amp breakers you will need for the installation.

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


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