Electric tankless sizing
Kilowatts for the flow you want, and the amperage that usually decides it.
Electric tankless units are cheap to buy and expensive to install, and the reason is amperage. The unit is not usually the problem. The electrical service is.
What each unit draws
| kW | Amps at 240 V | GPM at 40 F rise | GPM at 70 F rise |
|---|---|---|---|
| 11 | 46 | 1.9 | 1.1 |
| 14 | 58 | 2.4 | 1.4 |
| 18 | 75 | 3.1 | 1.8 |
| 24 | 100 | 4.1 | 2.3 |
| 27 | 113 | 4.6 | 2.6 |
| 36 | 150 | 6.1 | 3.5 |
A 27 kW unit draws 113 amps. On a 200 amp service that is more than half the whole house, and it needs multiple double-pole breakers and heavy cable. On a 100 amp service it is simply not possible without an upgrade, and a service upgrade commonly costs more than the heater.
Where electric tankless does work
- Point of use. A small unit under a sink, sized for one fixture, is a genuinely good application and draws a manageable current.
- Warm climates. At a 40 degree rise the same kilowatts deliver far more flow, so a southern house can be served by a unit that would fail in the north.
- Where there is no gas. Sometimes the alternative is a tank, and the comparison is different.
Gas and electric, compared at the same job
3.6 GPM at a 80 F rise:
Energy required144,209 BTU per hour
Gas unit at 0.93 efficiency155,063 BTU input
Electric equivalent42.3 kW
Current that draws at 240 V176 amps
Frequently asked questions
How many amps does an electric tankless water heater use?
A lot. An 18 kW unit draws 75 amps at 240 volts and a 27 kW unit 113 amps. Whole-house electric tankless commonly needs 150 to 200 amps of dedicated capacity, which is why a service upgrade is often part of the quote.
Can I run an electric tankless water heater on a 100 amp service?
Not a whole-house one. A small point-of-use unit for a single sink is feasible, but a unit capable of a shower at a winter temperature rise will exceed what a 100 amp service can give it alongside everything else in the house.
Is electric or gas tankless better?
Gas handles a larger temperature rise for a given installation cost, which matters in cold climates. Electric is cheaper to buy, simpler to vent because it does not vent, and well suited to point-of-use or warm climates. The deciding factor is usually the electrical service you already have.