Amps = watts ÷ volts. At 240 volts: 8 kW is 33 A, 13 kW is 54 A, 18 kW is 75 A, 24 kW is 100 A, 27 kW is 112.5 A and 36 kW is 150 A. Makers split large units across several double-pole breakers and state the breakers and wire in their manuals and listings. A licensed electrician confirms the panel can carry it. Run your unit through the calculator.
Amps by size
| Rating | At 240 V | At 208 V (75% of rated power) |
|---|---|---|
| 3.5 kW | 15 A | 13 A |
| 6 kW | 25 A | 22 A |
| 8 kW | 33 A | 29 A |
| 11 kW | 46 A | 40 A |
| 13 kW | 54 A | 47 A |
| 18 kW | 75 A | 65 A |
| 24 kW | 100 A | 87 A |
| 27 kW | 112 A | 98 A |
| 36 kW | 150 A | 130 A |
What the makers ask for
Listings in our catalog state the breakers directly, for example Atmor AT-36N-XTP 36 kW Electric Tankless Water Heater. Follow the exact model's manual: it sets the number of circuits, breaker sizes and wire gauge.
On 208-volt service
A heating element rated at 240 volts delivers (208 ÷ 240)², about 75%, of its rated power on 208-volt service, so it heats less water. Stiebel Eltron's sizing guide says to increase one model size when a 240-volt unit goes on 208-volt service.
The panel question
A 36 kW unit at 150 amps is three quarters of a 200-amp service on its own. Whether the service, panel and wire can carry it with heating, cooking and drying loads is an electrician's calculation under the electrical code, not a number on a listing.
Products mentioned
Atmor AT-36N-XTP 36 kW Electric Tankless Water Heater
36 kW
Eemax EEM24036 36 kW Electric Tankless Water Heater
36 kW · 240 V
Stiebel Eltron Tempra 20 Plus 19.2 kW Electric Tankless Water Heater
19.2 kW · 240 V · 3.8 GPM max