Section 5
Electric whole-house tankless water heaters: power, amps and flow
This table lists power, amp draw, breakers, main service size and flow rate for 12 electric whole-house tankless models from Stiebel Eltron and EcoSmart. Power ranges from 8 to 36 kW. Flow is listed at a 63 F rise and a 43 F rise.
| Manufacturer | Model | Power | Amp draw | Breakers | Minimum main service | GPM at 63 F rise (42 F water) | GPM at 43 F rise (62 F water) |
|---|---|---|---|---|---|---|---|
| Stiebel Eltron | Tempra 12 Trend / Plus | 12 kW | 50 A | 1 x 50 A | 100 A | 1.3 | 1.9 |
| Stiebel Eltron | Tempra 15 Trend / Plus | 14.4 kW | 2 x 30 A | 2 x 30 A | 100 A | 1.6 | 2.3 |
| Stiebel Eltron | Tempra 20 Trend / Plus | 19.2 kW | 2 x 40 A | 2 x 40 A | 125 A | 2.1 | 3.1 |
| Stiebel Eltron | Tempra 24 Trend / Plus | 24 kW | 2 x 50 A | 2 x 50 A | 150 A | 2.6 | 3.9 |
| Stiebel Eltron | Tempra 29 Trend / Plus | 28.8 kW | 3 x 40 A | 3 x 40 A | 200 A | 3.2 | 4.7 |
| Stiebel Eltron | Tempra 36 Trend / Plus | 36 kW | 3 x 50 A | 3 x 50 A | 300 A | 4.0 | 5.8 |
| EcoSmart | ECO 8 | 8 kW | 33 A | 1 x 40 A DP | Not listed | 0.8 | 1.2 |
| EcoSmart | ECO 11 | 13 kW | 54 A | 1 x 60 A DP | Not listed | 1.4 | 2.0 |
| EcoSmart | ECO 18 | 18 kW | 75 A | 2 x 40 A DP | 150 A | 1.9 | 2.8 |
| EcoSmart | ECO 24 | 24 kW | 100 A | 3 x 40 A DP | 200 A | 2.6 | 3.8 |
| EcoSmart | ECO 27 | 27 kW | 113 A | 3 x 40 A DP | 200 A | 2.9 | 4.2 |
| EcoSmart | ECO 36 | 36 kW | 150 A | 4 x 40 A DP | 300 A | 3.9 | 5.7 |
Sources: power, amp draw, breakers and service size come from Stiebel Eltron's technical specifications[1] and EcoSmart's product pages[2][3]. Flows follow the method in section 2. EcoSmart's ECO 8 and ECO 11 pages list no panel size.
5.2 Model size by region
An electric tankless heater turns almost all of its rated power into heat in the water, so its flow at any rise follows from its kW rating. In the north, one 2 GPM shower at 105 F needs a rise of about 63 to 68 F. At a 68 F rise, only the 24 kW models and larger deliver 2 GPM. At a 63 F rise the 19.2 kW Tempra 20 also delivers it. In Gulf states the same shower runs on 13 to 18 kW. A one-bath ADU on a 100 A subpanel therefore gets a different result by region, and the sizer starts with the state.
Stiebel Eltron's guide says its recommendations assume 240 V service, and that the next model size up applies at 208 V[4]. Apartment-style buildings and some commercial services run at 208 V.
5.3 Electrical requirements
The amp draw, breaker and service sizes above are the manufacturers' numbers. Stiebel Eltron's sheet ends with "These are our recommendations. Check local codes for compliance if necessary."[1] Whether a panel can carry 100 A or more of new load is a load calculation for a licensed electrician. Section 10 lists the published service sizes side by side.