The short answer
Power needed = GPM × 500 × rise ÷ 3,412. For one 2 GPM shower: about 13 kW at a 45°F rise (72°F inlet to a 117°F mixed shower), and about 21 kW at a 70°F rise. A 1.5 GPM low-flow head needs a quarter less.
kW for one shower
| Shower flow | 45°F rise | 60°F rise | 70°F rise |
|---|---|---|---|
| 1.5 GPM | 9.9 kW | 13.2 kW | 15.4 kW |
| 2.0 GPM | 13.2 kW | 17.6 kW | 20.5 kW |
| 2.5 GPM | 16.5 kW | 22.0 kW | 25.6 kW |
Units that fit
Stiebel Eltron Tempra 20 Plus 19.2 kW Electric Tankless Water Heater, Stiebel Eltron Tempra 20 Trend 19.2 kW Electric Tankless Water Heater, Mount Steel MST-TLS18A 19 kW Electric Tankless Water Heater and Stiebel Eltron DHX 15-2 Plus 14.4 kW Electric Tankless Water Heater.
Products mentioned
Whole-house electric tankless · Stiebel Eltron
Stiebel Eltron Tempra 20 Plus 19.2 kW Electric Tankless Water Heater
19.2 kW · 240 V · 3.8 GPM max
Whole-house electric tankless · Stiebel Eltron
Stiebel Eltron Tempra 20 Trend 19.2 kW Electric Tankless Water Heater
19.2 kW · 240 V · 2.0 GPM max
Compact electric tankless · Stiebel Eltron
Stiebel Eltron DHX 15-2 Plus 14.4 kW Electric Tankless Water Heater
14.4 kW · 240 V
Questions readers ask
What size electric tankless for one shower?
About 13 kW in a mild climate and 18 to 21 kW in a cold one, for a 2 GPM head.