The short answer
Power in a heating element is voltage squared over resistance, so a 240-volt element on 208 volts delivers (208 ÷ 240)² = 75% of its rating. A 27 kW unit becomes about 20 kW, and its flow at any rise drops by a quarter. Stiebel Eltron's sizing guide says to increase one model size when a 240-volt unit is installed on 208-volt service. Apartments and some commercial buildings run 208 volts.
Rated vs actual
| Rated at 240 V | Delivers at 208 V | Amps at 208 V | GPM at 45°F rise |
|---|---|---|---|
| 13 kW | 9.8 kW | 47 A | 1.5 |
| 18 kW | 13.5 kW | 65 A | 2.1 |
| 24 kW | 18.0 kW | 87 A | 2.7 |
| 27 kW | 20.3 kW | 98 A | 3.1 |
| 36 kW | 27.0 kW | 130 A | 4.1 |
Products mentioned
Questions readers ask
Can I use a 240V tankless water heater on 208V?
Many are rated for both, but output falls to about 75%. Size one model up, and have an electrician confirm your service voltage.