What size generator to run a tankless water heater (electric)?
Tankless water heater (electric) sizing at a glance: it draws 18000 running watts.
| Spec | Value |
|---|---|
| Running watts | 18000W |
| Starting (surge) watts | 18000W |
| Recommended class (this load + margin) | 22000W |
Why the startup surge matters
A tankless water heater (electric) is a resistive load, so it has no meaningful startup surge. If you size a generator only for the running watts, it can stall the instant the tankless water heater (electric) switches on. Always size for the surge.
Sizing the whole system
The number above covers the tankless water heater (electric) on its own with a safety margin. In a real outage you'll run other things too — and the right method is your total running load plus the single largest surge among everything, not the sum of every surge. Use the full calculator to size your actual setup:
Common questions
How many watts does a tankless water heater (electric) use?
A typical tankless water heater (electric) draws about 18000 watts, with no significant startup surge.
What size generator runs a tankless water heater (electric)?
For the tankless water heater (electric) alone plus a safety margin, a 22000W (≈22.0kW) class generator is appropriate. Add your other essential loads to size the full system.