What size generator to run a tankless water heater?
Electric tankless water heater draws about 18,000W while running, and being a resistive load it has no startup surge. Allowing 20% headroom, size for at least 21,600W of starting capacity.
Effectively impossible to run on a portable generator. Gas tankless units need only their ignition and controls, at about 100W.
Units that cover it
- Generac GP17500E
26,250W starting · 46% headroom
Comfortable headroom without overpaying for capacity.
Check price - Westinghouse WGen14500DF
18,000W starting · 0% headroom · $2,749
Sized close to your load with a little room to spare, runs on propane as well as gasoline.
Check price
Running more than one thing
Most people are not powering one appliance. Add everything you need at once and the calculator works out the real peak — your total running load plus the single largest surge, not the sum of every surge.
Size the whole load