Generator Sizing Calculator
Size a generator from running watts plus motor starting surge.
Generator connection to a building requires a transfer switch and must comply with the NEC. Backfeeding a panel through an outlet is illegal and can kill utility workers. Use a licensed electrician.
How this calculator works
capacity = running watts + (motor hp × 746 × (surge factor − 1))
One horsepower draws about 746 watts running, but three times that for a second or two at startup. Only the largest motor matters — the odds of two large motors starting in the same instant are low enough to ignore.
Worked example
With total running watts 5000 , largest motor 1 , starting surge multiple 3 , this calculator returns 6492 .
Frequently asked questions
What size generator do I need for my house?
Add the running watts of everything you want powered, then add the starting surge of the largest motor. A typical home running essentials with a 1 hp well pump needs about 6,500 watts.
Why do motors need extra starting watts?
An induction motor draws three to six times its running current for the first second as it overcomes inertia. A generator that only matches running load will stall or trip when the motor kicks in.
Do I add surge for every motor?
No, only the largest. Motors rarely start simultaneously, and sizing for that coincidence would double the generator cost for a scenario that almost never occurs.
What is the difference between running and peak watts?
Running or rated watts is what a generator sustains indefinitely. Peak or starting watts is a brief surge capacity lasting a few seconds. Always size against the running figure, never the peak on the box.