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.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- NIST Special Publication 811 2008 — the electrical horsepower as exactly 746 watts
- NFPA 70 (National Electrical Code) 2023, Article 445 — generator conductor and overcurrent requirements
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
Worked example
With total running watts 5000 , largest motor 1 , starting surge multiple 3 , this calculator returns 6492 .
Generator size for common backup loads
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Generator capacity needed |
|---|---|---|
| Fridge, lights and a furnace fan | 2000 × 0.5 × 3 | 2,746 |
| Adding a well pump | 3500 × 1 × 3 | 4,992 |
| Most of a house, no A/C | 5000 × 1 × 3 | 6,492 |
| House with central air | 7000 × 3.5 × 3 | 12,222 |
| Job site — saw and compressor | 4000 × 2 × 3 | 6,984 |
Only the largest motor counts. Two big motors starting in the same instant is a coincidence you can plan around.
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.