Size a generator from total running watts, the largest motor’s starting watts and a safety margin, with a recommended size.
Recommended generator
How to use this tool
Sum the running watts of everything you will run together.
Its running and starting watts, from its nameplate.
20% in advanced options is the default.
And stagger motor starts in use.
Formula & worked example
3000 W running, largest motor 500 W running / 1500 W starting, 20% margin: 3600 W continuous, 4000 W peak — a 4000 W generator.
Limitations — read this before you rely on it
Assumes one motor starts at a time and a surge rating of about 125% of running watts; altitude, temperature and the exact model’s ratings change real capacity.
Frequently asked questions
Running watts are steady draw; motors briefly draw 2–3× more at start-up, which is the surge the generator must survive.
For extra loads, hot weather, altitude derating and future additions — 20% is a sensible default.
No — this calculation assumes the largest motor starts alone; stagger starts in practice.
You are beyond portable generators; get a standby unit sized by an electrician.
They are quieter and cleaner for electronics, but the wattage maths is the same.