Step one: list what runs at the same time
Write down every load, its running watts, and whether it will genuinely be on at the same time as the others. Nameplate data on the appliance or tool is the reliable source; catalogue estimates are not.
Total only the concurrent loads. Summing everything you own produces a generator far larger and more expensive than the job needs.
Step two: add the largest starting surge
Motors — compressors, pumps, air conditioners, hoists — draw a brief surge on startup, often three to seven times their running figure. Take the single largest surge on the list and add it to your concurrent running total.
Only the largest matters, provided motors do not all start simultaneously. If they do, stagger them or size for the combined surge.
Step three: leave headroom
Around 20 percent headroom above the calculated figure covers measurement error, altitude and temperature derating, and the load nobody mentioned until day three.
Running a diesel engine at a very low fraction of its rating for long periods causes its own problems, so oversizing dramatically is not a free choice either.
When to hand the sizing to someone else
If the job involves three-phase equipment, transfer switching, medical equipment or anything connected to a building panel, give the provider your load list and let their engineer confirm the size.
Choosing 'help me determine the size' on a quote request is a normal answer. Sizing is the single most common reason people contact us.