1. List and total your critical loads
Make a simple load list of the machines and services you really must keep running during an outage: key production lines, air compressors, pumps, HVAC, lighting, IT and safety systems.

Write down the kW or HP of each item. Convert HP to kW using kW ≈ HP × 0.746. Add the kW values of all critical loads to get your total running demand.

2. Convert to kVA and add margin
Most industrial generators are rated in kVA. Divide total kW by power factor (often 0.8) to estimate the kVA you need.

Example: 200 kW of critical load ÷ 0.8 ≈ 250 kVA.

Then add 20–30% margin for motor starting, voltage stability and future machines.

Example: 250 kVA × 1.25 ≈ 310 kVA → you would usually choose a 315–350 kVA set instead of 250 kVA.

3. Pay attention to motors and starting sequence
Large motors (compressors, pumps, cranes) draw 3–7 times their running current at startup. If several start together, they can pull the generator voltage down.

Try to start big motors one by one, and use soft starters or VFDs where possible. This often lets you use a smaller generator than a “worst‑case all at once” calculation.

4. Check rating type and growth
Decide whether you need a prime‑rated generator for continuous use or a standby‑rated unit just for emergencies. Then think about the machines you may add in the next 2–3 years and include some capacity for that.

For many factories, the final choice is one size above the minimum kVA from the calculation, to avoid overloading when the plant grows.

5. When to get professional sizing help
If your factory has several large motors, sensitive CNC equipment, or you plan to run multiple generators in parallel, it is worth asking a generator engineer to review your load list and confirm the kVA and configuration.

A short consultation can prevent expensive mistakes like chronic overload, nuisance trips, or buying an oversized unit that wastes fuel.