How do you size a Generator for a UPS system?

For most installations, using the latest UPS technologies with a high input power factor and low input THDi, it is standard practice to recommend at factor of 1.25:1 (generator:UPS). Older UPS systems with a poor input waveform will need from 1.5 to 2:1 as a sizing factor. It can also be important to known the age of the generator and specifically the characteristics of the alternator/governor arrangement. It is possible to ‘pseudo’ match a UPS to around 1:1 but only if the UPS is down-rated; normally with a firmware change and the battery is sized appropriately.

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How do you size a Generator for a UPS system?