We have two chillers at work to cool the offices.
We know the start current and running current and they are star delta start.
How do we calculate the KW rating of the motor so we can size a gen set to keep them running during a power cut?

Hello normanundercroft

Welcome to the forum.

It is not really practical to calculate the KW rating of a motor from it's running current and start current.
To size your generator, there are two major considerations in terms of the load side.
The alternator is loaded by KVA (Amps) and the engine is loaded by KW.

To size your generator, measure the continuous KW drawn by the motors, and the peak KW drawn during start. This will determine the continuous rating of the engine and the peak rating of the engine.
You need to make an allowance for the KW losses in the alternator and cables during start.
Some engines have a good overload margin such as 300% for 10 seconds, while others will only allow 120% for 10 seconds.

Measure the starting current and the run current of the motors and calculate the KVA (V x I x rt3 / 1000). This will determine the continuous rating and the overload rating of the alternator.
If the alternator is fitted with permanent magnet excitation and a three phase averaging AVR, it is possible that the alternator may be able to withstand 300% for 10 seconds or even more. The more common single phase peak AVR and self excitation will probably only allow an overload of 20% (120% load) for 10 seconds or less.

The problem with the star delta starter for this application, is that there is a high current transient as the starter switches from star to delta. This can cause problems with heavily loaded generator sets.
I would suggest that a soft starter may be a better option.

Best regards,
Mark.