You need to look at the the alternator rating (kVA), and the engine rating (kW). Alternators are traditionally rated at 0.8 PF, so we have a relationship between kW and kVA.
You may have to deduct the power needed to drive the radiator fan from the engine power (kW).
However, it also depends on the type of load. Straight linear loads, you should be able to use up to 100 %. Rectifier or UPS loading is a different story. This type of non-linear load causes voltage distortion, which can cause problems for the load, and for the alternator AVR, of which there are various types, some of which are more suited to non-linear loads than others.
There are many other relevant points, overload, motor starting, voltage dips, effect of one load on another, etc., etc.
Is the room adequately cooled ?
Have you asked the genset supplier ?