As NYOJ says make sure that you account for transient loads such as starting currents, particularly if you have one or more loads which consttitute a significant proportion of the generator capacity.
Another point to consider is harmonic current. Today, installations often include many non linear loads such as Variable Speed Drives, switched mode power supplies etc. These all produce harmonic currents. You will need to measure, or make an assessment of, the harmonic current content in the system and advise the generator supplier of this as he may need to make an allowance for the additional heating these will cause in the generator.