Genset Rating Calculation and Feasibility

Derating factors and efficiency are different concepts, as recently explained on another thread. In your example, the engine's efficiency (as well as capacity) will be reduced, but probably not by so low as the 85% factor. If you specify the altitude to the vendor, they may adjust the engine tuning and can also provide an oversize radiator. The 100 KVA selection sounds right.

Yes! Your calculation is theoretically correct. We are at an altitude of 2000m. Our DG set requirement is 75kVA; we have gone for a 125kVA DG set. You should go for a 100kVA DG set. The de-rated genset would not cause any overheating as long as the load is within the limits. And, yes, partially loaded DG sets have lesser efficiency. This has nothing to do with altitude. Even at a lesser altitude, partially loaded DG Sets would have lesser UPL (Units per litre).

Derating Factor - CR4 Discussion Thread (globalspec.com)

The manufacturer of the <...diesel genset...> is not on CR4 right now. Please pick up the telephone and ask these questions directly.

Your genset should be operating in a range of temperatures, and that will also effect what might be needed for smooth reliable operation....first you didn't say if this a turbo-charged diesel or not...I would assume it is, this will increase the air supply compensating for the altitude somewhat....in any case the engine must be tuned properly as far as timing and fuel mixture, and special consideration for starting might be necessary to preheat the glow plugs and intake...the duty cycle is also important...Each brand and model is different and requires experienced personnel to properly setup...

https://www.dieselgeneratortech.com/generator-sets/Guides-to-Use-Diesel-Generator-Set-In-Plateau-Areas-and-High-Altitude.html

Ambient temp is low so rise can be higher as most alternators have 40 Deg ambient .Deration is mainly for altitude as standard machines are rated for 1000 m and below.I would suggest you to go in for 110 kva which is a standard rating than 100 which is non standard rating or as we call it non preferred rating .You should take the lowest temp possible from the point of bearing lubrication and also from engine side factors.Is 18Deg C throughout the year?