As a general rule, Utilities will size a distribution transformer at 1.2-2HP connected per KVA of transformer capacity, depending upon the anticipated load factor or load diversity of the load. More information is needed to accurately size a transformer, but a liquid filled transformer should be able to serve its rated KVA in load 24x7 assuming a specified ambient temperature and a specified wind. It only cares that its rated hot spot insulation temp is not exceeded.
I commonly see 20-40DegC ambient specs and 2-4MPH wind specs.
Losses are commonly broken into no load (core) losses, and load (conductor) losses. Since conductor losses vary as the square of the current, a reduction from 1PU current to .8PU will reduce load losses from 1^2 to .8^2, or 100% to 64%. Load losses usually account for 70-85% of total losses depending on the transformer design. Core losses are pretty constant 24/7.
You are on the right track. Load lose only have to do with is the efficiency at those given load points. And, yes, both the insulation class and the hot spot as allowed are the target points. I included some of the following in one of my posts a few weeks ago or so using a dry type transformer as an example. No matter what the kva rating at a given temperature rating is, 80degC, 115degC, or 150degC one thing will always stay the same is the 220degC insulation class and the 30degC hot spot allowance.
What will vary is the ambient temperature up to 220degC insulation class less the hot spot allowance of 30degC which is 190degC. No matter what the 190degC should never be exceeded. Now take the 190deC-150degC rated transformer and you end of with 40degC which is the max ambient temperature. Reduce the ambient temperature and the transformer is capable of some overload without exceeding the 190degC. A transformer that is rated 115degC has the advantage of 150-115=35degC. As such its rating starts 35degC lower than the 150 so you have 35degC to work with which equated to the ability to be overloaded 15% w/o exceeding the 190degC target. Get get that ambient down some such as 25degC and you gain some more overload capacity.
The, is you start with transformer rated a 80degC....well you should understand how it works. Yes, you get a basic 30% overload capacity w/o playing with the ambient temperature.
It used to be that you could expect the transformer life expectancy to be some 30year and sometime more. Today, however, not as much overload capacity is specified and the transformers run warmer for longer periods of time. Transformers running a close to that 190degC mark are expected to only last maybe 3-5 years.