How do you find capacity of transformer?

If the load is single equipment, calculating the capacity of the generator or transformer is fairly straight forward. If the load is mix of various several different equipment it would be more complicated to arrive at transformer / Generator capacity. Complexity increases if you have to estimate some future unknown loads.

The rating of the generator or transformer should be such that it is loaded optimally, average being say 80 to 95 %. Since maximum efficiency of generator or transformer generally occur at these range. Also this ensures that it is not overloaded continuously and there is room for occasional short time full load too. You need to check the data sheet of the transformer or generator for more correct information.

One need to know the maximum load or peak load, average load and diversity factor and load factor. These values drive the decision on the rating of the transformer or Generator.