I want/need an isolation transformer, but they are not especially cheap. And as I have many other power transformers lying around this place, I should use two of those in a back to back configuration if it will provide a viable result. But I was concerned about voltage "sag" at the output under load, and temperature rise.
So I tested three handy pairs of transformers, using light bulbs as a dummy load, and with a variac to adjust the input voltage for a 240V output at the load. I was going to do power and efficiency calculations, but found the temperature rises to be quite small, and so just went with the "Input-voltage-for-240V-output" as the metric to compare them with.
Here are the transformer pairs used, with estimated VA ratings of 90VA, 150VA, and 2KVA, with weights of 2Kg, 4.5Kg and 22Kg.
Those very large ones served as a reference as what the very best I could come up with would be.
Here are the results of the testing:-
I have decided to make a general purpose isolation transformer box using the 150VA pair.
And I'll attach handles to the large ones, along with some connector arrangement to enable them to be quickly configured for any purpose, including as an isolation transformer, but still allowing them to be general purpose high power units.
Video:-