[Amps] Transformer Efficiency Question?

[Colin Lamb]

I have a 6.5 KW 120 volt Onan generator.  I have been told that because it uses a transformer as part of a regulator system that it is not recommended to reconnect to produce 240 output.  

Question: What sort of efficiency am I looking at to use a 120 to 240 transformer?  Is there a particular type that might be more efficient?  Would I be looking at 15% losses?

Note:  This is an amps question, because I may run my 240 amplifier off the generator.  

73,  Colin  K7FM