Efficiency is not affected by insulation , voltage rating or primary voltage.

Efficiency is affected by winding resistance and Leakage Inductance or it's equivalent Coupling ratio with primary inductance.

There are also eddy current losses.

So it has more to do with copper losses and magnetic coupling ratio which becomes more important with higher power (MVA) so the design of those tends to be > 99%.

Since Excitation current is mostly reactive, it is not true power lost . This VAR excitation current is needed to coupling the magnetic fluxes in primary and secondary and may be as much as 10% primary current. This has some primary loss with no load.

Efficiency is not affected by insulation , voltage rating or primary voltage.

Efficiency is affected by winding resistance and Leakage Inductance or its equivalent Coupling ratio with primary inductance.

There are also eddy current losses.

So it has more to do with copper losses and magnetic coupling ratio which becomes more important with higher power (MVA) so the design of those tends to be > 99%.

Since Excitation current is mostly reactive, it is not true power lost . This VAR excitation current is needed to coupling the magnetic fluxes in primary and secondary and may be as much as 10% primary current. This has some primary loss with no load.