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I was thinking it may be possible to establish the max efficiency achievable with an opa in the same way you can define RF transistor efficiency without any values assuming the operating bias point.
As an example,
the max theorical efficiency for a Class B amplifier is 78,5%
Class A amplifier is 50%
I thought class C was 78, class B was 50 and class A was 25%? Anyway, if you have a low power opamp with a high current output stage then I guess you could approach class B efficiency at maximum output swing.
An ideal rail-to-rail OP in class B (zero output bias current) also achieves 78.5% with sine waveform, it's basic mathematics. A real OP achieves less. You can calculate the efficiency by putting in the maximal output voltage and bias current.
I guess my memory is failing! It is a long long time since I have looked at power amplifier efficiency. It seems 25% is for a resistive load class A, 50% for inductive class A and 78% for class B, I think, although the 50% is probably only applicable to RF amplifiers.
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