When it comes to energy efficiency, just think in terms of light bulbs Dom.I must admit I know nothing of the different classes. 😁
The old filament light bulbs used a lot of mains power and most of that was wasted as heat.
LED lightbulbs use much less power, because they don' t waste much as heat.
Think of Class A amps as filament bulbs and class D amps as LED.
(However, plenty of people prefer the light output from filaments - and plenty of people prefer the sound output of class A amps).
Your amp is probably class A/B?
So its energy efficiency will be somewhere between class A and D.