I seen manufactured published specs on an amp and was curious on something to see if this is common among all amps or just this particular one.

 

They listed the current draw for voltage tested and output power so calculating efficiency was easy.

 

Here is what it shows at a 1 ohm load-

 

14.4v- 78%

16v- 77%

18v- 71%

 

Is this normal performance among 18v amps?

I would think it would go up. That's interesting.

It can't be answered unless every variable was exactly the same for each test. A simple temperature differential could cause that

As an engineer, if efficiency is the primary design objective you'd focus on having the best at the voltage the device was designed to run at. Outside of that scenario is a bunch of compromises.