Start / Terms / Turbo Compressor Efficiency
Compressor efficiency is a measure of how much boost a compressor makes as opposed to how much heat it creates.
If a compressor were 100% efficient, it would create boost without heating the air at all. (It would also break the laws of physics.) A compressor that is 75% efficient uses 75% of its input power to create boost, and the other 25% of the power that is driving the compressor ends up heating the air as it is being compressed. This heating is the result of friction between the compressor wheel and the air.
You should be able to get compressor efficiency specifications from your turbocharger manufacturer. If you cannot because it is a factory, car manufacturer-installed turbo, you can estimate 75%.