Calculating the efficiency of an audio amplifier
This part is unique with no filter and optional dual class D or AB with differential drive , no pops and 4W from 5Vdc into 2 Ohms.  

Since both Pch,Nch  class D drivers are each less than 10% of 2 Ohms the losses are 20% max with 2Ohm speaker and 10% max with 8 Ohms.

Datasheet:
η Efficiency Class D Mode f = 1kHz

RL =8Ω,THD=10%
90%
RL =4Ω,THD=10%
85%
RL =2Ω,THD=10%
80%

Class AB is always much less than 65% depending on peak to RMS crest factor