E7 actually runs over 100 hours on single charge - but it is not because it is power efficient. It is because it doesn't have a lot of output power or as many features (which all need to be powered regardless whether you use them or not). If an amp doesn't output a lot of power, then naturally it doesn't consume a lot of power, as simple as that.
Ok, that's really simple. But for E17, even if you set the volume low, the battery runs out quicker than E7. Why is that?