It is important to remember that the terms "power" and "energy" are related to physical energy. In many systmes we will be interested in examining power and energy in signals over an infinte time interval.
Thos signals for which Einf<inf must have zero average power.
If Pinf>0, then Einf = inf. This makes sensce because there is a nonzero average energy per unit time, then integrating or summing this over an infinite time interval yields and infinite amount of energy.
