Power, Energy, and Battery Life
The terms "power" and "energy" are often used interchangeably. Although these terms are related, they describe different concepts. Power consumption is the rate at which a device consumes energy. In other words, if a device consumes a fixed amount of power P over time t, then energy = P x t. An obvious (but important) point is that energy consumption can be reduced by minimizing P, t, or both.
While it is common to hear discussions about low-power design, in general low-energy design is a more useful concept, because in most systems energy is the fundamental resource that must be conserved. Batteries have a fixed nominal energy capacity, and thus battery life depends primarily on the energy consumption of a system. Although energy consumption is the primary determinant of battery life, power consumption may also play an important role. For example, lithium ion batteries are less efficient and deliver less total energy when discharged too quickly, so designers must sometimes consider limiting the power consumption of a system in order to maximize battery efficiency.