In this chapter, the reader is introduced to concepts of power, energy, and efficiency. Direct current (DC) power and efficiency discussion sets the stage for the more complex concepts of single-phase alternating current (AC) power and AC energy. And, discussion of single-phase AC serves as a suitable segue for consideration of the more complex subject of three-phase AC power. Significance of efficiency and energy is explained within the context of DC and AC powers. Once the basic understanding of AC and DC powers is clear, the reader is better poised to examine the flow of power and energy from the point of generation to the point of consumption. In this chapter, the flow of power and energy from some common sources such as steam and water to electricity is described, with illustrations in the form of example problems and self-assessment problems. The flow of power from utility to pumps is discussed in the “Wire to Water” section. Conversely, the flow of power and energy from water to wire and steam to wire is illustrated through hydroelectric and steam turbines, respectively.