The quality of electrical energy, or simply the quality of energy, involves voltage, frequency, and waveform. Good power quality can be defined as a constant supply voltage that remains within the prescribed, constant range a. Frequency close to the nominal value and smooth voltage curve (resembles a sine wave). In general, it is useful to consider the quality of energy as the compatibility between what comes out of an electrical outlet and the load that is plugged into it. The term is used to describe the electrical energy that drives an electric charge and the ability of the charge to function properly. Without proper power, an electrical device (or load) may malfunction, fail prematurely, or fail to function at all. There are many ways in which electrical power can be of poor quality and many more causes of such poor quality power.
The electric power industry comprises the generation of electricity (alternating current), the transmission of electrical energy and, ultimately, the distribution of electrical energy to an electricity meter located at the end-user's premises. The electricity is then moved through the end user's wiring system until it reaches the load. The complexity of the system to move electrical energy from the point of production to the point of consumption, combined with variations in time, generation, demand and other factors, provide many opportunities for quality of supply to be compromised.
While "energy quality" is a convenient term for many, it is the quality of the voltage - rather than the electric or electrical current - that is actually described by the term. Power is simply the flow of energy and the current demanded by a charge is largely uncontrollable.