Electric Power vs Electrical Energy

Most of us have heard the concept of electric power since we all use it (to turn on the lights in a room, charge the computer, heat our food in the microwave oven, etc.) But, what is electric power exactly?

Most of us have heard the concept of electric power since we all use it (to turn on the lights in a room, charge the computer, heat our food in the microwave oven, etc.) But, what is electric power exactly? The most general definition is S = P ± Qj; where S is complex power (measured in kVA), P is real power (measured in kW) and Q is imaginary power (measured in kVAR).

The easiest way to understand the relationship between these three types of electric power is to use the “Beer Analogy” shown below:

Beer consists of a gaseous part and a liquid part; the complex power is also composed of two parts: an imaginary power and a real power. In most applications, it is desired to maximize the real power (or the liquid part of the beer) and to minimize the imaginary power (or the gaseous part of the beer). The apparent power (measured also in kVA) is the magnitude of the phasor of the complex power (S) and it is what the utility companies supply their customers. There are two types of utility companies’ customers: residential and commercial/industrial. The difference between these two types of customers is that residential loads demand mostly real power (P). On the other hand, commercial/industrial loads demand real power (P) and imaginary power (Q). But in the utility bill, either customer is not charged for electric power (whether real or imaginary) but charged in electrical energy consumed. Electrical energy is based on the amount of time by which electric power is used and it is measured in kilowatt-hours (kWh) for real power consumption and in kilowatt-hours (kVARh) for imaginary power consumption.

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