Electrical power in alternating current (AC) systems is measured using the kilowatt (kW) and the kilovolt-ampere (kVA). Both units represent electrical power capacity, but they measure distinct aspects of the power flowing through a circuit. Understanding this difference is important for properly sizing and operating electrical devices, such as generators, transformers, and Uninterruptible Power Supplies (UPSs).
Real Power (kW) and Apparent Power (kVA)
Kilowatt (kW) is the unit of measure for “Real Power,” also called active or working power. This is the electrical power converted into useful output, such as heat, light, or mechanical motion. Real power performs the work, is consumed by the load, and is what utility companies bill customers for.
Kilovolt-ampere (kVA) measures “Apparent Power,” representing the total power flowing in the electrical circuit. This total power is the product of the circuit’s voltage (kV) and current (A), determining the overall demand placed on the electrical infrastructure. Apparent power includes both the real power (kW) and the reactive power (kVAR), which is needed to establish magnetic and electric fields in devices like motors and transformers.
A common analogy to visualize this relationship is a glass of beer, where the total contents of the glass represent the Apparent Power (kVA). The liquid beer itself, which quenches thirst, represents the Real Power (kW) that performs useful work. The foam at the top of the glass, which takes up space, represents the Reactive Power (kVAR).
The Role of Power Factor in Electrical Systems
The relationship between Real Power (kW) and Apparent Power (kVA) is mathematically defined by the Power Factor (PF). Power Factor is the ratio of kW to kVA, indicating the efficiency with which total electrical power is converted into useful work. It is expressed as a value between 0 and 1, or as a percentage.
This difference exists because in alternating current circuits, the voltage and current waveforms can become misaligned, or “out of phase.” This misalignment is primarily caused by inductive loads like motors. When the waveforms are perfectly aligned, the Power Factor is 1.0 (or unity), and the Apparent Power is equal to the Real Power (\(kW = kVA\)).
When the voltage and current are out of phase, the Power Factor drops below 1.0. This means a portion of the total supplied power is reactive and does not contribute to useful work. The fundamental relationship linking these quantities is \(kW = kVA \times PF\). This equation shows that the actual usable power (kW) will be less than or equal to the kVA rating, depending on the system’s efficiency.
Selecting Equipment Based on kVA and kW Ratings
Electrical equipment that supplies power, such as transformers, Uninterruptible Power Supplies, and generators, is typically rated in kVA. This is because the physical components of this equipment, like the wiring and cooling systems, must be designed to handle the total current flow, which is determined by the Apparent Power. The kVA rating dictates the maximum current capacity and thermal limits of the machine.
The kW rating, on the other hand, determines the actual mechanical work the equipment can deliver or the amount of energy it will consume. For instance, a generator’s engine is rated in kW because it is the actual output power that the engine can sustain. To select the correct equipment, one must calculate the required kVA based on the necessary kW load and the expected Power Factor of the system.
Since most general-purpose generators are designed to handle a mix of loads, they are often rated with an assumed Power Factor, commonly 0.8. For example, a 100 kVA generator with a 0.8 PF rating can reliably supply 80 kW of Real Power (\(100 \ kVA \times 0.8 = 80 \ kW\)). Selecting the proper size requires converting the load’s required kW into the necessary kVA to ensure the total power capacity is not exceeded.