Power Factor
Basic definitions
In an AC circuit, power (or energy, when integrated over time) can be described by three main components:
- Active Power: This is the real power that performs useful work—such as generating movement or heat.
- Reactive Power: A form of "virtual" power, created or absorbed by components like inductors (e.g., motors) or capacitors. It does not result in any net energy transfer.
- Apparent Power: The combination of active and reactive power components.
In sinusoidal systems, these power contributions result from the phase shift between the current and the voltage waveforms.
These quantities are typically illustrated using a vector (phasor) diagram:
In AC systems, the apparent power Papp (also sometimes denoted S) is defined as the product of the RMS (effective) values of voltage and current:
Papp = Ueff × Ieff, expressed in [kVA].
The Active power Pact is the average power over a full sinusoidal period, obtained by integrating the instantaneous product of voltage and current. It is given by:
Pact = Ueff × Ieff × cos(φ), expressed in [kW], also sometimes written [kWac].
The Reactive power Pre (also sometimes denoted Q) represents the orthogonal (vectorial) component:
Pre = Ueff × Ieff × sin(φ), expressed in [kVAr].
The power factor is defined as the ratio of active power to apparent power:
Power Factor = cos(φ)
It is crucial to understand that reactive power is not useful power : it does not produce movement, heat, or any tangible energy output.
Conversely, reactive power can be generated without consuming any real (active) power—this is the case with capacitors and inductors.
Reactive power produced by an inverter
The active energy produced by an inverter always originates from the input DC energy supplied by the PV array. Any real energy difference between the inverter’s input and output is converted into heat, representing the inverter's inefficiency.
With modern technologies such as Pulse Width Modulation (PWM), it is now possible to generate voltage and current signals with any desired phase shift—without any additional energy cost. Traditionally, this compensation is handled using large banks of capacitors. The ability to generate such a phase shift is the result of explicit control within the inverter, defined by a parameter set in its control system.
For this reason, grid operators may request that PV systems produce reactive energy to help compensate for the reactive power consumed by other elements connected elsewhere to the grid, such as motors or switching power supplies.