Reactive power, apparent power and active power describe different aspects of electrical power in AC systems. Distinguishing between them is important because electrical equipment is affected not only by usable active power, but also by the total current flow and therefore by apparent power.
Reactive Power, Apparent Power and Active Power at a Glance
- Reactive power QQ describes the periodic exchange of energy between a source and electric or magnetic fields. At the same active power and voltage, additional reactive power increases the required current flow.
- Apparent power SS results from the interaction of active and reactive power and describes the total electrical load placed on equipment.
- Active power PP is converted into heat, light, mechanical work or other usable forms of energy.
- Inverters, transformers and cables must be sized according to their apparent power or current-carrying capacity.
What Is the Difference Between Reactive, Apparent and Active Power?
The three power quantities describe different aspects of the same electrical system:
| Quantity | Symbol | Unit | Meaning |
| Reactive power | Volt-ampere reactive (var) | Describes the periodic exchange of energy with electric or magnetic fields | |
| Apparent power | SS | Volt-ampere (VA) | Describes the total electrical load |
| Active power | PP | Watt (W) | Is converted into a usable form of energy |
Resistive loads such as electric heaters primarily consume active power. Inductive loads such as motors, transformers and reactors require magnetic fields and typically consume inductive reactive power. Capacitors and appropriately designed or controlled inverters can provide capacitive reactive power and thereby compensate for inductive reactive power.
Reactive power is not inherently useless or undesirable. It is associated with the creation of electric and magnetic fields and can be used for voltage control. However, at the same active power and voltage, additional reactive power results in a higher current and therefore places a greater load on cables, transformers and other equipment.
Where Are These Three Power Quantities Relevant?
In commercial and industrial installations, reactive power is often caused by motors, pumps, fans and transformers. Suitable reactive power compensation can reduce current flow within the installation and relieve existing equipment.
In photovoltaic systems and Battery Energy Storage Systems, inverters can provide active and reactive power within their technical limits. Reactive power can, for example, be used to maintain a specified power factor or influence the voltage at the grid connection point.
Apparent power is also decisive when sizing equipment. An inverter with a maximum apparent power of 100 kVA can, for example, provide 80 kW of active power and 60 kvar of reactive power simultaneously. The apparent power limit is therefore fully utilised. If additional reactive power is required, the active power must be reduced unless the inverter permits a higher apparent power.
Benefits, Limitations and Technical Requirements
Measuring threse types of powers provides transparency regarding the actual loading of an electrical system. Targeted reactive power control can reduce the load on equipment, influence reactive power exchange at the grid connection point and support voltage control.
Suitable energy meters or measuring devices are required. Depending on the application, values may need to be measured at the grid connection point, at individual systems or separately for each phase. For active control, inverters or compensation systems must also be controllable and provide suitable communication interfaces.
The simple relationship between PP, QQ and SS is not always sufficient in systems with harmonics, unbalanced loads or non-sinusoidal waveforms. In these cases, the overall power factor can also be affected by distortion and is not necessarily equal to the displacement power factor cosφ\cos\varphi.
Manufacturers may also use different sign conventions for inductive and capacitive reactive power. Data points, units and reference directions must therefore be checked during each integration.
How Does EcoPhi Support Monitoring and Control?
EcoPhi can collect, historise and visualise available measurements for reactive power, apparent power, active power, power factor and voltage. This requires the connected meters, inverters or other components to provide the corresponding values through a suitable interface.
For controllable inverters, EcoPhi can transmit project-specific reactive power or power factor setpoints. The exact implementation depends on the current device integration, available interfaces, permitted operating limits and intended control architecture. This does not automatically replace grid protection functions or a certified plant controller that may be required for grid connection.
Reactive Power, Apparent Power and Active Power Summarised
Active power describes usable power, while reactive power represents the periodic exchange of energy with electric or magnetic fields. Under sinusoidal conditions, their interaction determines apparent power. Apparent power is a key factor in the electrical loading of inverters, transformers and cables.
Frequently Asked Questions
Why Does Reactive Power Place a Load on Electrical Equipment?
At the same active power and voltage, additional reactive power results in a higher current. This places a greater load on cables, transformers and inverters.
Can an Inverter Provide Active and Reactive Power Simultaneously?
Yes, provided that the inverter supports this function. However, the apparent power resulting from active and reactive power must not exceed the inverter’s permissible apparent power limit.