Simple Explanation for Getting Started

When a power rating is given for a power generator, it initially sounds clear. In practice, however, this number alone is not enough. This is because electrical power is not just electrical power: one part is actually used, while another part arises from the behavior of certain consumers in the circuit.

This is precisely where the terms active power, reactive power, and apparent power come from. Understanding these relationships allows for much safer selection of generators and avoidance of common mistakes.

What is Active Power (kW)?

Active power in kW is the power that is actually used. This is the portion of electrical energy that is converted into a real effect.

• A heating device gets warm.
• A lamp illuminates.
• A motor drives a machine.

This power is the most tangible in everyday life because it describes the actual work performed. When speaking of "usable power," this is usually the portion referred to.

What is Apparent Power (kVA)?

Apparent power in kVA is the total electrical power that a generator must provide. It therefore includes not only the usable portion, but also the power required for certain electrical fields and interactions in the system.

Apparent power is composed of active power and reactive power. Therefore, the kVA rating is often the decisive factor for selecting a generator.

What is Reactive Power?

Reactive power occurs with consumers that need to build up electric or magnetic fields. These primarily include motors, transformers, switching power supplies, or ballasts.

• Electric motors
• Transformers
• Switching power supplies
• Ballasts

This energy is not consumed permanently but oscillates back and forth between the source and the consumer. For the generator, this means it still has to supply this power, even though it is not directly used.

What is the Power Factor (cos φ)?

The power factor cos φ describes the relationship between active power and apparent power. It shows how much of the total power provided actually arrives as usable power.

• Pure resistive load: cos φ ≈ 1.0
• Motors: cos φ ≈ 0.8
• Unfavorable loads: < 0.7

A simple example: A generator with 10 kVA and a cos φ of 0.8 delivers approximately 8 kW of usable power. The remaining portion appears as reactive power.

Why is This Important for Power Generators?

A power generator must always supply the entire apparent power. In practice, this often leads to misunderstandings because many users primarily look at the kW number and assume that this alone is sufficient for selection.

• The generator seems large enough on paper, but motors still don't start.
• Under load, the voltage drops, and consumers react sensitively or unstably.
• The generator is overloaded despite seemingly sufficient power.

Technical Explanation for Expert Users

Technically speaking, the three types of power are intrinsically linked:

This relationship shows that the available active power always decreases as the reactive power component increases. This is relevant for generators because the design of the machine must react to the total electrical load.

Typical Load Types and Their Influence

1. Resistive Loads

• Heaters
• Incandescent lamps
• cos φ ≈ 1
• No reactive power

The generator is used particularly efficiently here because almost all power is active power.

2. Inductive Loads

• Motors
• Pumps
• Compressors, e.g., in refrigerators or air conditioning units
• cos φ usually 0.7–0.85
• High reactive power and additional starting current

These loads typically require a significantly larger generator in practice.

3. Non-linear Loads

• Switching power supplies
• IT devices
• LED lighting
• Distortions and harmonics
• Additional strain on the generator and control system

Especially for sensitive applications, a clean voltage quality is therefore particularly important.

Importance for Sizing Power Generators

When designing a generator, several influencing factors must be considered simultaneously. Simply adding up the nominal power of consumers is often insufficient for many applications.

Generator Specifications: kVA vs. kW

Many manufacturers specify power generators in kVA, often referring to a reference value of cos φ = 0.8. For example, if a device has 10 kVA at cos φ = 0.8, approximately 8 kW of continuous power can be derived from it.

It is important to note that if the power factor of the connected loads deteriorates, the usable active power decreases further. Those who only read the kVA value without considering the load structure can quickly overestimate the actually available reserve.

A generator that is too large is usually uncritical within certain technical limits, but not always economically sensible. Prolonged underload conditions should also be avoided.

Special Features of Inverter Power Generators

Inverter power generators behave differently in some respects than classic generators. They often react more sensitively to high starting currents but simultaneously frequently provide very stable voltage and cope better with certain non-linear loads.

The power ratings for inverter devices are often given directly in kW because the electronics actively regulate the voltage, and the focus is more on usable active power. Nevertheless, even here, the actual load behavior remains crucial for selection.

Practical Example

This example precisely illustrates why merely adding up kW values often leads to insufficient calculations. Only by considering reactive power and starting behavior can a realistic generator size be determined.

Frequently Asked Questions