what is kva
KVA (kilovolt-ampere) is a unit used to represent **Apparent Power** in the power system, and it is commonly found in the
specification markings of equipment such as generators, transformers, and UPS systems. The following are its core concepts,
calculation methods, and application scenarios:
# I. Basic Definition
- **Apparent Power (S)**:
It refers to the product of **Voltage (V)** and **Current (I)** in an alternating current circuit, without considering the electrical
energy conversion efficiency (i.e., the power factor). The unit is **VA (volt-ampere)** or **kVA (kilovolt-ampere, 1 kVA = 1000 VA)**.
- **Formula**:
**S (kVA) = Voltage (V) × Current (I) ÷ 1000**
II. Differences between KVA and Active Power (KW)
| Parameter | Definition | Unit | Relationship |
| Apparent Power | Total input/output capacity (including active power and reactive power) | kVA | S = P / cosφ (φ is the power factor) |
| Active Power | The power of actual work done (such as driving a motor, heating) | kW | P = S × cosφ |
| Reactive Power | The power to maintain the magnetic field/electric field (such as inductors, capacitors) | kVar | Q = S × sinφ |
III. Calculation Examples
1. Single-phase Circuit
- **Given**: Voltage 220V, Current 10A, Power factor 0.8
- **Apparent Power**:
**S = 220V × 10A ÷ 1000 = 2.2 kVA**
- **Active Power**:
**P = 2.2 kVA × 0.8 = 1.76 kW**
2. Three-phase Circuit
- **Formula**:
**S (kVA) = √3 × Line Voltage (V) × Line Current (I) ÷ 1000**
- **Given**: Line voltage 400V, Line current 15A
- **Apparent Power**:
**S = 1.732 × 400V × 15A ÷ 1000 = 10.39 kVA**
IV. Application Scenarios
1. **Capacity Marking of Generators/Transformers**
For example: A **10 kVA generator** indicates that its maximum output capacity is 10 kVA, and the actual load it can carry
depends on the power factor (for example, when cosφ = 0.8, the active power is 8 kW).
2. **Power System Design**
It is used to calculate the cable specifications and the rated current of the circuit breaker to avoid overload. For example:
- For a 10 kVA load, a transformer with **≥10 kVA** and a circuit breaker with **≥50A** (220V single-phase) should be selected.
3. **Equipment Selection**
- **UPS**: It is marked with kVA instead of kW because the reactive power of non-linear loads (such as computers) needs to be considered.
- **Welding Machine**: The nameplate is marked with kVA because the power factor is low (about 0.3-0.6) during welding.
V. Precautions
1. **Power Factor (cosφ)**
- For pure resistive loads (such as heaters): cosφ = 1, kVA = kW.
- For inductive loads (such as motors, transformers): cosφ < 1, kVA > kW.
- **Example**: A 10 kW motor (cosφ = 0.8) requires a **12.5 kVA** power supply for support.
2. **Three-phase vs. Single-phase**
When calculating the kVA of three-phase equipment, it is necessary to multiply by √3 (approximately 1.732), otherwise the
capacity will be underestimated.
**Conclusion**: KVA is a unit for measuring the **total capacity** of electrical equipment. When selecting the equipment, it is
necessary to convert it into active power (kW) in combination with the power factor to ensure that the equipment matches
the load requirements and avoid overload or waste.
