In the technical parameter table of the inverter, KW and KVA are the two most common power indicators, but they are often confused by unprofessional designers and users. Accurately understanding the difference between these two parameters is directly related to the rationality of equipment selection, the stability of system operation and the effectiveness of cost control. This article will deeply analyze the core differences between KW and KVA and explore their importance in practical applications.

1、The essential difference between active power and apparent power

Power is a physical quantity that describes the rate of energy conversion. In electrical systems, due to the presence of inductive or capacitive loads (such as motors, transformers, capacitors, etc.), the expression of power is complex. The measurement methods of inverter output power are two key parameters, KW and KVA.

KW is the unit of active power, which represents the power that actually does work in the circuit, that is, the power that can be converted into useful energy such as mechanical energy, heat energy, and light energy. For example, when a 10KW inverter supplies power to an electric water heater, the 10KW power will be completely converted into heat energy for heating water. At this time, the energy conversion is "effective". Active power is the core indicator to measure the inverter's actual ability to output "useful energy", and its size directly determines the actual working power that the load device can obtain.

KVA is the unit of apparent power, which represents the product of voltage and current in the circuit, and includes both active power and reactive power. Reactive power is the energy that maintains the magnetic field or electric field of the circuit. It does not directly work externally, but it is an indispensable part of the normal operation of inductive or capacitive loads. For example, when the motor is running, a rotating magnetic field needs to be established, and this process consumes reactive power. Apparent power essentially reflects the maximum current load that the internal circuit of the inverter (such as transformers, switches, wires, etc.) can withstand, and is a direct reflection of the inverter capacity.

2. What is the relationship between KW and KVA?

KW and KVA do not exist in isolation, they are closely related through the power factor (PF). The power factor is the ratio of active power to apparent power, and the calculation formula is: PF=KW/KVA. Its value range is between 0 and 1. Inductive loads (such as motors) are usually 0.8~0.9, and capacitive loads (such as photovoltaic inverters) may be close to 1.

When the load is a purely resistive device (such as an electric water heater, incandescent lamp, electric oven, etc.), the current and voltage are in phase, and the reactive power is zero. At this time, the power factor PF=1, and KW and KVA are equal in value. For example, when an inverter marked as 5KVA supplies power to a purely resistive load, its actual output active power is 5KW.

When the load is an inductive or capacitive device, there is a phase difference between the current and voltage, the reactive power is not zero, and the power factor PF<1. If the load power factor is 0.8, a nominal 100kVA inverter actually outputs only 80kW of active power, and the rest circulates in the circuit in the form of reactive power. If you mistakenly select the model according to kVA=kW, it will cause the equipment to be overloaded.

The power factor directly affects the numerical relationship between KW and KVA. When selecting an inverter, if you ignore the power factor, it may lead to two extreme situations: one is that the KVA capacity of the selected inverter is sufficient, but due to insufficient active power, it cannot drive the load to work normally; the other is that the active power meets the demand, but the KVA capacity is insufficient, causing the internal components of the inverter to be damaged due to overcurrent.

3. Application scenarios: parameter selection logic under different loads

The parameter marking of the inverter is closely related to its application scenario. The selection of KW and KVA needs to be comprehensively judged according to the load type and system requirements.

In scenarios dominated by pure resistive loads, KW is the key parameter. Household photovoltaic inverters carry refrigerators, TVs and other equipment, with a low reactive power ratio and a power factor close to 1. The inverter KW directly determines the load capacity. For example, if the total household load is 5KW, a 5KW inverter can be selected without excessive attention to KVA.

There are many inductive loads in the industrial field, and KVA is the core. A chemical plant has a 200KW motor (power factor 0.8), corresponding to an apparent power of 250KVA. If a 200KVA inverter is selected, it will trip due to insufficient apparent power overload, and 250KVA and above must be selected.

Off-grid systems are more cautious in selecting parameters. Remote communication base stations have mixed loads, and the reactive power demand is large when the air-conditioning compressor starts. If the total active power of the base station is 5KW, the power factor is 0.7, and the apparent power is about 7.14KVA, the selection of a 5KVA inverter will cause the air conditioner to fail to start or even damage the equipment due to insufficient apparent power. Therefore, the selection should be based on the apparent power, taking into account the active power demand.

The parameter marking of the grid-connected inverter must adapt to the grid specifications and must have the ability to adjust the power factor (usually 0.9 leading to 0.9 lagging). KVA reflects the capacity limit under different power factors, and KW corresponds to the active output capacity under different power factors. For example, for a 10KVA grid-connected inverter, the maximum active output is 9KW when the power factor is 0.9, and 8KW when it is 0.8.

Accurately understanding the difference between KW and KVA can not only avoid economic losses caused by incorrect selection, but also optimize system design and improve energy efficiency.