Relationship between kilo volt ampere ( kVA ) and kilo watt ( kW ) ratings of an electrical devices.

In an electric device, the power rating are mentioned on the label. Few devices have power specifications mentioned in kilo watt ( kW ) while few are specified in kilo volt ampere ( kVA ) ratings. The value may look similar but they differ according to the electric circuit.

what is the difference between kilo watt( kW ) and kilo volt ampere ( kVA )?

Kilo watt( kW ) – In an electric device, the power which is used in the circuit is measured in terms of kilo watt ( kW ) and is also called as useful power or the actual power of the electric circuit.

Kilo volt ampere( kVA ) – It can be said as the apparent power of the electric device. It may also be calculated as the product of the current and voltage ratings of the electrical circuit. The values of kilo volt ampere ( kVA ) are not the same as kilo watt( kW ) for every device.It depends upon the loads in the circuit and the phase difference of the circuit. It also depends on the factor by which the values may change known as the power factor of the electrical device.

What is power factor and how it differs for every device?

In an electric power system, the power factor is the ratio of power which can be utilised in the circuit to the apparent power of the an electric circuit.

Power factor ( pf ) =

Power factor is always equal to unity in DC circuit devices because the current and voltage are always in phase. So, kilo watt ( kW ) rating is always equal to kilo volt ampere ( kVA ) rating in DC voltage devices. In AC circuit devices, the current and voltage are not in phase due to which the kilo watt ( kW ) and kilo volt ampere ( kVA ) ratings would not be same. This also due to different type of loads acting on it such as inductive loads and capacitive loads, etc. The value of Power factor may vary from 0-1 in AC circuit devices. We can understand the relationship between active power and apparent power in an explaination given below :

Conversion of kilo volt ampere ( kVA ) and kilo watt ratings ( kW ) of AC circuit devices.

Conversion of kilo volt ampere( kVA ) to kilo watt ( kW ) is very simple. By the relation given below we can convert the values.

pf =

kW = kVA x Pf

1) Let us consider a device having an apparent power of 10 kVA having power factor of 0.8. We can convert it into kW by simply using the above relation.

kW = kVA x pf

So , kW = 10 x 0.8 = 8

As per the power factor of 0.8, we get the value of 10 kVA as 8kW.

2) Similarly we can convert the kilo volt ampere ( kVA ) into kilowatt( kW ) for a different device. Lets say an electrical device having an apparent power of 25 kVA having a power factor 0.6. We will have to apply the same relation for converting.

kW = kVA x pf

So , kW = 25 x 0.6 = 15

As per the power factor of 0.6, we get the value of 25kVA as 15kW.

This is how we can calculate the value of kilo watt for any electrical device by understanding the relation of kilo volt ampere and kilowatt.

Causes of low power factor in an electrical device.

1) Due to inductive loads in an inductive machines such as generators, heating furnaces etc the power factor gets decreased.

2) The voltage and current drawn from the device is more when the circuit is loaded lightly which results in low power factor.

3) Power factor becomes low due to improper wiring because the device will heat up easily.