# Why are Generators and Alternators Rated in kVA, Not in kW?

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• 26 Aug 2024

## Why are Generators and Alternators Rated in kVA, Not in kW?

As we already know that why is a transformer rated in kVA instead of kW, same is the case and reason as well here i.e. the power √3 VL ICos Φ delivered by the alternator and generator for the same value of current, depends upon P.F. (Power Factor = Cos ϕ) of the load. In other words, generators and alternators don’t deliver Watts but both of active and reactive powers which depends on the power factor of the connected load to it.

Keep in mind that the conductors of an alternators are calculated for a definite current and the insulation at the magnetic system are designed for a definite voltage independent of P.F. (Cos Φ) of the load. For this reason apparent power measured in kVA is regarded as the rated power of the alternator.

In very short, a generator generates both active and reactive power which is rated in volt-amperes (VA) instead of watts (W).

The main factors manufacturers consider while designing electrical devices and appliances which provide electric power like transformer, UPS, alternators and generators etc, are load and power factor. As they don’t know exactly what is the power factor and which kind of load (resistive, inductive, capacitive or a mixture of all of them) will be connected to the device and appliances.

So they simply design and rate the electrical device according to its maximum current output that the conductors can safely carry while they consider unity power factor (In case of pure resistive load).

If we connect inductive or capacitive load (When power factor is not at least unity), The output would differ as there are losses due to low power factor.

For this reason, KVA is an apparent power which does not take into account the PF (Power factor) instead of KW (Real Power).

Where:

KW = KVA × Cos ϕ

And

kVA = KW ÷ Cos ϕ

### For example,

In case of unity Power factor (1) i.e. pure resistive load. A 100kVA generator or alternator would provide exactly 100kW according to the following formula.

P = V x I x Cos Φ

kW = V x I x Cos Φ

kW = 100 x 1 = 100 kW

If we put the value of Cos Φ as 1, The real power (kW) would be equal to the apparent power (kVA).

In case of inductive or capacitive load, suppose the power factor is 0.80.

Now the real power would be

kW = KVA X Cos Φ

kW = 100 X 0.80

kW= 80 kW.

Now you know why alternators, generators, transformers and UPS etc are rated in kVA instead of kW.

## FAQ

### Why generator rating in kVA instead of kW?

Generators are rated in kVA instead of kW because they measure different aspects of the power output. kW is the real power that a generator can supply based on its engine’s horsepower. kVA is the apparent power that a generator can supply if it was 100% efficient

### Why is the motor in kW not kVA?

Since the power factor is unknown both generator and transformer are rated in kVA. But in the case of a motor, it has a fixed power factor. That is why the power factor is included in the nameplate data. Hence, Electric Motors are rated in kW and not in kVA

### Why are inverters rated in kVA not kW?

Because the AC output may have the voltage and current with a phase difference. Only at zero phase the KVA is equals real KW, otherwise VA is only the apparent power

### Why is kVA different from kW?

kW is the amount of ‘actual power’ an electrical system has. This shows you how much power is being converted into useful, working output. kVA, on the other hand is the measure of ‘apparent’ power. If kW is how much power you can work with, kVA tells you how much is being used in the system overall

### Why are alternators in kVA or not in kW?

Ans. Alternators/generators are rated in kVA (kilovolt-ampere) because they are designed to handle both real power (kW) and reactive power (kVAR). The kVA rating represents the total apparent power that the alternator/generator can deliver, while the kW rating represents the real power or the actual work done

### Why engine power is measured in kW?

This power measurement is becoming increasingly common in the age of the electric car, as it is a purer signifier of electrical power. Certain countries, like Australia, also use this as their preferred figure in car reviews and manufacturer literature, even for vehicles with internal combustion engines

### Why is VFD rated in kVA?

A correctly sized kVA rating ensures that the VFD can manage the required load without overheating or experiencing electrical stress. When a VFD operates within the appropriate kVA range, it minimizes power losses and maximizes performance, leading to more efficient energy use and lower operational costs

### Why does a 6.6 kW system have a 5kW inverter?

If the solar system outputs more than 6.6kW, the inverter will need an increased capacity if it’s a single-phase site. But if the inverter is larger than 5kW, it would then no longer be compatible with grid requirements. In this case, you can set the inverter to export 5kW maximum back to the grid

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