kVA: apparent power
kVA is a measure of apparent power: it tells you the total amount of power in use in a system. In a 100% efficient system kW = kVA. However electrical systems are never 100% efficient and therefore not all of the systems apparent power is being used for useful work output.
kW: actual power
kW is the amount of power that is converted into a useful output. kW is therefore known as actual power or working power.
Power Factor: a measure of electrical efficiency
You can convert between kVA and kW if you know the efficiency of the electrical system. Electrical efficiency is expressed as a power factor between 0 and 1: the closer the power factor is to 1, the more efficiently the kVA is being converted into useful kW. Generators have a power factor of 0.8.
The formula for converting kVA into kW is:
Apparent power (kVA) x power factor (pf) = actual power (kW)
e.g. 100 kVA x 0.8 = 80 kW
The formula for converting kW into kVA is:
Actual power (kW) / power factor (pf) = apparent power (kVA)
e.g. 100 kW / 0.8 = 125 kVA