In general terms, an AC machine produces an output which is dependent upon the load. Basically, an AC generator has to be capable of producing more power, apparent power, than is actually used in real terms. Units of apparent power are in VA or kVA, while those of real power are in kW. kVA have to do with voltage regulation and field strengths. A generator is usually rated in a value of kVA, and occasionally in a value of kW. A WATT/NAR (VAR) meter is located on the flight deck of some aircraft, and by selection indicates the relevant reading on a generator.
In simple words, the measured output power components of a constant frequency AC system are kW, kVA and kVAR.
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