Alternators are rated in kilovolt-amperes (kVA) because their rating needs to encompass both real power (kW) and reactive power (kVAR). The rating in kVA provides a measure of the overall power-carrying capacity of the alternator, considering both the active (real) power and the reactive power components in an AC electrical system.
Explanation:
When a load has voltage V across it and current I through it, the electrical power that appears to flow to it is VI.
If the load comprises of both resistance and reactance, this product represents neither real power nor reactive power. Since VI appears to represent power, it is called apparent power. Apparent power is given the symbol S and has units of volt-amperes (VA).
For small electrical equipment, VA is a convenient unit. However, in case of heavy power apparatus, VA is too small and kVA (kilovolt-amps) is frequently used.
For same value of electric current, the power √3 VLIL cosϕ delivered by the alternator depends on power factor of load. Alternator conductors are calculated for definite current and insulation and magnetic system are designed for definite voltage that is independent of PF of load.
Due to above reasons, alternators are rated in kVA.