What is Kilo-Volt_Ampere used for? What is kW used for?
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VA is used for apparent power and reactive power. Watt is used for active power. An inductor or a capacitor in an AC circuit does not dissipate energy, because the current and the voltage are 90° out of phase: energy flows into them during half the cycle, but it flows back during the next half cycle.
The power lines have to supply that current however, and they have transport losses since their resistance is not zero. The dissipated power in a supply line is equal to I²R: if the current is I+d during the first half of the cycle and I-d during the second half, the dissipated power during the whole cycle is: (I+d)²R/2 + (I-d)²R/2 = (I²+d²)R.
The rated VA of a load also determines the required current capacity of the line, the fuse needed to protect it etc...
The ratio of active power to apparent power in a circuit is called the power factor, or sometimes the "$cos \phi$". Large customers (factories,...) may be charged extra when their $cos \phi$ is too low. Usually there is an excess of inductive load (electric motors for example), and the $cos \phi$ will be corrected by adding capacitive load.
Power ratings for transformers, AC motors, AC motor controllers are usually expressed in (k)VA. Purely resistive loads like heating elements will have a rating in (k)W.