Kilowatts may not be the best unit to use unless you just want to know the actual power consumption, as for calculating the cost to operate the compressor. Amps may be more useful for practical purposes, for such things as what wire size would be needed for the circuit and the circuit breaker rating needed.

On paper, 1 horsepower equals 746 watts, so 7.5hp times 746 watts equals 5,595 watts. Close enough to 6kW. Watts are the product of volts times amps. Assuming you have a 240 volt main, then amps would be calculated by watts divided by volts, so 5595 watts divided by 240 volt will equal 23 amps. Starting current for a compressor or a similar motor under load, is roughly twice the run current, which in this case would be roughly 50 amps at 240 volts. Using three times the run current (a liberal figure), you will draw roughly 75 amps for the first few seconds it takes to start the compressor turning.

To answer your question more directly, since starting the motor will draw 2 to 3 times the current it takes to run the motor, it would be using 12 to 18 kilowatts for the first few seconds, and then settle back to using 6 kilowatts.