How can I calculate the power consumption of an A/C unit?
It will only draw that level of power when the compressor is running.  In normal circumstances it will cycle on and off to chill the cooling element as needed; the rest of the time only the fan will be running to blow air over them.

The amount of power consumed will vary depending primarily on how hot it is and what temperature you have the AC set to.

To find out how much power it's actually using, you'll need to use a power meter of some sort.  I'm partial to the Kill-a-Watt if they're available where you live.