Capacitors make use of certain laws of electricity to decrease the amount of power consumed.

Here's a good website for calculations and formulas:
http://www.nepsi.com/formulas.htm

The pertinent term to lock in on is "power factor correction." This term means you are trying to mathematically offset some of the electrical inefficiencies of the system by adding capacitors.

Once you understand the losses associated with induction loads, you'll begin to see how the capacitor offsets some of those losses. This "offset of loss" equals a reduction in usage, and therefore a lower bill for large consumers (factories, hospitals, etc..) but for the design engineer of the HVAC system, it means lower amp draw, allowing smaller wires, both in the motors, and feeding the motors.

Here is a website that explains it much better than I can, and (combined with the formulas you get from the first website I posted) allows you to calculate power correction factors as if you had taken the class in college.

http://www.lmphotonics.com/pwrfact.htm