To start off, I'm not a HVAC specialist, but as a home owner I try to keep my bills down for AC. Here's the sort of stuff that I read:
Air Conditioning Systems and Humidity
In theory, an air conditioning system should be able to remove moisture from the air. When humidity levels spike, however, most systems can’t cope. Humidity affects air conditioning negatively because it cancels out the cooling effect.
When the humidity is too high, your home will feel warmer than it actually is. You’ll have to keep your air conditioning system running but won’t derive nearly as many benefits from using it. In other words, you’ll pay more to cool your home but won’t actually cool it that effectively.
http://byrdheatingandair.com/articles/how-does-humidity-affect-air-conditioning-and-heating
How humidity affects your Air Conditioning System
In all the coastal regions of the United States our buildings struggle with far greater levels of moisture in the air.
This moisture laden air enters our homes and businesses causing our HVAC systems to run much longer. The increased run times cause our utility bills to also increase significantly.
http://www.advancedinsulationla.com/humidity-and-air-conditioning.html
PRINCIPLES OF AIR CONDITIONING
Dehumidifying equipment for air conditioning usually consists of cooling coils within the air conditioner. As warm, humid air passes over the cooling coils, its temperature drops below the dew point and some of its moisture condenses into water on the surface of the coils. The condensing moisture gives up latent heat that creates a part of the cooling load that must be overcome by the air-conditioning unit.
For this reason, the relative humidity of the air entering the air conditioner has a definite bearing on the total cooling load. The amount of water vapor that can be removed from the air depends upon the air over the coils and the temperature of the coils.
http://www.free-ed.net/free-ed/Courses/05 Building and Contruction/050207 HVAC AC/AC_Fund00.asp?iNum=01