One of the guys I work with is taking a course in HVAC and since my boss used to teach it, we got a little lesson today (helping with the homework). The result of the conversation was the realization that it costs significantly more to air condition a building in Philadelphia than it does in Las Vegas. Surprising, huh? Here's the math though: Indoor design conditions are for 75F and 50% relative humidity. That corresponds to an enthalpy of about 28.5 (btu/lb). In Philly, the design conditions (from NOAA data) are 92F dry bulb and 75F wet bulb (that's about 48% relative humidity). That corresponds to an enthalpy of 39.0. In Vegas, the conditions are 112F wb and 68F db (about 8% rh). Thats an enthalpy of about 33. So the difference between the two enthalpies means its 18% hotter! in Philly than in Vegas on a typical August day and requires 57% more energy (10.5 vs 4.5 btu/lb of air) to cool outside air (your house is more complicated since much of the load is actually solar).