There are a number of reasons why air conditioning might seem expensive to run. One factor could be that the system is inefficient and it's challenging to save energy with an overly-large unit or one that has not been properly tuned. Another culprit could be poor insulation in the home which means, given the same outside temperature, more energy is needed because there's less efficient heat transfer from inside to outside. And finally, some people forget about adjusting their thermostat back after they turn off their AC before they go to bed.