High Power Bills Related to Air Conditioner
If the company that serviced your A/C is not able to give you the answer you are looking for, I would recommend trying another company to get more opinions. I would recommend you ask them to measure the air coming out of the A/C to make sure its at least around 30 degrees cooler than the ambient temperature. For example if it is 90 degrees outside, then the air coming through the ducts by the AC unit should be 60 degrees. If the difference is much lower than that, then your system might be working extra hard to achieve your desired temperature, keeping your monthly bill higher than normal.

It is hard to know for sure without looking at your unit, and your setup, but here are some reasons why your A/C bill can be out of control:


Your house has poor insulation
The thermostat switch is always on (can you find times during the day to not have it on? such as when you are at work?)
Faulty AC Ducts such as blockages, disconnects, holes, etc.
Inefficient AC Unit due to lack of routine maintenance
Very Old AC Unit. As an AC unit ages so does its efficiency.
Faulty thermostats
Low refrigerant charge
Refrigerant flow-related problem
Bad compressor valves
Dirty air filters
Poorly insulated house, or air leaks
Duct work that was not properly sized