Check the cheap stuff first
Before you pay anyone: change the filter if it's been more than a couple of months, make sure the outdoor unit isn't choked with grass clippings or cottonwood fluff, and confirm nobody moved the thermostat setpoint. Those three fixes cost almost nothing and explain a surprising share of high-bill complaints.
The causes you can't see from the thermostat
High static pressure. When the duct system restricts airflow, the blower works harder, the system runs longer to move the same heat, and efficiency falls. It's a sixty-second reading most companies never take. Duct leakage. Supply ducts leaking into an attic are literally air-conditioning the outdoors — in a Tennessee Valley summer, that's money straight through the roof deck. Refrigerant charge off spec. Even a modest over- or undercharge costs capacity and efficiency, and the symptoms hide until the bill arrives. Short-cycling. An oversized system cools the thermostat fast, shuts off before it dehumidifies, and restarts over and over — and in our humidity, a clammy house gets the setpoint pushed lower, which compounds the bill.
Putting a number on it
All four causes show up on instruments in a single visit. The $149 System Evaluation measures static pressure, airflow, charge, and duct leakage, and the Home Comfort Report ranks what's worth fixing — you can see a sample of the report before booking. The point isn't that something is always wrong; it's that you stop guessing about where the money is going.