Delta T is the most commonly used measurement in the HVAC industry. HVAC professionals measure the difference between the temperature in the return duct a few feet before the evaporator coil and the temperature a few after the evaporator coil.
SmartAC.com uses Cooling Performance as a proxy for Delta T. Our sensors are located on the supply vent and intake vent. There will be a difference between Cooling Performance and true Delta T as heat is lost or gained in the duct work, but not much.
Monitoring over time can clue you in to things like sub-optimal system performance, unnecessary energy usage or impending equipment failure, helping you address these issues proactively instead of reactively.