When using EER in the field, a technician can effectively calculate improvements made to systems being serviced using this simple equation:
EER = Capacity (or Btu/hr) / Power (or Watts)
This will give a snapshot of the system based on the test conditions that day, or time of day. We know that in New England, if you don't like the weather to just wait five minutes, right? So, it is possible that a technician could have made an improvement to the a/c, but since the sun came out the EER may have still dropped. One would need to normalize the answers to a known point in order then compare apples to apples. This is where computers and tablets come in handy.
The above equation explains why there is a benefit to add refrigerant to an undercharged system. The capacity gained by correcting the superheat and/or subcooling will outweigh the additional watts used by the compressor. We all know that if we make the compressor work harder by introducing additional refrigerant, the amperage draw will increase during run load. As long as the EER rises, the customer will benefit. Adjusting refrigerant charge is not the only way to improve this ratio, you could also fix airflow issues, seal duct systems, or even reduce watt usage by replacing components like blowers with more efficient alternatives. In fact, then maybe the system will actually reach temperature and shut down. There really is no better energy savings than that, right?
When a technician overcharges a system, using the EER equation, you can see the condenser will use additional wattage without a benefit of additional capacity. It does not take much refrigerant to overcharge a high SEER condenser these days. I was working with a contractor just last week on a micro-channel unit. We removed 4 oz. of R-410A and reduced the subcooling from 20F to the correct 10F. This is the amount of refrigerant that could fit into a 6' charging hose! Be careful out there, it is amazing what you find when you start measuring the invisible!