U.S. governmental standards require that newly produced air conditioners have a SEER (seasonal energy efficiency ratio) rating of over 13, Federal Register (2001), . This rating is closely tied to the COP (coefficient of performance). In fact, the SEER is 3.792 times the COP. Since COP varies with temperature loads, a standard testing method requires the unit to be tested at standard conditions of temperature and humidity. This requires the use of expensive climate control chambers, where the system can be loaded to the specified temperatures. The scope of this paper proposes a simpler, less expensive method to test spot AC (air conditioning) units, as an alternative field test to ASHRAE (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) Standard 128 (2001), . By taking temperature measurements of the appropriate control volume, the COP can be calculated. To obtain steady state, the control volume will be treated as very large (or infinite), placing the unit to be tested outdoors or in a room big enough so that delta T will remain constant. Themocouples in conjunction with data logging software are used to take the temperature measurements, and the mass flow rate is measured by assuming uniform flow and placing a flow meter in the center of the air exhaust, on the evaporator side. The entire system can be assembled into a portable unit composed of a computer, thermocouples, flow meter and a digital multimeter, alternatively, a handheld relative humidity and temperature sensor can be used, ASHRAE (2003), . This would allow not only testing of units before they go into production, but having technicians in the field test the efficiency of units already in operation. The need may be there since there could be a significant drop in the SEER between factory conditions and installed unit, due to variations in duct sizes, losses due to non-ideal installations. Owing to the fact that the COP varies with loading, and our testing method requires no artificial control over loading temperatures, the current study is being conducted to find if the AC unit can perform up to its rating. The second law COP at environmental loading conditions is also evaluated for each of the five AC units tested. The calculated COPII (based on exergy) of the AC units tested do not vary as much (percentage-wise) as the rated COP. Their relative detrimental effects to the environment are probably not that much different from each other.