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SEER's EER's and Errors . . . To rate equipment efficiency, engineers calculate the coefficient of performance which is a defined ratio of output divided by input. Simply put, it is a measure of what goes in and what comes out in a strictly controlled environment. While useful to engineers, coefficient of performance data is cryptic and confusing to the average consumer. So, to improve comparison shopping, the EER (energy efficiency ratio) was established. For air conditioning equipment, the EER, a rating standardized by ARI (Air Conditioning and Refrigeration Institute), is an index of the steadystate efficiency at 95F outdoor and 80F indoor temperature. A higher EER means the system is more efficient. The formula for calculating EER is: EER = BTU of cooling / Watts used at 95F outdoor and 80F indoor temperatures For instance, if you have an air conditioner that uses 7500 Watts of electricity to produce 60,000 BTU of cooling, it would have an EER of 8.0 (60,000 divided by 7500). A unit using 6000 watts to produce the same amount of cooling would have an EER of 10 and would be more energy efficient. Note that the EER is a precisely defined ratio of cooling capacity in BTUs divided by the electrical power in Watts at specific outdoor and indoor temperatures. But not all days are 95 degrees outside and not all rooms are 80 degrees inside. The actual efficiency of an air conditioner varies under different climatic or room temperatures Therefore, a new rating, the SEER (seasonal energy efficiency rating) was established. The SEER is an EER adjusted to be an average rating for operation of the machine over a wide range of conditions. And that is where the trouble starts . . . The SEER is defined as the total cooling air conditioning output (in BTU/hours) during a normal annual usage period divided by its total energy input (in Watthours) during the same period. The SEER is prominently displayed on a yellow label affixed to the A/C unit. Higher SEERs are better. The "formula" for calculating SEER is: SEER = seasonal Btu/hour’s of cooling / seasonal watthours used. As with EER, a higher SEER reflects a more efficient cooling system. The difference between a 10 SEER and a 12 SEER reflects about a 20% improvement in energy efficiency. Except . . . . In a study published by Southern California Edison , researchers plotted the EER and the SEER of 7000 air conditioners. They found that for a given SEER level, there was a significant variation ( + or  15%) to the EER. Likewise, for a given EER level, there is an even more significant variation (+ or  25%) in SEER. Southern California Edison researchers attributed these large variations to the varied methods by which manufactures use to obtain the highest possible SEER rating. They conclude, "This raises a concern that these steadystate efficiency ratings may not provide a reliable indication of seasonal performance . It follows that these same units will exhibit a great deal of variation in seasonlong performance under dynamic load and temperature effects. Perhaps different or additional rating and performance data should be required to more reliably indicate true seasonlong performance. " The difference between a 10 SEER unit and a 12 SEER unit is 20% but the margin of error can be as high as 25%! So, will a 12 SEER air conditioner cost less to operate than another rated a 10 SEER? Probably . . . we can only hope . . . but, then again, maybe not. Company A’s 12 SEER unit might only rate a 10 SEER or even less if it were tested at company B’s facility. Or it might be a 14 SEER. Or 11, 12, 13 . . . Who knows? If a person can’t trust the SEER rating, how does one make an intelligent decision when purchasing air conditioning equipment? I suggest the best bet for the informed consumer is to evaluate both the EER and the SEER with more emphasis on the EER. Don’t worry that the EER is always a smaller number than the SEER number. This is normal because the EER is measured at tightly defined peak load conditions while the SEER factors in some cool April afternoons, breezy summer nights and cloudy September mornings. An EER of 14 is probably about the highest efficiency you will see today. SEER’s currently go as high as 18 and a 20 SEER unit is on the drawing board. As of 2002, Federal law mandates the minimum efficiency to be 12 SEER. So, look for an air conditioner with a SEER of at least 12 COMBINED WITH an EER no less than 9.5. The higher numbers IN BOTH RATINGS the better. But beware of an SEER that seems disproportionately higher than it’s corresponding EER compared to other competing units. Now, I have to ask: Don’t you think it is a little silly to be basing lofty Department of Energy mandates and Federal Regulations on such a sloppy rating system as the SEER? I do . . . 2003.05.31 