Central air conditioners are far more efficient than models installed just over a decade ago. Manufacturers continue add new features that help increase comfort levels and lower operating costs.

Energy consumption for air conditioners is measured in both theoretical and practical terms. A universal standard was developed by the Air Conditioning, Heating & Refrigeration Institute (AHRI) to provide a method for the direct comparison of different brands and models of AC equipment under laboratory conditions.

Understanding EER and SEER ratings

The Energy Efficiency Ratio (EER) was introduced in 1975 as a rating mechanism for cooling units that operate according to the principles of the refrigeration cycle. A unit’s EER rating is determined by dividing its BTU output by the rated wattage input at specific temperature and humidity levels.

In 1978, Congress passed a law requiring certain appliances to be tested under conditions that included a number of seasonal variables that affect BTU capacity and energy consumption. The Seasonal Energy Efficiency Ratio (SEER) replaced EER as the accepted method for calculating air conditioning efficiency.

Units are tested at a standard temperature of 82 degrees Fahrenheit outdoors and 80 degrees Fahrenheit dry bulb indoors. SEER ratings are assigned to every unit produced so that consumers can determine the value of each product relative to cost and efficiency. Since the 82 degree outdoor temperature is unrealistic in many southern climates, homeowners are encouraged to ask for extended ratings data, which offers a much more accurate view of the unit’s capacity under actual conditions.

The energy consumption characteristics of two different air conditioners can be compared by dividing the difference between the SEER ratings by the larger SEER number.

For example: 16 (SEER) – 10 (SEER) = 6 / 16 = 37.5. Therefore, a 16 SEER air conditioner is 37.5 percent more efficient than a 10 SEER unit.

In 2015, the government will adopt a new regional approach to SEER rating standards. Air conditioners in northern climates will be required to have a minimum 13 SEER rating while equipment installed in southern climates must be at least 14 SEER. Replacing an older 10 SEER unit with a 14 SEER model can save up to 29 percent on monthly utility costs, depending on the thermostat setting and the efficiency of the duct system.

Calculating Actual Power Consumption

The nominal power consumption of an air conditioner can be calculated to within 10 percent of the actual usage based on a relatively straightforward formula. The required information can be found on a label attached to the outdoor condensing unit. This performance data includes the number of amps the unit draws at a specific voltage. Consumer-grade air conditioners operate at 220-240 volts. To calculate actual electricity consumption, write down the Rated Load Amps (RLA) for the condenser as well as the Full Load Amps (FLA) for the fan. The final piece of information required is the amp draw for the blower, which can be found on the inside of the service panel of the air handler or furnace.

The three figures for amp draw should be multiplied individually by the operating voltage for each component. In a conventional split system, the blower in the air handler typically operates at 115 volts. After adding the resulting sums together, the new figure is multiplied by the number of hours the unit runs over a specific period to identify total watt consumption.

I (amps) x V (volts) = P (watts) x Runtime Hours = Total Watts Consumed

The total watts consumed must be further divided by 1000 in order to establish kilowatt hours. This figure is then multiplied by the prevailing electric rate to provide an accurate representation of an air conditioner’s electricity consumption on a daily, monthly or annual basis.

A simpler method to calculate AC power usage involves averaging several consecutive months of electric bills during the winter season when the air conditioner is not operating. This will establish a baseline for power consumption that includes everything but the air conditioner. Subtracting the winter average from individual monthly summer utility bills will provide a reasonably accurate estimate of the cost of operating your air conditioner.

Homeowners can also take advantage of several online energy consumption calculators that make the process painless by doing all the complicated number crunching.

How to Lower Cooling Costs

There are numerous factors that affect equipment efficiency. In addition to the SEER rating of the unit, the energy conservation characteristics of the building also play an important role. Before replacing a working unit, always check the following structural elements and upgrade where necessary:

* Insulation: Substandard insulation levels in the walls and ceiling force your air conditioner to work longer and harder to meet the indoor load requirements. Insulation continues to be the single best energy efficiency investment a homeowner can make.

* Duct Sealing: Although it is largely neglected, the duct system in a home has a substantial impact on AC efficiency. Leaky ductwork can account for as much as 50 percent of your home’s total HVAC system operating costs. The vast majority of homes in the U.S. were constructed prior to the implementation of national duct design and installation standards. Ductwork issues can be resolved by hiring a professional HVAC contractor to seal the system.

* Low E Windows: Low E Windows are coated with a metallic oxide compound during the manufacturing process. The coating is placed on different interior surfaces based on the climate where the windows will be installed. It is estimated that replacing single-pane windows with a low e model can save up to 35 percent on cooling and heating costs.

Theoretical numbers are helpful, but determining your unit’s actual operating expenses will provide a basis for meaningful equipment comparisons. Once you have established how much your air conditioner costs to run, you will have a better understanding of where to invest your energy saving dollars.