SEER Rating Comparison

The Seasonal Energy Efficiency Rating or SEER rating is used to compare the efficiency of air conditioners and heat pumps.

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It is the ratio of total cooling season output, measured in Btu (British Thermal Units), to the total electrical input measured in watt-hours.  Below, we show you how to calculate actual savings and pay back for a replacement unit by comparing this rating.

The SEER rating measures cooling system efficiency as the unit cycles on and off throughout the season.  It is one the primary standards the HVAC industry uses to compare the efficiency of various makes and models and can be expressed as:

SEER = Total Btu Output per Season
             Total Watt-hours per Season

The Energy Efficiency Rating or EER also represents a ratio of Btu output to watt-hour input for central air conditioners and heat pumps.  It differs from the SEER rating in that it is measured only with the unit powered on, not cycled on and off. The SEER rating will always be higher than the EER rating for any given piece of cooling equipment.  This relationship is relatively constant and can be expressed as:

EER = 0.875 x SEER

If you have an older central air conditioner, how can you determine if it makes good sense to replace it with a new, higher efficiency model?   The quick and dirty method is to do a simple comparison of the SEER ratings.

Let's say you have an old central air conditioner with an rating of 9 and are considering an upgrade new unit with a rating of 13.  The energy savings you can expect can be expressed as:

% Efficiency Increase = 1 - 9 (Old SEER) =  30.8%
                             13 (New SEER)

This tells us that we can expect our power consumption to drop by more than 30 percent with the upgrade while maintaining the same cooling capacity.   If we decide to do this, how can we determine the time it will take to recover our investment in the new central air conditioner or heat pump?

First, we must determine the seasonal Btu output of the system.  This is a combination of the rated Btu output and average daily run time.  If you do not know the output of your system you can usually determine it from the model number on the placard.

Embedded in the model number are usually two digits that are divisible by six such as 18, 24 ,48, etc.  Heat Pump Placard - 4 Ton Unit These numbers represent the total output of the unit in thousands of Btu's. If you divide this number by 12 you will have the tons of cooling capacity. This term comes from the fact that it takes approximately 12,000 Btu's to melt one ton if ice in 24 hours.

For example, if your model number reads PSH1BD048K the unit would have a capacity of 4 tons (48/12).  This can sometimes be confusing to discern as other digit sets in the model number can be divisible by six.  A quick way to verify this is to look at the manual and see which digits change as different size models are listed.  The changing digits represent the different levels of output in thousands of Btu's.

Next, we need to determine the amount of kilowatt-hours consumed by the central air conditioner during the cooling season. This varies with outside air temperature and the thermostat setting.  Ideally, we could sub-meter the whole system for an entire season and have an accurate number for the year, but that takes a long time.  

A quicker method is to use cooling degree data for your city, nearest major airport or zip code.   Measure a month of cooling system kilowatt-hour consumption with your home energy monitor using load profiling, sub-metering or net-metering. Divide this reading by the total cooling degree days for that month in your area to determine the kilowatt-hours per cooling degree day.  Multiply kilowatt-hours per cooling degree day by the total cooling degree days for the year to get the seasonal energy usage for your cooling system.

KWh per CDD = Kilowatt-hours (KWh) per Month
                         Cooling Degree Days (CDD) per Month

   KWh per CDD x CDD per year = KWh per Cooling Season

From the data you gathered take the average power reading when the central cooling system is operating.  Divide the KWh per year by this power reading to determine seasonal run time.  Multiply the run time by the Btu output to obtain Btu's for the season.

Hours of Run Time for the Season = KWh per Season
                                                          System Power (KW)

Btu's for Season = Seasonal Run Time x Btu Output

We can now calculate the actual SEER rating as:

SEER = Btu's for Season
           Kwh per Season x 1000

Apply the kilowatt-hour rate from your electric bill and you can determine the seasonal cost to operate your central air conditioner:

Kwh per Season x $ per Kwh = Seasonal Cooling Cost

Compare the SEER rating of your current system with the that of the upgrade you are considering to determine your efficiency gain.  Multiply this by the seasonal cooling cost to determine your annual savings.  Divide this annual savings into the cost of installing a new cooling system to determine how many years it will take to recover your investment.

% Efficiency Increase = 1- Old SEER  =  % Annual Savings
         New SEER

Total Annual Savings = % Annual Savings x Seasonal Cooling Cost

Years to Pay Back = New System Cost
                                  Total Annual Savings

The current minimum SEER rating for central air conditioners and heat pumps is 13 which will likely be the most cost effective choice as you go through this analysis.   Energy Star rated homes require a minimum rating of 14.  Residential units can have ratings as high as 20 but come at a premium price.  These highly rated systems often operate at much greater refrigeration pressures which can lead to increased maintenance costs.

For additional energy saving ideas for HVAC equipment, please visit the following:

Central Air Conditioning  l   Programmable Thermostats  l   Outdoor Thermostats  l   AFUE Ratings

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