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What is SEER rating?

SEER stands for Seasonal Energy Efficiency Ratio. SEER is a rating system that was devised to provide a way to determine the relative operating cost of an air conditioning system or to compare the energy efficiency of air-conditioning systems. The SEER rating is calculated by dividing the cooling output during a complete cooling season by the energy used to achieve that cooling. It is measured in a laboratory at various temperatures between 65 degrees F and 104 degrees F. This is done to mimic the efficiency over a complete cooling season (from the cooler part of the cooling season to the hottest part). The SEER rating (similar to the mileage rating on a vehicle) gives the best rating you can expect from the air conditioning system. Your system will not always (and maybe not frequently) perform at that peak level.

The higher the SEER rating, the more energy efficient the system is. New air-conditioning systems have a SEER rating of around 14 to 17, but on super high-efficiency systems, the SEER rating can be as high as about 25. These super high-efficiency systems are not typically installed on homes. Older systems (about 20 years old or so) will normally have a SEER rating in the neighborhood of 6 to 8, so there is a significant difference in efficiency between newer systems and older systems. Since 2015, standards have required that in the hotter areas of the United States, new systems must have a minimum SEER rating of 14.

By dividing the SEER rating of one system by the SEER rating of another system, you can determine the relative costs of the two systems. For example, a system with a SEER rating of 16 should cost half as much to operate as a system with a SEER rating of 8. A system with a SEER rating of 15 should cost only 1/3 of what it would cost to operate a system with a SEER rating of 5. If you are considering getting a new system, you can use the SEER ratings to calculate how much money you will save with the newer system. As you compare a new system with your current system, please note that the SEER rating of a system decreases over time. A rule of thumb is that the SEER rating goes down by about one unit every five to seven years. Therefore, if you have a 20-year old system that initially had a SEER rating of 8, you can assume that its current SEER rating is around 5.

What is the Difference Between SEER and EER?

SEER is used for central air conditioning systems, while EER is typically used for window units. SEER is designed to measure the efficiency of a system over a complete cooling season – taking into account the multiple starting and stopping cycles that it will go through during the cooling season. EER measures the steady state efficiency of a system. That is, it only measures the efficiency of a running system and does not take into account the starting and stopping of the system like SEER does. (Systems use the most electricity when they first start up and are getting up to speed.) SEER measures the efficiency at temperatures from 65 – 104 degrees F, while EER only measures the efficiency at one temperature, 95 degrees F. You CANNOT compare SEER to EER. They are two totally different numbers that do not mean the same thing. It is like comparing apples to oranges

What is the Best SEER Rating for Your Home?

This is not an easy question to answer because there are a lot of variables to consider. There is a point of diminishing returns – the point at which what you are spending is not returned by energy savings equivalent to the cost of the system. Remember, the time value of money. Is spending $5000, $10000, or $15000 today worth it if it takes 15, 20, or 25 years for that investment to pay off in energy savings? That is a question that you will have to answer. Also, there are some systems out there that will cost so much over time (accounting for the cost of the system, its reliability, and its maintenance costs) that you will likely never recover your costs. All of these things must be taken into account as you choose the next system for your home.

Some things that affect the ideal SEER rating for your home are the following:

  1. How well is the home insulated?
  2. How much is the home shaded by trees?
  3. How leaky is your home?
  4. How many windows does the home have?
  5. What color is the roof? (Dark roofs absorb more heat.)
  6. What is your priority? Saving money or helping the environment by using less electricity?

These are just some of the factors that must be considered when determining the ideal SEER rating of the system to purchase for your home. As an example, if your home is “perfectly” insulated, then you would save very little in energy costs by going from an average system to a very high-efficiency system because the system will not need to run a lot.  On the other hand, if money is no object, and your number one priority is helping the environment, then by all means, buy the highest efficiency rated system out there. However, if you’re like me and somewhere in the middle of these two extremes, then I would recommend that you go with a 14 to 16 SEER rating for your new air conditioner. This is an efficient system and it will not cost you a fortune.

One thing to keep in mind is that maintenance adds even more to the cost of very high efficiency models. First, the technology is still being developed so the ultra-high efficiency models are not as reliable as the 14 to 16 SEER units. Also, when something goes wrong with the ultra-high efficiency models, it can cost two to three times as much to have them repaired.

Calculating SEER

As stated above, the SEER rating is calculated by dividing the cooling output during a complete cooling season by the energy used to achieve that cooling. Let’s look at an example. A 3-ton air conditioner is designed to produce 36,000 Btu/hr of cooling (see post, What is a Ton of Air Conditioning, for an explanation of tons of air conditioning). Let’s assume that the air conditioner runs for a total of 1200 hours during the cooling season (this would be an average of ten hours per day for 120 days).

36000 Btu/hr x 1200 hrs = 43,200,000 Btu’s produced during the cooling season. Now, let’s assume that your air conditioner is a 10 SEER unit. The units for SEER is Btu/Watt Hr.

If you divide the total Btu’s produced by the SEER rating of the A/C unit, the result will tell you how many watt hours of electricity are needed to provide that cooling.

43,200,000 Btu /10 Btu/W Hr = 4,320,000 W Hrs.

Electricity is normally measured in kilowatt hours (KW hrs), so

4,320,000 W hrs / 1000 w hours/KW hrs = 4,320 KW hrs.

If your cost of electricity is $0.12/KW then your total cost to run your air conditioner for the entire season should be

4,320 Kw hrs x $0.12/KW = $518.40.

I hope that I’ve given you some useful information and some things to think about if you are considering getting a new air conditioner for your home. If you have any specific questions or comments, please feel free to post a comment or to contact me directly.

© 2019 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.


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