What Is The Difference Between SEER and EER?

You’ve heard of EER and SEER being talked about and you’re wondering “what are these numbers and how are they important to me?”

In this article I’ll explain what each one means, how they calculated and why they are important to you when choosing an air conditioner.

What Is The SEER – Seasonal Energy Efficiency Ratio?

The seasonal energy efficiency ratio is a measure of an air conditioner’s cooling efficiency. The higher the SEER the more cooling you get for your money. It is a ratio of its cooling power output (in BTUs) divided by the amount of energy in watts (Watt/Hours) that is consumed to produce this cooling power. In other words, it’s a measure of how efficiently the system uses electricity to cool the air. There is one important point the cooling power is measured over a “season” hence the word seasonal.

Woman Researching OnlineFor example, if an air conditioner was rated at 750w and ran for 8 hours a day during a 125 day season the amount of watt hours consumed would be 750x  8 x 125 = 500,000 watt hours = 750kWh.

The BTU value is actually a measure of the heat energy that the unit can remove from the air each hour. Let’s say that the BTU value of the portable air conditioner was 12,000. This can also be multiplied by 8 hours a day and 125 days to calculate the total amount heat energy removed from the air over the season. This would be 12,000,000 BTU.

The SEER = the total BTU removed over the season / the total watts used to power the unit over the same period = 12,000,000 / 750,000 = 16.

The SEER is normally displayed on a tag which is attached or printed directly onto your unit. The range of SEER values starts at about 8 and can go up to 30. The higher the SEER the better as it means the unit produces more cooling power for the less electricity. The US government regulation issued by EPA (Environmental Protection Agency) states that any new AC unit produced after 1st January 2015 must have a SEER greater than 14.

In reality most portable a/c units operate in the high teens. Ducted central systems provide more cooling for your money and operate in low 20’s. Ductless and geothermal systems can range from mid 20’s to over 30!

The Problem With SEER

There is a small problem with the SEER value. To allow equal comparisons to be drawn between A/C units the “season” has to have a standard definition. This definition is based on the unit being used for a fixed number of hours each day, over a fixed number of days. It also assumes that the indoor temperature is held constant while the outdoor temperature changes over a set range, ranging from 60°F to over 100°F.

This allows the SEER rating of models to be compared like-for-like which is good. However, if you live in a climate where the average temperature range is well outside that used for calculating the SEER then it can be misleading.

For example, if you live in cities along the south, and particularly in the Florida area, then you can have a very high average temperature. This means that your unit will be using more power than the average over the season.

Likewise, if you live in a city where you have shorter summers and longer cooler winters it might be opposite.

SEER Adjustment Rule of Thumb

The average range for SEER is around 83 degrees. Check out the average summer temperature in your area. For every 10 degrees above 83°F subtract 2 points from the SEER rating.

For example, if you live in a climate where the average summer temperature is 93°F degrees then the adjusted value would be 14 in the example above, because it’s 10 degrees more that 83°F so 2 points have been subtracted from the SEER rating.

How Is The SEER Used?

The SEER can be used to estimate the cost of running the unit over a period of time, usually a year and also it’s useful life (i.e. how long will you keep the unit for before it will need to be replaced). The useful life is usually 10-15 years.

SEER = BTU Cooling In A Season / Total Watt Hours

Let’s look at an example.

  • The size of a typical portable air conditioning unit is 12,000 BTU/h.
  • We will assume it is operated for 1000 hours a year.
  • It’s rated at 750 watts.
  • Electricity costs an average of 12.68 cents per kilowatt-hour.

The full calculation is therefore:

12,000 (BTU/h) x (1000 hrs) x ($0.1268/kWh) / [16 BTU/Wh) * (1000 W/kW)] = $95.10

If the SEER was lower i.e. 13 the figure would be $117 so you’d save an extra $21.19 a year in energy costs. Over the 15 year lifetime of the unit that would be around $328.50. So it would be well worth your while paying a bit more for a higher SEER rated unit.

There are of course other factors to consider such as expected costs of electricity in the future and number of hours you think you’ll be running the unit for each year, but you can use the equation above to input the variables that best suit your situation.

The other consideration is the time value of money but that is too much detail for the scope of this article!
Over the last few years all air conditioning units and systems have increased in efficiency as manufacturers implement new technology to drive up their mechanical efficiency. The example above is for a small portable air conditioner and the sums of money are not that big. However, if you’re running a central air conditioning unit at say 72,000 BTU (3 tons) the different is SEER value could really start to add up. This means that is could well be worth carrying out a quick calculation to understand if replacing your current unit might be worth the money.

What is the EER – Energy Efficiency Ratio?

We’ve covered the SEER, now it’s time to talk about the EER. The EER is more of a conceptual number that is used mainly from an engineering or HVAC technician point of view. It’s a ratio of the cooling power in BTU divided by the wattage rating of the system. So for example, if the portable ac unit was rated at 12,000 BTU and ran at 750 watts the value would be 12,000/750 = 16.

EER = BTU Rating of the Unit / Wattage Rating of the Unit

The EER is a pure ratio based on BTU amount of cooling that an air conditioner will provide and the watts used to power this cooling. It makes the assumption that the outside temperature is 95°F and the indoor temperature needs to be maintained at 80°F. It also assums a humidity of 50%, no concept of seasonal variations is factored into this calculation.

When To Use The EER And The SEER, And Which Is Best?

Because the SEER factors in a range of outdoor temperatures over a period of time known as a season, it tends to be used indicate actual performance of the unit or system. You will need to adjust the SEER for your average temperature and then use it to calculate an estimated cost of running the unit over a year.

The EER is a pure ratio. This makes it useful for simply carrying out a like-for-like comparison of how efficient the unit will be but it doesn’t take into account the likely power consumption over a period of time.

Hopefully you’ll have learnt an insight into what the SEER and EER are and how best to use them. If you have any questions or comments I’d love to hear them. Please leave a comment below.

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