SEER vs. EER (and COP)

Efficiency in air conditioning (for both traditional air conditioners and heat pumps, which serve as air conditioners in the summer, but have a reversing valve to act as heaters in the winter) measures how little energy is needed to transport heat from the inside to the outside.  Energy (and heat) can be measured in watts or BTUs.  Often, when someone first looks at the Coefficient of Performance (measured as the number of watts of heat removed from a space divided by the number of watts needed to do the removing), they're surprised that the value is above 1.  Wouldn't this make an air conditioner or a heat pump like a perpetual motion machine or something?  No, because moving the heat, not creating it.  Think of an air conditioner like a semi truck hauling gasoline down the highway.  The truck is needing a bit of energy to move, but it's carrying maybe 10,000x more potential energy to its next service station than it's requiring to operate.  So, yes, both air conditioners and heat pumps use some energy to move much more energy to the outside of the house.  

A mini split heat pump may have a rating of 12,000 BTUs/hr..  When calculating COP, we need to convert this output to watts.  1 watt = 3.41 BTU/hr.  So, 12,000 BTU/hr. x  (1 watt/3.41 BTU/hr.) = 3,519 watts.  So, a 12,000 BTU/hr. air conditioner could also be called a 3,519 watt air conditioner.  

To determine the efficiency using the COP formula of your 3,519 watt air conditioner, a run test is performed at 95 degrees Fahrenheit outside temperature, 80 degrees Fahrenheit inside temperature, and 50% humidity.  How much energy does the unit need to maintain this 12,000 BTU/hr under these conditions?  If it needs, say, 950w, then...

COP = 3,519w/950w = 3.7

So, for every watt you use to run your air conditioner or heat pump, you can kick 3.7w of heat outside.  Not bad.  

So, what's EER?  More of the same, really, except the formulate is BTU/hr. (transferred out of the space)/watts (to run the unit).  So, in the above example, we could translate the 3,519w back to 12,000 BTU/hr. to get...

EER = 12,000 BTU/hr./950w=12.63.

Think of COP or EER as geared for these very specific conditions...95°F, 50% relative humidity, 80°F in the space.  Maybe, depending on your location, this is one of your hotter days of summer.  But it's not measuring efficiency over the whole summer, and because of that, its utility, to many, is deemed dubious.  That's where SEER comes in.  

SEER is the Seasonal Energy Efficiency Ratio.  It measures how efficient a unit is not only when it's working its hardest but also when it's working at partial speed.  

SEER = (1 * EER @ 100% max load+ 42 * EER @ 75% max load + 45 * EER @ 50% max load + 12 * EER @ 25% max load)/100.

Sounds messy, but the idea is to see how efficient the unit is when it's not running at 100% (or, put another way, when it's not super hot).  The 100% max load is at 95°F again, the 75% max load is at 86°F, the 50% max load is at 77°F, and the 25% max load is at 68°F.  So, this gives a better indication of how much you might actually spend during a cooling season.  Building off of the example above...

@95°F, the unit required 950w (from before), @86°F, the unit requires 600w (assume), @77°F, the unit requires 350w (assume), and @ 68°F, the unit requires 295w (assume).  Then, 

SEER = {1 * (12,000/950) + 42 * (12,000/600) + 45 * (12,000/350) + 12 * (12,000/295)}/100 

           = {13 + 840 + 1543 + 488}/100 = 28.84.

So, this unit's SEER value (in this albeit hypothetical case) is super high because, although it is very efficient at peak load, its efficiency is outstanding at partial load.  And, arguably, one wants to know how efficient their air conditioner or heat pump is not only on the hottest days, but on any and all days it's running.  And so, arguably, SEER is more important than COP or EER.

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