How Efficient is your Air Conditioner?

Alright, so you've made it to the end of our HVAC energy efficiency discussion, and now that you have a good understanding of this whole HVAC efficiency thing, how about we perform a little experiment and take all that we have learned and calculate some energy costs ourselves with a real air conditioner?

We're actually going to cover a few different ways of performing energy cost calculations today in order to compare exactly which method is most accurate and also show the differences between what's listed on labels and actual energy consumption. Keep in mind, all of these methods are still technically predictions based off of real data, but it's still interesting to see just how accurate the official energyguide labels are when compared to actual performance readings.

So far we've learned how to calculate the cost of energy consumption for our American Standard condenser based off the information listed on our condenser's data plate, which only listed data gathered by testing the condenser under maximum operating conditions, and ultimately resulted in the maximum prices we should every have to pay for running our condenser.

But there's another rating on the condenser we can calculate energy consumption based off of, and it's probably one you've seen on an energy guide sticker or heard of before when discussing your air conditioning equipment with your AC guy: The SEER rating.

Like the RLA and FLA ratings we gathered off our outdoor condensing unit's data plate in our previous calculation, the SEER rating is also another calculation that is determined in a laboratory setting, but instead of gathering data from tests performed under the air conditioner's maximum operating conditions, the SEER rating is based off of a range of conditions that attempt to simulate a summer scenario home owners experience in the United States.

And, although the summer season itself differs depending on where you are in the United States, the SEER rating bases its averaged results off various indoor and outdoor temperatures ranging from 80°F to the 95°F between 40%-50% relative humidity, and tests the equipment under these conditions and simulates an environment that the equipment is much more likely to operate within when it's installed at a home.

It's not a perfect science since summers vary across the U.S., and it's definitely not a calculation that reflects the air conditioner's actual energy consumption, but it does yield much more reasonable results for us to compare the energy consumption of air conditioning equipment to versus simply comparing the RLA and FLA information we collect off of equipment data plates.

So let's go ahead and get started and see how basing energy calculations off SEER ratings compare to our previous example where we based our calculation off of information listed on the data plate.

As mentioned earlier, the SEER rating is a ratio of our condenser's cooling output measured in BTUs, divided by its energy input in Watt Hours, as shown below:

Since our SEER equation above consists of a SEER rating, BTUs, and Watt Hours, we're going to have to at least collect 2 of these variables to solve for the other. The easiest variables in the equation to collect are the SEER rating, and the BTUs.

If you recall from our Understanding BTUs, Heat, and Heat Transfer section, a Ton of Air Conditioning is equal to 12,000 BTUs, and manufacturers of air conditioning equipment happen to hide the tonnage of the equipment somewhere in the model number of the equipment, which is listed on the data plate. So let's examine our American Standard condenser's data plate, locate the model number, and extract the tonnage information we need for our calculation. If you know your air conditioner's tonnage already, then you can skip this part:

Do you see the tonnage information in our model number: 2A7A8048B1000AA?

Don't feel bad, manufacturers of HVAC equipment "hide" the tonnage information in the model number, so you have to really know what you're looking for in order to locate it because the model number of any piece of HVAC equipment contains a bunch of different information about the unit, and all model numbers are going to differ from manufacturer to manufacturer.

Let's step our way through dissecting the model number to locate the tonnage information so we can understand how to locate it across various types of HVAC equipment.

We know that 1 Ton of air conditioning is equal to 12,000 BTUs, and it just so happens that instead of listing "1 Ton" or "2 Tons" on air conditioning equipment, manufacturers list the BTUs, but they also list it a certain way within the model number.

Residential air conditioning equipment typically comes between 2-5 Tons. Below is a table that converts tonnage into BTUs:

If you examine our table above, you'll see that each ton is exactly 12,000 BTUs larger than the previous, therefore, a ton is equal to 12,000 BTUs, which means if tonnage is listed in BTUs and it's hidden in the model number, then the number we are looking for is obviously divisible by 12,000, or is it?

Most likely not!

At this point, you have to start to think like the manufacturer who has a limited amount of physical space on their label and a ton (no pun intended) of information they want to fit inside a model number that they also have to fit on their tiny label. So instead of listing all of the BTUs, they simply list the first 2 numbers, and shave off all the extra zeros.

So, 24,000 BTUs appears simply as "24" within a model number, and "48,000 BTUs" appears simply as "48" and so on.

So essentially, what you're looking for in a model number to determine tonnage, is a number that is divisible by 12.

Let's look at our American Standard condenser's data plate one more time and zoom in on the tonnage information we're looking for within the model number:

Now that we have located the BTUs in our model number, which is actually listed as 48 but therefore translates into 48,000 BTUs, we now need to locate the actual SEER rating of our condenser, which is required by law to be listed on air conditioning equipment as a bright yellow energy star label like the one shown below.

We can see from the picture above that the SEER rating for our condenser clearly rates our condenser at 18 SEER. American Standard was also nice enough to list the SEER rating within our condenser's name: The Allegiance 18. Now, this definitely isn't always the case, especially with other manufacturers who seem to go out of their way to make us really dig for information like this.

And, even though all manufacturers are required to list the yellow energy star tag with the SEER rating clearly listed on it, over time this sticker will deteriorate and fade away because it's made of paper and isn't made to withstand the elements like a data plate is. Often the sticker is also removed by the company who installs the equipment for some reason, I've never figured that one out..

Unfortunately, if the energy star tag is gone, and the SEER rating is nowhere to be found on your condenser's data plate, you'll then have to reference the model number online, or with the manufacturer and have them tell you what the SEER rating is. So, if this sounds like you, try searching Google for the model number of your piece of equipment first, you should easily be able to find the SEER rating that way, and if not, you can always look through the manual that came with your air conditioning equipment, or call the manufacturer and ask them to tell you what the SEER of your model number is.

Now that we have both variables off of our American Standard condenser, we can easily solve for the energy consumption the SEER rating testing gathered at the laboratory when the equipment was tested by dividing the BTUs by the SEER rating as shown below:

Once again our calculation yields an ugly number of 2,666.666666666667 Watt Hours worth of energy, but, before we round it, let's stop and think real quick. Right now, we have energy consumption based in Watt Hours, but the electric company bases our bill off of Kilowatt Hours worth of energy, so before we round our number, let's first convert Watt Hours to Kilowatt Hours.

Converting Watts to Kilowatts isn't so bad - We know a Kilowatt consists of exactly 1000 Watts, so we simply divide our Watt Hours by 1000 to convert it to Kilowatt Hours. Easy, right? Let's do it:

Alright, now that we have converted our Watt Hours into Kilowatt Hours, we can now go ahead and round our 2.666666666666667 Kilowatt Hours to 2.666 Kilowatt Hours just to make life a little easier on us.

Wow, that's a big difference from our previous calculation that we based off of the data plate, which had come out to 4.896 Kilowatt Hours. So based off the SEER rating, we're using about half as much energy this time, and so our energy cost should definitely reflect that change in energy consumption.

Let's go ahead and see how much it costs to run our condenser per hour based off our SEER rating by multiplying our Kilowatt Hours worth of energy consumption by the price we pay per Kilowatt Hour worth of energy used listed on our electric bill again, which is $0.14 cents per Kilowatt Hour.

Nice! So just to recap, before we were paying $0.68544 per hour to run our air conditioner when we based our calculations off the condenser's data plate, and now we're only paying $0.37324 per hour when we use the calculations determined by the SEER rating. Let's go ahead and make another cost analysis table and compare both methods, dollar to dollar.

Wow, so what a huge difference that made now that we can see it all out before us in the table. Remember, as stated previously, no one runs their air conditioner for entire months or years at a time, the table is simply something for us to compare continuous run times, and now that we have the data plate calculations next to the SEER method calculations, we can definitely see the difference!