How Efficient is your Air Conditioner?

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Calculating Energy Costs with Real HVAC Equipment

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.

The first cost calculation we're going to perform is going to go be formulated using the data collected off the actual data plate of the AC equipment we're concerned with - We're going to grab the HVAC equipment's rated amperage, voltage, and wattage data printed on the AC equipment itself and perform our calculation that way, projecting it into the future to see how much it costs to run our air conditioner based off the official data printed on the equipment.

The 2nd method we're going to use to calculate energy costs involves using the SEER (Seasonal Energy Efficiency Ratio) energy efficiency rating of our air conditioning equipment, which is the energy star efficiency rating that is actually required to be listed by law on the equipment.

And finally, the third method we're use invovles actually collecting the energy usage ourselves using an electrical multi-meter, and calculating our energy costs based off actual readings we collect as our AC equipment operates.

So grab your calculator, sit down, strap in and hold on because the results may surprise you as we examine the accuracy of these 3 different methods for calculating energy costs.

Method 1: Calculating Energy Costs with HVAC Equipment Data Plate Labels

So our first method of calculating the cost of operating of our HVAC equipment involves collecting the electrical data that is actually listed on the HVAC equipment itself. We're going to be looking at an 18 SEER American Standard 4 Ton Condenser made in 2006, and based on the data that's printed on the label we're going to calculate the cost of running the equipment over a period of time without using any tools or taking any measurments ourselves - We're simply taking what's listed on the data plate, and converting it into cost.

So, let's get to it, shall we?

First things first, in order to start calculating energy cost, we first need to figure out how much energy we are consuming, which means we need to gather some electrical data from our outdoor condenser, including the Voltage, Amperage, and Wattage.

Luckily for us, most of this information, with an exception to Wattage, is all required to be labeled on data plates/stickers that are located somewhere on each piece of equipment we're calculating energy cost for.

An American Standard Allegiance 18 Condenser
Finding out our HVAC equpiment's Wattage (Power) is our main agenda here, once we have our HVAC equipment's Wattage, we can easily figure out how many Kilowatt Hours worth of energy the unit is using. But as mentioned earlier, Wattage usually isn't listed on our HVAC equipment, but that's ok, right? Because if we think back to earlier where we discussed how to calculate Wattage, we can see it's simply what we get when we multiply Amperage by Voltage.

This means that, even if Wattage isn't listed on our equipment, we know we can calculate Wattage ourselves just by gathering the Amperage and Voltage off the equipments data plate, and simply multiply them together.


Watts = Voltage x Amperage

So let's gather the Amperage and Voltage data we need to perform our calculation from the outside condenser unit, and determine just how much it's going to cost us to operate our outside unit based on what's listed on the data plate below:

An American Standard Allegiance 18 Condenser Data Plate
If we examine the data plate of the American Standard condenser above, we can easily extract both the Amperage and the Voltage from all the electrical components that make up the unit.

We can see Wattage isn't listed on this piece of equipment, but that's ok because we can calculate Wattage by multiplying Amperage by Voltage.

If we take a look at the red highlighted area in the picture, we can see all of the electrical components that make up the electrical load of the entire condenser, which are each separately listed - The Compressor is listed as "COMPR. MOT", and the Outdoor Fan Motor is listed as "O.D. MOT".

Next to each component listed on the data plate is also it's respected rated Amperage, which is listed as "RLA" and "FLA" (more on that in a sec), and next to the amperage we'll notice the Voltage, which is listed as "208/230 V".

"Wait a minute... What is RLA and FLA again? I thought we were looking for Amperage?"
RLA and FLA are amperage approximations, and are technical ratings which are referring to Underwriters Laboratory test results performed at the factory where the labeled componenets are designed and tested. Technically, they may or may not be the exact amperage the component is actually using during operation.

RLA stands for "Rated Load Amperage" and essentially is an approximation of what the maximum amperage should be when the compressor is operating under maximum load conditions, which usually isn't the exact case - Actual amperage is typically lower than what's listed as RLA for compressors, and actually changes depending on the evaporator coils temperature, the condenser coils temperature, and the line voltage..

FLA stands for "Full Load Amperage", and technically it's the same definition as RLA - FLA was changed to RLA in 1976. It's typically listed for outdoor fan motors because unlike compressors, outdoor fan motors are very likely to be much closer to their FLA ratings because fan motors are designed to move a constant amount of air across the condenser's coil, wheras a compressor's load changes much more frequently because there are so many more factors involved with a compressors work load, such as refrigerant pressure, current state of matter, and temperature, which vary depending on indoor and outdoor temperature.

Actual compressor and motor amperage vary because operating conditions vary, for example, the outdoor temperature may be very hot, the indoor air filters may not be clean, or the outdoor unit's coil may be very dirty, which will change how hard HVAC components are working, thus increasing their amperage. It's a lot like your miles per hour rating on your vehicle, if your driving your vehicle like Dale Earnhardt while there isn't enough air in your tires, and you haven't changed your oil in a year, then you're actual MPG rating is going to be lower than what was listed on the sticker when you bought the car. However, if you're driving conservatively with your tires adequately filled with air, and your oil has been recently changed, then you're MPG will be less than or equal to the factory MPG label.

So for this Data Plate energy calculation, we are going to be using the RLA and FLA ratings that are listed on the condensers data tag, which means we are calculating cost based on maximum load conditions.
"Ok.. Well what about the Voltage? It says "208/230 V", what does that mean?"
The Underwriters Laboratory requires that manufacturers of electrical devices list the specific voltage tests the device was tested and confirmed to operate within. A voltage label of 208/230 V tells us that the motor has been ran under 2 separate tests: One test confirming the motor will operate with a supply voltage of 208 Volts, and another test confirming the motor will operate with a supply voltage of 230 Volts. However, it's important to note that these tests actually span a little higher and a little lower than either of the 208 or 230 voltages listed on the label.

Why 2 tests? When electricity comes into our homes from the power company it's usually not exactly 208 volts or 230 volts - Most homes in the United States actually get supply voltage at around 240 volts. In any case, if voltage were to be actually tested with an electrical voltimeter the odds are you will not have exactly 240 volts being supplied to your home - You may get 242 volts, or you may get 238 volts, either way the label is indicating a range of tests have been performed that actually verify that the component will operate within plus or minus 10% of what is actually listed on the label. So, even though it's listed as 208 Volts or 230 Volts, these tests include testing the components up to 264 volts and down to 187 volts.

Since most homes recieve somewhere around 240 volts as supply voltage, we are going to go ahead and round our voltage to 240 Volts for our Data Plate energy calculation.

Let's take a look at the data we need to focus on from the tag on the American Standard Condenser:

American Standard Allegiance 18 Condenser Data

Component Amperage Voltage
COMPR. MOT (Compressor) 17.6 240
O.D. MOT (Outdoor Fan Motor) 2.8 240

With the above information collected from our outside unit, we can now figure out how much energy our entire condenser consumes at maximum load.

Because we only have Amperage and Voltage without the Wattage for each componenet, we are going to have to calculate the Wattage ourselves, and this is typical on just about all HVAC equipment you'll come across besides Electric Furnaces or Electric Heat Kits, which typically list Kilowatts.

So, lets calculate our Wattage for our condenser's compressor by multiplying the compressor's Amperage by the Voltage:

Compressor Wattage Calculation

17.6 Amps X 240 Volts = 4,224 Watts

Since our American Standard condenser contains 2 electrical componenets that both run together when the air conditioning system is turned on, and therefore 2 electrical loads consisting of the Compressor and The Outdoor Fan Motor, we now need to calculate the Wattage for The Outdoor Fan Motor:

Outdoor Fan Motor Wattage Calculation

2.8 Amps X 240 Volts = 672 Watts

Now that we have the Wattage calculated for both the Compressor and The Outdoor Fan Motor, we can now combine the Wattages into a single value by adding both of the Wattage calculations together to get Total Wattage for the entire outdoor condensing unit:

Total Condenser Wattage

4,224 Watts + 672 Watts = 4,896 Watts

Now that we have our Total Wattage, also known as the Total Power required for the entire condenser to operate, we now need to convert our Watts into Kilowatts - Remember, the electric company charges us based on the number of Kilowatt Hours worth of energy we use, not Watt Hours worth of energy used. So, because Kilowatt Hours are made of Kilowatts which get used over time, we need to first convert our Watts into Kilowatts.

Kilowatts consist of exactly 1000 Watts, so in order to convert Watts into Kilowatts we simply divide our total wattage of 4,896 by 1,000, like so:

Convert Condenser Watts to Kilowatts

4,896 Watts / 1000 = 4.896 Kilowatts

Now that we have Kilowatts, we need to convert our Kilowatts of Power into Kilowatt Hours of Energy Usage, which simply means we need to multiply our Kilowatts by Time (Energy = Power X Time), and since a Kilowatt Hour consists of 1 single Hour worth of energy use, all we have to do is multiply our Kilowatts by 1 Hour, which will show no change in our previous Watts to Kilowatts conversion value since we are only multiplying by 1. Here's how you it:

Convert Condenser Kilowatts to Kilowatt Hours

4.896 Kilowatts X 1 Hr = 4.896 Kilowatt Hours

Gathering Kilowatt Hour Cost Information from Your Electric Bill

So now that we know exactly how many Kilowatt Hours our outdoor condensing unit uses, we need to find out how much we're paying the electric company per Kilowatt Hour worth of energy we use, right? In our Taxi example this would be like determining how much we pay the taxi company for each mile we travel.

The easiest way to do this is to examine your electric bill.

Now, electric bills can be pretty confusing to figure out on their own, let alone all this stuff we're calculating today, but electric bills get complex because electric companys change the cost of a Kilowatt Hour depending on the day, location, and other variables that they list on the electric bill.

So, what we're going to do is take the total amount of our electric bill shown below, and we're going to subtract all the base fees, surcharges, taxes, and any previous credits or outstanding balances and essentially strip our electric bill down to pure energy charges.

Once we have the total amount of dollars we spent on energy usage alone without all the extra charges included, we're simply going to divide it by the total number of Kilowatt Hours used that's listed on the electric bill for the entire month.

Lets take a look at our electric bill below and see how this is done:

An example electric bill for performing our calculations.
So, let's take a quick look at our electric bill above, and locate the information we're going to be using, and separate it from the information we're going to be stripping out, such as taxes, base charges, and surcharges.

We can see right from the beginning that our total charge for the month is $148.49, but this total includes the taxes, which are not apart of our energy usage, so let's go ahead and subtract the $0.02 we paid in taxes from our total and remove the taxes from the equation, as well as the Customer Charges fees of $8.45:

Isolating Energy Charges on our Electric Bill

Subtracting all extra fees from our electric bill.

There may be more charges on your own electric bill that aren't related to energy usage, so make sure you subtract anything that isn't dealing with the generation, distribution, or transmission of energy.

After we subtract all the extra charges and fees from our electric bill, we are left with a total of $140.02. This is now the total amount we spent on energy use alone, and as you can see, there's still quite a few categories left on the electric bill we couldn't eliminate, that's because the electric company charges us for Generating Energy, Distributing Energy, as well as Transmitting Energy, all of which are apart of our energy usage.

However, now that we have the total amount of money that we spent on energy usage, we need to divide it by the total amount of Kilowatt Hours we used for the entire month, and a quick glance at the upper right hand of our electric bill shows us that we used exactly 1,022 Kilowatt Hours worth of energy over the course of the entire month.

So let's divide the total amount of money we spent on energy, by the total number of Kilowatt Hours we used, and calculate how much an individual Kilowatt Hour actually costs accross the Generation, Distribution, and Transmission energy charges:

Calculating How Much We Pay Per Kilowatt Hour

$140.02 / 1,022 Kilowatt Hours = $0.13700587

So, after dividing our total energy cost by the total number of kilowatt hours we used for the month, we arrive at a really long and ugly number of $0.13700587084148727984344422700587 per Kilowatt Hour. Which, for the sake of simplicity, we are going to go ahead and round to $0.14 cents per Kilowatt Hour from now on.

So, we're paying $0.14 cents a Kilowatt Hour, now what?

Now that we know how much we pay the electric company per kilowatt hour used, how about we calculate how much we pay per hour that we run our outdoor unit? If we look back to how many Kilowatt Hours worth of energy our outdoor condensing unit used, we can see that it was using right at 4.896 Kilowatt Hours worth of energy per hour. So let's go ahead and multiply our outdoor unit's energy usage of 4.896 Kilowatt Hours by the $0.14 cents we pay the electric company when we use a single Kilowatt Hours worth of energy:

Calculating How Much We Pay Per Hour To Run The Condenser

4.512 kWh X $0.14 (Cost Per kWh) = $0.63168 (Cost Per Hour)

Alright! So another funky number pops out of our calculation of the Cost Per Hour to run our condenser at a rough $0.68544 cents an hour, which we're also going to go ahead and round up to $0.07 cents an hour. Now, since we know how much it costs to run our condenser each hour, we can easily figure out how much it costs to run the condenser per day, or per week, or per month, and so on. Below is a cost analysis table for us to think about:

Condenser Run Time Cost Analysis

Run Time Length Formula Cost
Hour 4.896 kWh (Condenser kWh) / $0.14 (Cost Per kWh) = $0.68544 (Cost Per Hour) $0.7 Per Hour
Day $0.68544 (Cost Per Hour) X 24 Hours = $16.45056 (Cost Per Day) $16.46 Per Day
Week $16.45056 (Cost Per Day) X 7 Days = $115.15392 (Cost Per Week) $115.16 Per Week
Month $16.45056 (Cost Per Day) X 31 Days = $509.96736 (Cost Per Month) $509.97 Per Month
Year $509.96736 (Cost Per Month) X 12 Months = $6119.60832 (Cost Per Year) $6119.61 Per Year

Hmm.. The costs don't add up, our electric bill looks nothing like this. I think our calculations are way off or something. Either we did something wrong, or the electric company did, right?

Well, not exactly. Remember, we are calculating our run time costs based off the air conditioner's RLA and FLA ratings, which are amperage ratings achieved from testing equipment in a laboratory where the equipment is operated under it's maximum load conditions, which isn't a scenario most homes ever replicate.

The table also only reflects costs based on the continous operation of our outdoor unit at these run times, and if you really think about, no one runs their air conditioner continuously (usually), and no one runs their air conditioner continuously under maximum load conditions.

After reviewing the table, it really brings up more questions than answers, doesn't it? Specifically:

"I don't watch my condenser 24/7, I know it's not running constantly (I hope) so how long does it actually run?"

Well, there are a couple of ways to monitor our air conditioning system's run time without physically watching it. One of the cheaper ways is to install a smart thermostat such as the Ecobee Thermostat, which will monitor and graph your air conditioning systems run time for you to review whenever you like. There are also more expensive whole home energy monitoring systems, which can be installed on each circuit of your home by an electrician, and will monitor the energy usage of every circuit in your home for a more complete picture if your real serious about monitoring your energy usage.

But, our cost analysis table definitely doesn't reflect real life usage at all. Not only do we not run our air conditioners continuously for entire days, weeks, or months at a time, but the electric company can also change their rates throughout the year, and during peak demand times.

Not to mention all of the other factors that vary from person to person, home to home, and location to location, such as:

Unknown Factors That Affect HVAC Run Time

Factor Explination
Seasonal / Weather Seasons and Weather forecasts change throughout the year which will have an impact on our air conditioners run time - Air conditioners run more often during the summer than during the winter. The weather also has an impact on our equipment's run time, which is a factor that is going to vary greatly from location to location, day to day, month to month, and year to year.
Temperature Setting Preference The human metabolism is the primary mechanism involved in human comfort and is a huge unpredictable factor when it comes to AC run time. People's bodies are simply different and also react differently when they do different things and it's difficult to predict changes in the human metabolism brought on by food, emotions, and activity. Some people feel comfortable keeping their air conditioners set at 76, while others may keep their air conditioner set on 69, which is going to affect how long our air conditioner operates.
Heat Load All homes have an internal heat load, which is defined as the amount of heat that is generated from within the home. Most of us think of heat as something that enters our homes through windows or open doors and windows during the day brought on by the sun, but heat is also created internally within our homes. Factors such as the number of people living in our homes, their activities, as well as their things, such as televisions, cooking and office equipment, showers, non-led light bulbs and computers, and just about everything that plugs into a wall outlet all create heat within our homes which can be referred to as a home's internal heat load. As a home's internal heat load increases, so does the internal temperature, and therefore the AC system will kick on and run longer to cool down a home with a higher internal heat load versus a home with a very low internal heat load.
Insulation Insulation plays an important role in controlling the temperature in our homes. Homes that are tightly sealed and well insulated will stay colder longer due to a higher insulation factor which will trap cold air inside the home, and hot air outside of the home, or vice versa, which means the HVAC system wont be running nearly as often when compared to a home with a poor insulation factor. Theoretically, if a home was absolutely insulated, the air conditioner would only have to work to remove the homes internal heat load, and would rarely operate. Factors such as opening and closing doors or windows also affect how much heat enters a home, which will affect system run time.
Equipment Maintenance Have you ever forgotten to change your air filter for a few months? What about if you didn't have your outdoor condenser coil cleaned for the entire year? Dirty air filters and dirty coils greatly reduce an air conditioners ability to transfer heat from the inside of your home (through the air filter) to outside of your home (through the condenser coil). If the AC system cannot pull all of your homes air through a dirty air filter, or transfer all of the heat outside of your home due to a dirty condenser coil, then it will run longer as it attempts to do so, and therefore use more energy.
System Design HVAC system design plays a huge role in how well the air conditioning equipment can perform it's job. Factors such as: Placement of ducts and vents, the number of ducts, the duct's insulation rating as well as the size of ducts, all affect how well air conditioning equipment can transfer heat and deliver conditioned air throughout a home. It's very typical for home construction companies to go by the bare minimum when it comes to duct design and install only 1 huge return air grille in a central location of the home. This is in fact very inefficient and creates the opportunity for heat to build up in areas all throughout the home specifically in rooms where the doors are closed (such as bedrooms). If an air conditioning system cannot pull the hot air out of a room because the door is closed, then heat will build up within these rooms, which is going to affect the systems run time and also tends to make people turn their thermostats lower and make their systems run longer in order to provide cool air to these hot areas, when in reality the problem isn't a matter of adding more cool air, it's a matter of removing the heat from these isolated rooms where the air simply cannot reach the return air grille easily.
Efficiency Equipment efficiency varies depending from system to system, especially when it comes to inverted systems which can modulate their capacity and therefore their work load. Variable speed components, such as variable speed fan motors and inverted compressors, all have the ability to "switch gears" depending on their settings and the way they are set up by the HVAC company who installed the equipment to satisfy the comfort of their clients. Components like these can be set up in such a way that they don't run at a constant speed, and essentially change speeds throughout the day, and as they change speed, they consume more or less energy.
Comparing Apples to Apples

But, even if we disregard all of the unknown factors listed above that affect our system's run time, one fact remains the same: Our data plate energy cost calculation is based off the maximum operating conditions of our condenser, and so we should look at it as such, even if we are monitoring our AC system run time with a smart thermostat, basing our cost analysis off maximum operating conditions is still what we are achieving here.

Therefore, data plate energy cost calculations should only be compared to other data plate energy cost calculations and viewed as a comparison of maximum energy consumption across HVAC products - They don't actually reflect our actual energy consumption, but they do give us a good understanding of comparing maximum operating conditions across pieces of equipment.

So, let's compare apples to apples, and oranges to oranges, and simply rely on data plate cost calculations as one of many perspectives we can use when we viewing the energy consumption of our HVAC equipment. Which brings us to our next energy cost calulation method: The SEER rating method.

Method 2: Calculating Energy Costs With The SEER Rating

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 calulate 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.

Our American Standard Condenser is has a SEER rating of 18

The Seasonal Energy Efficiency Ratio (SEER) is an efficiency rating that is required by law to be listed on cooling equipment and is used to express an air conditioners performance over the course of a typical summer in the United States, the higher the air conditioners SEER rating, the more energy efficient the air conditioner should be. It's technically a ratio of the cooling output of an air conditioner measured in BTUs, divided by it's electrical input energy measured in Watt-Hours.

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 conditioners 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 it's 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 yeild 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.

It's much easier than before this time.

Since the SEER rating is an energy efficiency rating, it's already done most of the math for us, so it's going to be much easier to calculate our cost this time.

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

SEER = BTUs / Watt Hours

Since our SEER equation above consists of a SEER rating, BTUs, and Watt Hours, we're going to have to atleast 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 conditioners tonnage already, then you can skip this part:

Condenser Model Number: 2A7A8048B1000AA

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

Uhhh.. That's a hard, and definite, NO! Where's the tonnage part listed, again?

Don't feel bad, manufacturers of HVAC equipment "hide" the tonnage information in the model number, so you have to really know what your 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 manufactuer.

Let's step our way through disecting 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:

Tons to BTUs

Tonnage BTUs Model Number Style
1 Ton 12,000 BTUs 12
1.5 Ton 18,000 BTUs 18
2 Ton 24,000 BTUs 24
2.5 Ton 30,000 BTUs 30
3 Ton 36,000 BTUs 36
3.5 Ton 42,000 BTUs 42
4 Ton 48,000 BTUs 48
4.5 Ton 54,000 BTUs 54
5 Ton 60,000 BTUs 60

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 your looking for in a model number to determine tonnage, is a number that is divisible by 12.

That's horrible! I thought you said this was going to be easier than before? How do you do this everyday without pulling your hair out?

You don't want to know.. Don't even get me started when manufacturers of much larger equipment *cough* york *ahem* carrier *cough* like to list tonnage as a letter instead of a number.. Anyway, so our tonnage in the residential arena is hidden in the model number, and consists of a pair of numbers that are divisible by 12, who would of guessed? 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 manufactuers 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..

Anyway, so what do you do if you can no longer read or find the bright yellow energy star sticker on your piece of equipment and your SEER rating isn't listed on the data plate like our American Standard condenser?

Unfortunately, if the energy star tag is gone, and the SEER rating is nowhere to be found on your condensers 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.

Try Searching Online First Try searching online if you can't locate your condensers SEER rating, it shouldn't take long to find!

Alright, so we have our BTUs and our SEER rating, now what?

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:

48,000 BTUs / 18 SEER = 2,666.666666666667 Watt Hours

Once again our calculation yeilds 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? Lets do it:

2666.666666666667 Watt Hours / 1000 = 2.666666666666667 Kilowatt Hours

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 came 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.

2.666 Kilowatt Hours X $0.14 Cents = $0.37324 Per 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 condensers 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.

Condenser Run Time Cost Analysis

Method Run Time Length Formula Cost
Data Plate Hour 4.896 kWh (Condenser kWh) / $0.14 (Cost Per kWh) = $0.68544 (Cost Per Hour) $0.70 Per Hour
SEER Hour 2.666 kWh (Condenser kWh) / $0.14 (Cost Per kWh) = $0.37324 (Cost Per Hour) $0.40 Per Hour
Data Plate Day $0.68544 (Cost Per Hour) X 24 Hours = $16.45056 (Cost Per Day) $16.46 Per Day
SEER Day $0.37324 (Cost Per Hour) X 24 Hours = $8.95776 (Cost Per Day) $8.96 Per Day
Data Plate Week $16.45056 (Cost Per Day) X 7 Days = $115.15392 (Cost Per Week) $115.16 Per Week
SEER Week $8.95776 (Cost Per Day) X 7 Days = $62.70432 (Cost Per Week) $62.71 Per Week
Data Plate Month $16.45056 (Cost Per Day) X 31 Days = $509.96736 (Cost Per Month) $509.97 Per Month
SEER Month $8.95776 (Cost Per Day) X 31 Days = $277.69056 (Cost Per Month) $277.70 Per Month
Data Plate Year $509.96736 (Cost Per Month) X 12 Months = $6119.60832 (Cost Per Year) $6119.61 Per Year
SEER Year $277.69056 (Cost Per Month) X 12 Months = $3332.28672 (Cost Per Year) $3332.29 Per Year

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, noone runs their air conditioner for entire months or years at a time, the table is simply something for us to compare continous run times, and now that we have the data plate calculations next to the SEER method calulations, we can definitely see the difference!

There's only 1 thing left to do, and that's calculate our actual energy consumption based off our condenser, so let's get to it and see how these methods actually compare to the real energy our American Standard condenser is using.

Method 3: Calculating Energy Costs With Actual Readings

Alright, so now it's time to actually measure how much energy our condenser is actually using, and compare it to the cost per Kilowatt Hour on our electric bill!

Now, for this method you'll need a digital multi-meter, and also a reasonable sense of safety around electrical equipment.

Remember: Air conditioning condensers are typically supplied with 240 volts of power, some are even 460 volts of 3 phase power, so if your not familiar with measuring electrical readings and electrical safety, now is probably not the time to learn. Only perform this method if you know what your doing and how to stay safe around high voltages.

So, with that part out of the way, let's go ahead and discuss what exactly we're going to be doing. This method is very similar to our first method, where we gathered the FLA and RLA ratings off the data plate of our condenser, except this time we're going to gather actual amperage with a digital multimeter that measures the amp draw of our condenser. Then once we have the amperage of our condenser while it's operating, we'll simply repeat the steps we performed in Method 1. Simple, right?

So, if we recall how we calculated our condensers power before, we simply gathered the Voltage and Amperage data listed on our condensers data plate and multiplied them together to find Wattage. This time we're actually going to gather this data with a digital multimeter, let's go ahead and gather the supply voltage information we need from our condenser's disconnect box so we don't have to actually open up the unit:

Our meter reads 248 volts alternating current

And look there, measuring across the leads shows us that our supply voltage is at 248 volts alternating current. Remember how when we examined the data plate earlier, we saw that our condenser was tested and confirmed to operate at either 208 volts or 230 volts, and our actual reading is 248 volts. Voltage is something that is going to differ at every single home due to the transformer on the pole that's owned by the electric company, and also the distance the electricity has to travel from the distribution center.

Now that we have voltage, let's go ahead and gather the amperage off the condenser using our multi meter and once again gathering the data from the disconnect box:

Our meter reads 5.9 Amps

Our meter shows us that our condenser is drawing 5.9 Amps, and that's interesting, especially if we compare it to the RLA and FLA data listed on the data plate which we used in our first calculation in Method 1 and came in at a whopping 20.4 amps. That's a huge difference from our actual readings of 5.9 Amps.

Now just to be clear, a condenser's amperage is going to vary depending on several factors, the most important factors that determine a condensers load happen to be the indoor and outdoor temperature, as well as how clean the evaporator coil and condenser coil are, as well as the air filters inside of our home. To be fair, our indoor temperature at the moment of testing our condenser is 74 degrees, and our outdoor temperature near the condenser is 82 degrees, both the condenser and the evaporator coil are both 11-12 years old, and either has ever been cleaned, the air filters however are clean. These combined factors have all lended a hand at forming the amperage readings we have gathered with our multi-meter at the time of testing.

So now that we have both voltage and amperage readings, we now need to calculate the wattage, which means we need to multiply the voltage reading by the amperage reading as shown below:

248 Volts X 5.9 Amps = 1,463.2 Watts

Awesome, so our condenser is using 1,463.2 Watts of power, which is so far better than any of the other methods we've actually used to calculate power thus far.

Now, if we remember from doing this earlier, the electric company doesn't bill us on power, they bill us on Kilowatt Hours worth of energy usage, so since right now we have Watts of power, let's go ahead and convert our Watts to Kilowatts by dividing our Wattage by 1000:

1,463.2 Watts / 1000 = 1.4632 Kilowatts

And once again, our calculation comes in at the lowest we've seen so far at 1.463.2 Kilowatts. Let's go ahead and multiply our Kilowatts by 1 Hour to convert it into Kilowatt Hours:

1.4632 Kilowatts X 1 Hour = 1.4632 Kilowatt Hours

So now we have our condensers energy usage, which is 1.4632 Kilowatt Hours. We can now calculate how much we pay to run the condenser per hour by taking our condensers energy usage, and multiplying it by the rate the electric company charges us per Kilowatt Hour worth of energy we use, which is $0.14 cents per kWh:

1.4632 Kilowatt Hours X $0.14 Cents Per kWh used = $0.204848 Per Hour of Operation

Great! So now we have calculated how much it actually costs to run our condenser per hour, and the results are pretty amazing (Way to go American Standard!) - Our actual price per hour is lower than any of the other calculations we've gone through so far, including the data plate method, and the SEER rating method, awesome!

Let's go ahead and update our cost analysis table to show all 3 methods of calculating costs so we can compare them side by side:

Condenser Run Time Cost Analysis

Method Run Time Length Formula Cost
Data Plate Hour 4.896 kWh (Condenser kWh) / $0.14 (Cost Per kWh) = $0.68544 (Cost Per Hour) $0.70 Per Hour
SEER Hour 2.666 kWh (Condenser kWh) / $0.14 (Cost Per kWh) = $0.37324 (Cost Per Hour) $0.40 Per Hour
Actual Hour 1.4632 kWh (Condenser kWh) / $0.14 (Cost Per kWh) = $0.204848 (Cost Per Hour) $0.21 Per Hour
Data Plate Day $0.68544 (Cost Per Hour) X 24 Hours = $16.45056 (Cost Per Day) $16.46 Per Day
SEER Day $0.37324 (Cost Per Hour) X 24 Hours = $8.95776 (Cost Per Day) $8.96 Per Day
Actual Day $0.204848 (Cost Per Hour) X 24 Hours = $4.916352 (Cost Per Day) $4.92 Per Day
Data Plate Week $16.45056 (Cost Per Day) X 7 Days = $115.15392 (Cost Per Week) $115.16 Per Week
SEER Week $8.95776 (Cost Per Day) X 7 Days = $62.70432 (Cost Per Week) $62.71 Per Week
Actual Week $4.916352 (Cost Per Day) X 7 Days = $34.414464 (Cost Per Week) $34.42 Per Week
Data Plate Month $16.45056 (Cost Per Day) X 31 Days = $509.96736 (Cost Per Month) $509.97 Per Month
SEER Month $8.95776 (Cost Per Day) X 31 Days = $277.69056 (Cost Per Month) $277.70 Per Month
Actual Month $4.916352 (Cost Per Day) X 31 Days = $152.406912 (Cost Per Month) $152.41 Per Month
Data Plate Year $509.96736 (Cost Per Month) X 12 Months = $6119.60832 (Cost Per Year) $6119.61 Per Year
SEER Year $277.69056 (Cost Per Month) X 12 Months = $3332.28672 (Cost Per Year) $3332.29 Per Year
Actual Year $152.406912 (Cost Per Month) X 12 Months = $1828.882944 (Cost Per Year) $1828.89 Per Year

Whew! We're done! And, at this point we've calculated energy costs based on our condenser's data plate, and we've calculated energy costs based on our condenser's SEER rating, but neither of these calculations came anywhere close to the results we saw using our condenser's actual energy consumption, and that's definitely a good thing!

Final Thoughts

"So how should we look at efficiency ratings and these cost calculations? Should we assume our actual energy consumption is always going to be lower?"

No, we shouldn't - Even though our actual energy consumption came out to be lower than what our calculations predicted we would pay, that may not be the case in your situation - There are still just too many factors involved in any home's HVAC energy consumption to make our calculations and predictions exact - Remember: EVERYTHING affects an HVAC system's energy consumption! Factors such as outdoor temperature, indoor temperature and humidity level, the home's insulation factor, the cleanliness of the home's air filters, and pretty much everything inside a home that puts off heat (Lights, Showers, Windows, People) are all factors we didn't include in our calculations. In other words: All 3 of these methods are still technically projections, or estimates, and even though we used actual data, we didn't take into account every single variable that affects an HVAC systems energy consumption, which makes our calculations fall under the category of prediction. So, it's important to remember that these methods exist because they are useful for us to go by when comparing the equipment we're planning on purchasing. After all, manufacturers don't let you bring home their equipment and plug it into your home and test it before making a purchase. So, it's important to remember that when we are shopping for new equipment and making these actual calculations, we should always consider both the minimum and maximum results we get from our energy consumption predictions and aim for somewhere in the middle with our expectations.

"What about my furnace? Doesn't it use electricity as well, even when the air conditioner is running?"

That's correct. Your furnace also consumes electricity even if your only running your HVAC system in cooling mode, so it's important to point out that we only calculated the energy consumption from half of the entire HVAC system in these examples - The Condenser. The condenser is only one half of an air conditioning system, and whenever the condenser is running outside, then your indoor fan is also running inside your furnace and blowing air throughout the ducts in your home. Since this fan motor inside your furnace operates on electricity, it is technically apart of your electric bill and could always be added to your calculations since the furnace is a part of the HVAC system. However, as we can see from our recent calculations, fan motors like the one inside your furnace don't use nearly as much energy as all the components located inside your outdoor condensing unit, but it does add to the cost of your energy bill and it's an extra cost we didn't include in our calculations.

Therefore, if you wanted to include your furnace's energy consumption within these cost predictions, these same calculations can be performed with the data from your indoor fan motor by examining your furnace and collecting the same information from Method 1 off your furnace's data plate (Just like we did with the outdoor condenser's data plate), which you could then include in your calculations to predict how much it will cost to operate your entire HVAC system as a whole, and not just the outside unit as we have done in this article.

Anyhow, that about wraps it up for our HVAC energy efficiency series. Thanks for reading, and we hoped you learned a little something about HVAC today! As always, Adams Air takes pride in sharing information of all kinds, and promoting a progressive learning environment for all to enjoy - If you have any questions, feel free to let us know!