Air Conditioning Terms and Concepts for Calculating Cost
Air conditioning can be responsible for over 50% of your electric bill during the hot Florida summer months, so you may ask yourself, how much does my air conditioner cost to use?
Before we can tackle the cost of running an air conditioner, we first have to become familiar with a few of the terms and concepts we will use to make the calculation.
A BTU stands for British Thermal Unit, which is the unit that is used in air conditioning and heating to measure the cooling power of an air conditioner. Simply put, one BTU is the amount of energy that is required for your air conditioner to cool one pound of water by one degree Fahrenheit. So how many BTUs does your air conditioner come in at? That is one of the things that you have to figure out, but it’s easier than you think: all you have to know is the size of your air conditioner.
Your air conditioner is measured in ‘tons.’ One ton is equal to 12,000 BTUs of cooling power. So, if you have a three ton unit (the average sized unit in Central Florida) then your air conditioner has 36,000 BTUs of cooling power:
3 tons X 12,000 BTUs/ton = 36,000 BTUs
If you don’t know what size your air conditioner is, feel free to call us with your model number located on the air conditioner data sticker.
An Ampere is a unit of measurement used to measure how many electrons (electricity) have passed a single point in a given unit of time. If you want to nerd-out on Amperes, visit the Wikipedia site: Ampere, but one Ampere (aka an ‘Amp’) is a really large, fixed number of electrons (6.241X10^18) crossing a circuit in one second. It’s a measure of how much electricity is flowing.
A Watt is defined as one Joule of energy per second, and is the amount of energy actually transferred over time. Again, we don’t need to go too in depth for today’s discussion but if you want to, feel free to on the Wikipedia page: Watt. 1,000 Watts is known as a “Kilowatt.”
A Volt is simply a measure of the electric potential between two points on a wire. That’s all you really need to know, but if you want to be technical, it is the amount of electric potential between two points when one Amp moves one Watt worth of electricity between those two points.
Calculate Cost per Kilowatt-Hour
So why the painful flashback to high school physics? Because I need to introduce you to something that you have probably already heard of, but never fully understood: the Kilowatt-Hour. We will be using the Kilowatt-Hour (kWh) to calculate the cost of running an air conditioner.
As you can see in this picture, your costs per Kilowatt-Hour (kWh) can vary based on the day and your location. You can use the map above to estimate the cost, or if your electric bill is handy, you can calculate an accurate kWh.
Electric Bill – How to Calculate Cost per Kilowatt-Hour
You want to total your energy charge and your fuel charge below and above 1000 kWh, leaving out any customer charges, late fees, discounts, or rebates.
Total the kWh and the cost for each separately.
The calculation would look something like this:
1,000 kWh + 458 kWh = 1,458 kWh
$70.06 + $38.33 + $26.79 + $16.85 = $152.03
Then divide the total cost by the total kWh used. This gives you the cost per kWh.
$152.03 / 1,458 kWh = 10.42 cents per kWh or $0.104
Now that you know what a Kilowatt is, you can figure out that a Kilowatt-Hour is just the number of Kilowatts used in one hour. This is the basis for how your electricity usage is measured. That little meter on the side of your house and the numbers on your electric bill are all Kilowatt-Hours, and now that you know what it is, you will be able to figure out how much it will cost to run an air conditioner.
Step 1 – How many amps are we using?
There are three methods to calculate the estimated energy costs of your air conditioner. Choose the best method available to you and and then go to the next step.
Calculate the Amps Drawn by Your Air Conditioner
This first method will vary greatly based on what the SEER rating of your unit is (see: SEER vs EER), and its size and manufacturer, so I will re-emphasize that this is a quick, layman’s guide to calculating the cost of running your air conditioner. For this guide, the average amps drawn by a modern air conditioner is illustrated below and based on a 16 Seer rating:
2-Ton Air Conditioner – 15 amps
3-Ton Air Conditioner – 18 amps
4-Ton Air Conditioner – 21 amps
5-Ton Air Conditioner – 25 amps
RLA and FLA
This second method involves looking at your air conditioner. The rated load amperage (RLA) is the the maximum number of amps that your compressor can use when running. The full load amps (FLA) is the max amperage the indoor and outdoor motors use when running. Technically, these motors should always be using less than these numbers, but if you do not know how many amps they are actually using, these numbers can be a good guide. It can be found on your condenser and air handler data plate (see pic). As you can see, the RLA for this unit is 19.9 amps and the FLA is 1.0 amps. These can be added together along with the FLA of the indoor blower motor and can be used instead of the rough numbers given in the above method.
Beacon Air & Heat’s Service Invoice
The third method involves actual amperage readings. During any service call or clean & tune maintenance, Beacon technicians take readings from your fan motors and compressor and log them on the invoice. We record the rated numbers that you find on the data plates, as well as the actual amps these components were using at the time of our service.
Step 2 – Calculate the Wattage, Followed by Kilowatt-Hours
The calculation for figuring out the cost of running an air conditioner includes the number of Watts that you use as well. To figure this out, simply multiply the amps from the method you chose to use in step 1 by the voltage of your outlet. For this general price estimate, we will use a standard 240 volt power outlet which is the average size for a central air conditioner. If you have a window air conditioner, then use 110 volts.
For instance, the information evaluated on the Beacon invoice above would look something like this:
Compressor RLA 12.6 amps + Cond Fan Motor FLA 1.2 amps + Blower Fan Motor FLA 2.1 amps = 15.9 amps
Do not add the LRA information. LRA is (locked rotor amperage) and is for a different discussion.
15.9 amps X 240 volts = 3,816 Watts
To calculate Kilowatt-Hours, divide the number that you just calculated by 1,000 to find out one Kilowatt-Hour for your air conditioner:
3,816 Watts / 1,000 = 3.81 Kilowatt-Hours
3.81 kWh X $0.104 per kWh = $0.396 an hour to operate your air conditioner
$0.396 X 24 hours = $9.50 per day,
$9.50 X 30 days = $285.00 per month
This calculates to $285 a month to run your air conditioning system… if your air conditioner ran 24 hours a day.
Step 3 – Find Your Multiplier
Like I said in step 2, you have just figured out the cost to run an air conditioner for a given period of time non-stop, but your air conditioner doesn’t run all of the time! It only runs when the temperature on the thermostat goes above the temperature you have set. In fact, if you were to set your thermostat for 100 degrees then your cost for air conditioning would be $0, because your air conditioner would never run. As such, the following multipliers can be used. There is no exact way to calculate the proper multiplier and this is where the equation loses accuracy and relies on mild assumption. Every day has a different outside air temperature, humidity level, etc.
However as a general guide, you can use the following multipliers based on an indoor household temperature of 75 degrees Fahrenheit, a properly sized air conditioner, and based on the assumption that the temperature drops 20 degrees at night:
Average Outside Daytime Air Temperature Multiplier
110 degrees – 0.8
100 degrees – 0.4
90 degrees – 0.3
80 degrees – 0.25
The purpose of the multiplier is to estimate how often your unit is actually running. For our example, we will use an average outside air temperature of 100 degrees (we are talking about Florida summer!):
$285.00 X 0.4 = $114 a month for the system we evaluated above.
Outdoor temperatures, thermostat settings, outdoor humidity, attic insulation, windows, heat loads and a variety of other variables can change this multiplier. This however is a good indication as to why electric bills are higher in June, July, and August.
This is a good skill to learn if you want to evaluate an air conditioner before you purchase it to see how much it is going to save you on a daily basis. It can also allow you to gauge the savings between different SEER rated air conditioners.
Call Beacon Services (352)726-7530, if you have any questions, or would like us to evaluate your air conditioner for you. We are here to help!
Leave a Reply