Stop that AC unit!
***Update - 8/20/2010 - The post below was originally posted in June of 2009 and has been incredibly popular. In that original post, I included a calculator to help people analyze how much electricity their AC unit consumes, but I'm not including it in this re-post because of some of the observations and updates I've made have added some uncertainty in the values given by the calculator. In any case, know that your AC unit is by far the biggest energy consumer in your home during the summer (if you live in a hot climate). I have a townhome with a split-level AC system, meaning I have 2 AC units: one for upstairs (2 ton) and one for downstairs (2.5 ton). For the purpose of estimation your AC electricity consumption in a central-AC configuration it seems that the tonnage is closely correlated to the kW consumption. So my 2 ton unit consumes a little more than 2 kW (including the AC unit and furnace fan). Bottom Line: Program and raise the temperature on your thermostat so your AC unit doesn't come on as often or stay on as long!
My recent post on lowering your energy consumption due to your Air Conditioning provided steps you could use to control your AC, but it didn't analyze how much energy your air conditioner uses when it is running. We will look at calculating AC energy consumption in this post.
In order to figure out the electricity consumption of your air conditioner unit, you have to first read the label to figure out how many Amps and at what Voltage your unit consumes electricity. You will find this on the unit's data tags. I found this helpful page from InspectAPedia describing the information on an AC unit's data tag.
The biggest reason your AC unit consumes so much electricity is to run the compressor. If you need to know why your AC unit has a compressor, check out HowStuffWorks on Air Conditioning. On the data tag, you will see energy data for the compressor and the fan.
Chances are if you have central AC that you have outdoor units and that those outdoor units run at 208-230 Volts. Find what Voltage your unit operates at on the tag. This is the easy part.
Next, locate the section on the data tag for the fan. This is just the fan that pulls air over the condenser coils and allows the heat from your home that has collected in the refrigerant fluid to escape into the surrounding outside air. Since this fan is just moving air, it does not require much power to do so. On my unit, the fan had an FLA (full load amp) rating of 1 Amp.
As mentioned, the big consumer of electricity on your AC unit is the compressor. This consumes much more power than the fan because it has to compress a fluid (your refrigerant) as opposed to just move air (like the fan). The value you want to look for on the data tag is the RLA value, this is the Rated Load Amps and is the amount of current (amps) your compressor should consume if it is operating normally. You may also have an LRA (locked rotor amps) value. This is how many amps your compressor would consume if for some reason it jammed up and it is the maximum possible amperage draw of the compressor. But under normal Air Conditioning, your unit will consume its RLA value. My RLA value is 14.1 Amps.
*** Update 8/20/10 - In practice, I haven't notice the compressor consume anywhere near this many amps. Surprisingly, what turns out to be the biggest energy hog is the air circulation system (furnace fan) inside the house (more on that in the update below). Do any HVAC professionals have any insight as to why the furnace fan (the system that moves air over the evaporator coils) seems to use more energy than the AC unit itself?
Once you have the amperage values for your compressor and fan, add them together. This is how many amps the unit as a whole consumes when it is running. From there, all you need to due is multiply the total current consumption by the voltage to find the power your unit consumes in Watts. If you've read my blog What's a kWh, you know that once you know how many Watts an item consumes, you just have to mulitply that value by the time the item is on to get total energy consumption (watts*time=power*time=energy).
So, how do you save energy by raising the temperature on your thermostat? When you raise the temperature on the thermostat, it means the AC doesn't have to stay on as long to get the air in your house at the desired temperature. If you look at the "energy equation" above, lowering the amount of time something is on means that you lower the total amount of energy it consumes!
Use the calculator below to figure out how much energy your AC unit consumes! Plug in the items in yellow and the cells in green will tell you how much energy your AC uses and how much it costs you. You will need an estimate of how much time the AC is on per day (in hours). You will also need to know how much you pay per kWh. If you dont know that value, then you really need to read my blog more often!
***Update 8/16/10 - Something has been bugging me for the last few months about this post. Mainly the fact that I only looked at the electricity consumption of the AC unit itself, not the entire AC system. If you have central air conditioning, there are two parts in the system that consume electricity: the AC unit and the fan that circulates the air through your home.
In my experience, the fans are actually the big energy hog. For example, when I turn the AC off (meaning the compressor won't come on) but turn the circulating fan on, my TED 5000 tells me my fan consumes around 2,000 watts (2 kW). That's a lot of electricity! This includes the fan that is on the AC unit (condenser fan) plus the fan that moves the air through the house (furnace fan). In actuality, it seems that the compressor isn't the main source of energy consumption, it is the fans (mainly the furnace fan that moves the air through your home). (I was mistaken when writing the text now crossed out. The reason I thought my furnace fan was using so much electricity is because my thermostat was wired incorrectly and every time I thought only my furnace fan was on, it was actually turning on the outdoor compressors as well, so 2 kW represents the entire system (compressor and furnace fan)).
In any case, your AC system is still the largest energy hog in your home during the hot, summer months. It's just harder to figure out how much AC it consumes unless you have a whole home energy monitor (like the TED 5000) that takes into account the electricity consumed by the fan and compressor on the AC unit (what the original post and sheet below helps you calculate) AND the electricity consumed by the furnace fan that moves the air through the house. If you ever use the fan feature on your AC system, I would stop (turn it to auto, so it only comes on when the AC compressor is on). Instead, use desk fans or ceiling fans in the rooms you are in. And finally, turn up your thermostat in the summer!