Window Fan vs Air Conditioner: How Much Energy do They Use?

by ‐ Tags: green living, alternative energy

With summer almost over and the August utility bills paid, you may be feeling a little poorer.  You feel like your electric bills were a lot higher during the summer than the spring, but why? Doesn’t the sun stay up longer, reducing your need to turn on lights? Well yes, but then you remember, you and your roommates have been running fans and window AC units all summer! How much of an impact did this have on your electricity bill? And which is the bigger energy hog, the window fan or the window AC?? 

As an energy nerd and a roommate in an apartment with four window fans and one window AC, I have wondered about how much of an impact running these appliances during the summer has on our electric bills.  I decided to do a couple of calculations to measure energy usage, and was easily able to realize how much of an impact each appliance has on my apartment's utility bills.  I have to admit, I was a bit surprised by just how much each uses!  Learn how you can figure out just how much your cooling appliances are costing you with these 7 steps.

1.    Gather a year’s worth of utility bills. 

This step was really easy for me because I use WegoWise to track my apartment’s energy use, so I clicked into my account and downloaded my data.  However, if you keep track of your bills online through your utility company's website or keep paper copies, you can organize your usage in a spreadsheet, or, if you're really old school, just write them down!

2.    Calculate your baseline energy use.

This isn’t scary, I promise! All you have to do is pick out the three months where you have the lowest usage, and then average the usage.  This average will serve as your baseline, and represents the amount of energy you use consistently each month.  For instance, it takes into account the energy you use to run your refrigerator, cooking appliances, TV, etc. For more information on calculating your baseline energy use, check out this blog article.

My three months with the lowest usage are March, April and May.  Averaging out the usage, my roommate’s and my baseline energy use is 138 kWh.  Not too horrible, huh?

3.    Figure out how many watts your cooling appliances use. 

In some instances, the watts will be right there on the appliance. In other cases, you may have to do some simple math to calculate the watts. Don’t worry, you learned this equation in physics class!  Remember Watts = amps x volts? Either way, you'll be able to find the information you need somewhere on the appliance, either as a sticker on the side or back of the appliance, or on the plug.


Sorry for the blurriness, but here's an example of where you might find the information you need.  In this case, I had the Amps (A) and the Volts (V) provided, so I was then able to calculate the Watts.

My roommates and I have a combined four window fans and one window AC unit.  The wattage of the AC unit is 515, while the window fans are 48, 60, 66, and 70 watts.

4.    Determine the proportion of total energy use each appliance contributes.

If you can add 2+2, and then divide by 4, you can do this step! Just divide each of your appliances’ wattage by the total wattage of all your appliances.  For instance:

515 +48+60+66+70 = 759 total watts for my apartment's cooling appliances

Window AC unit: 515/759 = 67% of total energy!

Window Fan #1: 48/759 = 6% of total energy

Window Fan #2: 60/759 = 8% of total energy

Window Fan #3: 66/759 = 9% of total energy

Window Fan #4: 70/759 = 9% of total energy

  5.    Identify which months you used the cooling appliances to determine usage.

After talking with my roommates, I realized we all used our fans and AC unit pretty much the same amount – we only had it on at night, and we only used them during the hottest days of summer, which were in July and August.  In July, we used 244 kWh, and in August, we used 225 kWh.     

 6.    Calculate excess usage.

Now that you know the monthly usage, you can use that usage and your baseline to determine the excess usage from your cooling appliances.  To do this, add the usage from the months, multiply your baseline by the number of months you used your cooling appliances, and subtract this value from the total usage.  For example:

244 kWh + 225 kWh = 469 kWh used in July and August

469 kWh – (138 kWh x 2 months) = 193 kWh from cooling appliances

7. Determine each appliance’s usage!

If you’re like my roommates and I and have used each appliance about the same amount, this is probably the easiest step! Just multiply your excess usage by the proportion of total energy each appliance uses. If you've used your appliances at different frequencies, you'll also want to take into account the total time you ran each appliance.

Here’s the breakdown for my apartment’s appliances:

Window AC unit: 130 kWh

Window Fan #1: 12 kWh

Window Fan #2: 15 kWh

Window Fan #3 & 4: 17 kWh

As you can tell, the window AC unit uses a great deal more energy than all the window fans combined! In fact, it’s more than two times the energy hog than the window fans! 

So while AC can be refreshing on the blistering hot days, keep in mind the impact its energy use has on your electric bills, and on the environment.  Try to minimize use, and always turn it off when you aren’t in the room!