It’s summertime and that means most of the United States is turning on our air conditioners. As of 2009 over 87% of all homes in the U.S. had some form of air conditioning, and that number is probably well over 90% now. Cooling accounts for 6% of an average home’s annual energy use. This might sound small, but when you consider that you’re only running your AC for about 25% of the year it’s pretty crazy. An average central AC draws around 3,500 Watts. This is probably more than any other appliance in the home, except maybe heating. It can easily add over $100 to our monthly electricity bill in the summer. You think it is just keeping you cool, but in reality your overused AC burns money.
How Much Does An Air Conditioner Cost You?
Tonnage (or BTUs)
To figure this out you need to know a few things about your AC. First, the total cooling capability of an AC is rated by a term called tonnage. A 1 ton AC doesn’t weigh 2,000 lbs, but it can remove 12,000 BTUs of heat from the air in your house every hour. Why 12,000 BTUs per hour? Well, apparently that’s the amount of heat required to melt 1 ton of ice in 24 hours, weird right? Most ACs have a label on them that notes their tonnage of BTUs, or you can also look it up from the model number. Many model numbers will have BTUs right in them, for example the FB4CNF036000 AC is rated at 36,000 BTUs. You can also use your house size and climate to estimate the tonnage of your unit, but that’s just an estimate and the actual unit installed at your home could be bigger or smaller.
Seasonal Energy Efficiency Ratio (SEER)
So now you know how much cooling your AC can put out, the next question is how efficiently it does this. The actual efficiency of air conditioners changes depending on the outside temperature and humidity. To average this out over a year’s use the industry came up with something called the Seasonal Energy Efficiency Ratio or SEER. SEER gives you the ratio of cooling your AC will provide versus energy it will consume over an average cooling season. Units with higher SEER ratings are more efficient.
The final number you need to know is how many hours your AC unit runs in a year. You can try and extrapolate this by estimating how many hours your AC runs on an average summer day and then multiplying that out by how many days like that you have where you live, but that’s a very rough estimate. To be more exact you could put a digital sound or temperature recorder next to your AC and then see how often it runs over 24 hour by looking at the recorded data. If you can get electric usage data 15 minute intervals from your utility you can probably figure it out from them as well (whenever there’s a huge spike your AC is on). Doing stuff like that is pretty hard so for now let’s just put down an estimate. The EPA did a study of this in 2002 and the averages it found for many cities in the US are in the Assumptions tab of this spreadsheet. For example Phoenix AZ is 2,141,Washington DC is 1,320, and Tampa FL is 3,068. We’ll just put in a rough 2,500 hours per year as our estimate for a hot area in the U.S. but feel free to use the spreadsheet to find a town closer to you.
Put Them All Together
Now, with all these numbers there’s a simple equation that can show you how much your AC costs to run each year. Just take the BTU rating of your AC times the hours it runs and divide that by SEER and you’ll get the watt hours the system uses over the course of one summer. Now you just multiple this by the price you pay per watt hour. The average utility in Florida charged just under 12 cents for 1,000 watt hours (aka 1 kWh) in 2017. As this page shows price can vary a ton depending on what state you are in. Many utilities also have time of use plans that charge more for electricity during peak usage times of the day. Here in AZ I pay an extra 18 cents for ever kWh I use during peak times (noon to 7 pm Monday through Friday). Since it is hottest between 12 and 7 there is a lot of AC runtime then. Back to our example though, here’s what it would cost to run a 3 ton (36,000 BTU) AC one summer in Florida with 12 cents per kWh flat energy pricing (no peak charges):
(36,000 BTU * 2,500 hours ) / 10 SEER * $0.12 per kilowatt hour / 1,000 watts per kilowatt = $1080
Upgrade Your Air Conditioner
If you upgrade this to a super efficient 30.5 SEER Heat Pump for $1700 it’ll only cost you $354 per summer. Of course, that heat pump is only 9,000 BTUs so you’d have to buy 4 of them to replace your 3 ton unit. With a combined cost of nearly $7,000 they’d take almost 10 years to pay their cost and that assumes you are able to install them without additional costs (unlikely). This quad zone 22.5 SEER unit for $3,000 would pay back in just over 5 years. How much you pay for electricity and how many hours your AC really runs each summer makes a big difference on the calculation here. The spreadsheet from the EPA can help you run the math on how long it will take for a more efficient AC to pay back its purchase price. That being said, that spreadsheet assumes you run your AC the same amount as an average American with a few changes you can drop the number of hours you run your AC a lot.
Before you replace your AC you should consider making some other changes to your home. At the very least these can reduce the number of hours your AC runs in a summer, and at best they can allow your house to get by with a lower BTU AC unit. Lower BTU units cost significantly less so they can save you hundreds, even thousands, of dollars.
Plant Some Trees!
First off, you can plant some trees. They’ll take years to grow up, but once they do their shade can cool your house by multiple degrees. Deciduous trees (not evergreens) are great too because their leaves fall down in winter so they don’t block the sun when you actually want it to heat your house. Use this site as a guide and plant some deciduous trees that will grow nice and tall on every side of your house but the south (make sure you leave some room so they don’t smash through your roof). Even if you don’t end up living in that house for the next decade the trees can help your resale value and help save energy for future owners.
Tighten Up Your Home
The next thing to look at is how well insulated and air sealed your home is. A home that is not well sealed or poorly insulated will allow hot air to come right in and cost a ton to cool. Since people generally don’t ask how much energy a new home will consume before they buy it, builders often skimp on this stuff. If you’ve never looked into this before it makes sense to pay for a professional home energy audit that includes a blower door test. Many utility companies offer deals on this sort of thing, so check with them first. Even if you pay full price it should only cost a couple hundred bucks and it could save you thousands. Once you have this test done they should provide a detailed explanation of how much heat is leaking into your home, a list of fixes and how long each will take to pay back. If you pay over $100 per month for energy it is likely a great idea to have a test like this done.
Improve Your Roof
Most homes in the U.S. are built with grey asphalt shingle roofs. These roofs are terrible in the summer because the dark shingles absorb the heat of the sun. This can help heat air in your attic up above 150 °F! If your roof still has years of life left in it, a simple solution could be just painting it white. You can DIY this for around $500 for a 2,000 square foot roof. If your roof is older and it’s almost time to replace it, you can simply get new white shingles. If your house is going to last a long time then an even better solution is to put a light colored metal roof on your house. A metal roof will cost more up front, but can last for over 100 years so you save in the long run. The final thing to consider is adding solar panels to your roof. These panels will be mounted a few inches above your shingles so they will effectively shade your roof. This makes your home cooler while also generating electricity for you.
Cheap Ways to Save
Insulation, new roofs and higher efficiency ACs will all help reduce the runtime of your AC, but they’re also all really expensive. Don’t worry, there are lots of cheap, even free, ways to reduce your AC bill. These range from the humble fan, to reducing electronic use, to just getting a bit tougher.
How Fans Keep You Cool
Over the thousands of years before the invention of AC humans relied on the amazing power of fans. Fans don’t actually lower the air temperature in a house, but they work in concert with humanity’s own natural cooling systems to cool our skin by multiple degrees. Basically, fans help evaporate your body’s own natural sweat. Evaporation of water is a phase change that absorbs tons of energy from the surrounding air and thus cools it. It only takes 1 BTU to raise a pound of water 1 °F, but around 970 BTUs to evaporate that same pound of water and this is with the water already at 212 °F! This amazing amount of energy is a big part of why we use water to put out fires and also why our sweat does such a good job of keeping our bodies cool. When sweat evaporates it takes so much energy that it actually cools your skin down. Without air movement the area right next to your skin will quickly reach 100% humidity and your sweat will start to pool up instead of evaporating. At this point you feel sticky and gross and are no longer being cooled as much; it sucks. A fan pushes enough fresh air over your skin to keep the humidity down so you stay drier and cooler than you would in still air.
Fan Energy Use
Most fans use under 100 watts with some really efficient or small ones using under 10. If your electricity costs 12 cents per kilowatt hour then you can run a 10 watt fan for 24 hours straight and it’ll cost under 3 cents (the 100 watt fan will cost under 29 cents). This means that if using a fan lets you keep your thermostat even 1 degree cooler you’ll save money. Now, this doesn’t mean you should leave fans on 24×7 in every room. If you are not in the room with the fan then you won’t notice any cooling from it and it’s motor will be consuming energy and adding a small amount of heat to your house. In this case running a fan will waste money and energy. But, if you only use them in the rooms you’re in and push your thermostat up even the tiniest bit, then fans will save you money.
Which Fans Should You Get
Ceiling fans look cool and can add value to your home, but they can be expensive and they’re easy to accidentally leave on. If your house doesn’t already have them there are probably better options. Energy Star ranks ceiling fans based on how many cubic feet of air they can move in a minute per watt. This is an important measure, but I think it’s just as important to look at how much of this air ends up blowing across your skin. These little portable misting fans only cost around $20 each and if you position them near you then pretty much all the air they move will be blowing across your skin. This, along with their misting feature, means that they’ll likely make you feel cooler than a large and distant ceiling fan. I like to set one up a couple feet away pointing straight at my head; it makes me feel cool even with my thermostat set to 82 °F. A ceiling fan can be better for big gatherings of people, but you could also just buy 10 of these little fans and let everyone use one for less money than a nice ceiling fan. Each fan only consumes 2-5 watts of energy depending on what mode it is in, so they use less overall energy than most any ceiling fan and if you accidentally leave one on the most it can waste is the 10 watt hours that its battery holds (which costs less than a penny to recharge). Finally, since they’re portable you can take them around with you outside, and they’re great at outdoor sporting events or thrown into the bike trailer with young children.
Harvest Coolness From The Night
There is one situation where running a fan in an empty room is good. If the night air in your area gets cooler than your thermostat setting then it makes sense to try and pull its coolness into your house. In this case running a fan next to an open window in an empty room will actually cool your house because it is pulling in the cooler outside air. You can use a regular stand fan, box fan, or get a fancy window unit with a thermostat. You need to make sure that there’s a pathway to another open window so the hot air that’s already in your house can exhaust out. Optimally this second window will be on the other side of your house so the air flow can grab air from other rooms along the way. At the second window you can set up a second fan pushing in the opposite direction so the first fan pulls cool air in and the second pushes hot air out. If you have windows open on multiple floors, pull cold air in on the lower floor and push it out on the higher one. This way your fans are working with the natural tendency of warm air to rise instead of fighting it.
To figure out when to open up the house simply stick your hand out a door to feel if it’s cooler outside. If you’re too busy for this you can buy a weather station with temperature alerts for under $40 that will beep when the outdoor temperature drops below a set level. Once it’s a few degrees cooler outside you can open up to let in the cool air. Don’t forget to open your doors if you have screens on them since doors have more area than your windows and can let in a ton of cool air. The fans will likely be able to replace all the hot air in your house with cool air in just an hour or less, but your walls can have absorbed a lot of heat that they need to radiate back out so you may have to run the fans for five or more hours to truly cool your house. Still, running two small fans for 10 hours is way more efficient than running a big central AC for even 20 minutes. Just be damn sure that you close your windows back up before your AC kicks on the next day or all your savings will literally go out the window.
When I lived in an old home without AC in upstate New York my summer ritual would be to open up three windows upstairs and two doors downstairs (with screen doors closed). I had a fan sucking air in through the front door and pushing it up the stairs and another at a window at the back of the house on the second floor exhausting hot air out. I’d shut the doors and turn off the fans before bed. Then, in the morning I’d shut the windows to lock in the coolness. This technique alone kept my house under 80 °F for pretty much the whole summer. This method doesn’t help during July in Arizona where temperatures are above 80 °F most of the night, but it’s great here in May and September when it can be 100 °F in the day but 70 °F at night.
Stop Heating Your House With Electronics
Anything that uses electricity is going to produce some waste heat. In the summer that heat is double bad because it adds to the heat the AC has to remove from the home. That means summer is the best time to curtail electronics use. Replace your bulbs with LEDs if you haven’t already. Hang dry your clothes instead of using the dryer, cook outside on the grill instead of with your stove, unplug energy vampires like your DVR. I mean, seriously, no good shows put out new episodes over the summer, right1, so why not just unplug for a few months? Every watt of waste you save in the summer get’s amplified because your AC doesn’t have to remove the waste heat it creates.
Higher Thermostats Shield Homes From Heat
Turning your thermostat up is like putting an invisible heat force field around your home. Sure your home itself will be warmer but less heat will be conducting into it. This is because the rate that heat conducts across a surface increases as the temperature difference between both sides increases. Turning your thermostat down will cause your AC to use some energy to bring your house down to the new lower temperature, but it will also cause more heat to conduct into your house once that temperature has been reached. Check out the “Your House is Like a Cup of Coffee” section of this previous post on heating for the nitty gritty details on why this is. Or just accept that turning your thermostat up even just a few degrees in the summer can save hundreds of dollars.
Turn Up the Temp When You’re Out
Just like in the winter it makes sense to reduce your HVAC use while you’re out of the house in the summer. As I just discussed, raising your thermostat setting will result in less outdoor heat conducting into your home so you want it as high as possible for as long as possible. There are a few things that you should consider though. Food items and electronics in your house probably won’t be too happy at 100 °F. They can survive the mid 80’s just fine and 90 °F probably isn’t the end of the world for them, but you’re probably going to want to leave some form of AC running. The other reason to do this is moisture control. An AC has a great side benefit of removing humidity from your house. If you live in a very humid area (like, say, Washington DC) then you’ll probably want to keep your AC running at least a little during the day to dehumidify your home. Alternatively, you could also save energy by just running dehumidifiers in the most humid rooms. Out here in Arizona I set my AC to between 80 °F and 82 °F while I’m home and 84 °F to 86 °F when I leave. I have a neighbor who keeps his AC at 94 °F all day while working from home, but I haven’t reached his level of toughness yet.
What About Mold?
Americans really freak out about mold and it’s one reason people love to keep their thermostats blasting year round. When people would tell me that I had to run AC to keep my house mold free, I would chuckle because my home had been built in 1930 before AC use was widespread. Keeping air flowing with open windows at night is all it really needed to stay mold free. The few times I did find some mold it was easy to clean up with vinegar or baking soda.
The actual harms of mold in a house are pretty low unless you have a huge allergy to it. That doesn’t mean that people won’t freak out if they realize it’s there (maybe they’ve seen mold ghosts). So, it probably makes sense to keep your home’s humidity under 50% to inhibit mold growth. Honestly, you can probably get away with above 50% humidity; my old basement was at 60% most of the time and seldom had issues. If you’re worried about rooms in your home being too humid you can buy a humidity sensor for under $10 to test them. If only one or two rooms are bad then you would probably be better off running a dehumidifier in them specifically instead of running your whole house AC to drop the temperature further. Dehumidifiers use a few hundred watts, but that’s still far less than the few thousand watts your AC will use. Check out this site for great reviews of dehumidifiers and details on their energy use.
Another free way to cut down on AC use is to wear less clothing around the house. When you get home from work put on a t-shirt and shorts (or less if you’re daring2). Do not walk around your own home in summer wearing a full business suit or sweater. Do not watch TV on the couch under a Snuggie in the middle of July! When you get home change into lighter clothes. If this makes you feel cold that’s a good thing, it means you can turn up the AC.
Embrace Your Adaptable and Amazing Body
In general, human bodies enjoy temperatures around 70 °F. There are plenty of people who simply set their AC unit to 70 °F and leave it there all summer long. These people are ignoring the fact that human bodies are also incredibly adaptable and perceive temperature relatively. If you’re outside in 100 °F heat and you come into a 80 °F building, it will feel cool. Conversely, if you keep your house at 70 °F and then walk out into 100 °F, it’ll feel oppressive. By keeping your house warmer in the summer you help your body adapt to better handle the hotter outside temperatures. This is literally making you a tougher person, someone who can actually survive in nature and not just run from icebox to icebox. Try turning your thermostat up 1 °F every other day until you truly can’t stand it and see where you end up; I bet you’ll be amazed. Remember: humans lived for thousands of years without AC. You can still live in amazing luxury with the temperature set just a little higher, give it a shot and see!