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The Most Energy-Efficient Way to Cool Your Home

(Bloomberg) —

It’s getting hot out there. Already this year, people in Thailand, IndiaPuerto Rico and even Portland, Oregon, have experienced record high temperatures, straining power grids and public health. To deal with the heat, more and more people are turning to an obvious solution: air conditioners. 

Balmy Seattle, for example, was once the least air-conditioned city in the US — but after years of sweltering temperatures and wildfire smoke, the city lost its crown. More than half of Seattle homes were air-conditioned in 2021, up from around a third in 2013. Worldwide, the number of AC units increased by 267% between 1990 and 2022, according to data from the International Energy Agency, which anticipates another billion air conditioners by the end of this decade. 

As air conditioning cools our homes, it helps to warm the world. In cities, AC amplifies what’s known as the Urban Heat Island effect, whereby a preponderance of heat-absorbing surfaces, like concrete and roads, make cities hotter. The refrigerants in air conditioners, particularly hydrofluorocarbons or HFCs, are potent greenhouse gasses, and the electricity used to run AC, at least for now, often comes from fossil fuels. Air conditioners are responsible for around 4% of global greenhouse gas emissions, and extreme heat will send that share higher. 

This doesn’t mean we should forgo using air conditioning out of some sense of stoicism. Excessive heat can cause heart attacks and stroke, and can even be fatal. (One study found that, since the 1960s, air conditioning in the US has reduced heat-related mortality by 75%.) But we do need to be smarter about how and when we use air conditioning. Here are some tips on how to stay cool while reducing the environmental impact of your AC. 

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Stay cool, literally

There are things you can do to stay cool before even looking at your AC unit. 

As temperatures climb, limit how much time you spend outdoors during the sunniest time of day (usually around noon) and the hottest time of day (usually between 3 p.m. and  5 p.m.) Wear light clothing that breathes: linen, lightweight cotton or even lightweight wool. If you have to go out, avoid strenuous exercise — stroll, don’t run — and consider going old-school with a parasol or sun umbrella. Stay hydrated with cool and cold drinks and foods, but don’t go crazy with the frozen margaritas: Alcohol can make it harder for your body to regulate its own temperature. 

Make sure your AC is the right size

Bigger isn’t always better, especially when it comes to air conditioners. An oversized AC unit doesn’t just use more electricity than necessary — it also costs more money for less comfort. 

That’s because air conditioners don’t just lower temperatures, they also remove moisture. An oversized AC will cool a space quickly, but it also leaves behind humid air. You might find yourself lowering the temperature even more to boost comfort, which requires even more power. Large AC units also turn on and off quickly, leading to a truncated cooling period known as short cycling. This adds to the wear and tear on your unit, causing it to burn out more quickly.  

You can avoid all of this by buying an appropriately sized air conditioner. For window AC units, the US government’s Energy Star efficiency program has a helpful size guide. If you have central AC, Robert Bean, a fellow at the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), says it’s best to hire an expert to do what’s known as a Manual J Calculation. It’s a formula that factors in everything from building size to insulation levels to give an accurate measurement of cooling needs. 

Don’t add excess heat

If you’re running the AC to stay cool, it’s not the best time to, say, bake bread or roast a chicken. That’s just adding extra heat to your home that the air conditioner will have to work harder to remove. “If you have to generate heat, isolate those rooms, if you can, that are sources of heat and moisture,” Bean says. In other words if it’s hot in the kitchen, you might cordon it off from the rest of your home with a thermal curtain. 

Cool people, not spaces

This is easier to do with window AC units, which is one reason research has found those units tend to use less electricity than central air. If you can avoid or limit cooling unoccupied spaces (or intermittently occupied spaces like bathrooms) by closing doors and vents, it can reduce energy consumption and in turn costs. Ditto turning the AC off entirely when you aren’t home. For central air conditioning, smart thermostats can help control when and how you cool. Some window AC units also come with software that allows for a similar level of control.  

Consider a fan 

We feel cooler on breezy days because fast-moving air helps remove body heat. Fans basically mimic a breeze while using about 1% of the electricity of an AC unit. 

The EPA’s Excessive Heat Events Guidebook warns against relying on fans alone when the heat index is above 99F, but that doesn’t mean there’s no use for fans in that kind of heat. Using a fan in combination with air conditioning lets  you set the AC at a higher temperature — say, 78F instead of 75F — and still feel comfortable, while also using less energy. 

Consider upgrading your AC

According to the US Department of Energy, if your air conditioner dates back to the 1970s, you can cut its energy use in half by upgrading to a newer model. Even if your air conditioner is only 10 years old, you might save 20% to 40% on cooling costs by upgrading. Stick to energy-smart models, and pay close attention to their estimated costs: Forking over a little more cash upfront can bring significant savings in the long term. You might also consider a heat pump which despite its name, cools in addition to heats your home.

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Rethink the design of your home 

“Air conditioning is a response to bad architecture,” Bean says. “If you look around the world, and you look at vernacular architecture [in places like] Bangkok, Hawaii, Florida, the Middle East… if you go back in time, the inhabitants of those places survived without air conditioning.”

Much of modern home design ignores where a home is built — a townhouse in Florida looks a lot like one in Massachusetts, even though these places have quite different climates. Most homes are also designed with the assumption of artificial heat and cooling, which exacerbates reliance on mechanized temperature control.

Approaching building design with temperature in mind can mean, for example, reducing how much sun hits a structure in the first place. Bean notes that before the rise of central heating and AC, architecture employed all kinds of structural techniques to reduce heat naturally. 

“They kept the sun off the building [or] built large rooms that allowed for nighttime ventilation,” he says. “In moist climates — Thailand is a good example — they built the houses off the ground so they didn’t get wet.” 

Of course, few people can rebuild their homes from the bottom-up. But pre-AC techniques can be applied in other ways, too, like by giving rooftops reflective coatings or installing exterior shades to block sunlight. 

To contact the author of this story:
Kendra Pierre-Louis in New York at

© 2023 Bloomberg L.P.

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