The most energy-efficient ways to cool your home

By K endra P ierre- L ouis Bloomberg

It’s getting hot out there. Already this year, people in Thailand, India, Puerto Rico and even Portland, Ore., 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 U.S. — 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 gases, 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 U.S. 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.

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 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 U.S. 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, 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.

Lee Reich | In the Garden