Turning Off the Air Conditioner Completely When You Leave Does Not Save as Much Energy as You Think

Spain is facing one of its most extreme summers in memory, with the first half of July on track to become the hottest since national records began. Barcelona’s Fabra Observatory has recorded 40.9 degrees Celsius, its highest temperature in 112 years of measurements.

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An air conditioner cools at a more or less constant rate. Adjusting the temperature from 24 to 18 degrees does not make the space cool faster.

During the heatwave, air conditioning has become an essential tool for millions of households. Yet William Hutzel, a mechanical engineering professor at Purdue University in the United States, says many people still use it inefficiently.

Lowering the thermostat from 24 to 18 degrees does not make a home cool faster, Hutzel explains. An air conditioner cools at a more or less constant rate. Setting a lower temperature only makes the equipment operate longer, increasing electricity consumption. His advice is to choose the highest temperature that remains comfortable. Spain’s Institute for Energy Diversification and Saving recommends keeping air conditioning between 24 and 26 degrees Celsius during summer.

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The lower the temperature you choose, the more you will pay to cool your home.

Hutzel also challenges the idea that the system should always be switched off when nobody is home. Cooling a house that has accumulated heat throughout the day takes considerable time and does not necessarily produce major energy savings. He says it can be more efficient to raise the temperature slightly while the home is empty, then lower it shortly before people return, particularly with a smart thermostat.

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Cooling a house that has accumulated heat throughout the day takes a long time and does not represent a major energy saving.

Those devices, he says, can learn household habits and automatically optimize the system’s operation. Air conditioning also removes humidity, an important part of how people experience comfort. A home may feel pleasant at a somewhat higher temperature when humidity is low, while humid air can remain uncomfortable even when the thermometer shows fewer degrees.

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In addition to cooling, it removes humidity, and that reduction in humidity is a very important part of our feeling of comfort.