An air conditioner or heat pump with a cooling function keeps the room temperature comfortable and can mitigate the impact that heat has on health, such as dehydration or heat stroke (5). Rising temperatures, a growing middle class in developing economies, and increasing urbanisation is driving a strong global growth in residential AC: The number of AC units worldwide is projected to double from 1.6 billion units in 2016 to approximately 3 billion units in 2030 (6). Plus, with many parts of the world likely to spend more time indoors than usual this Spring and Summer due to the Covid-19 situation, AC use will likely additionally grow. But ACs consume a lot of energy. The more AC we use, the more hot air (and hazardous refrigerants) they pump out. Studies have shown that during hot weather spells, AC use can raise a city’s temperature by over 1°C (7).