"It has been taken for a fact that living in the hot areas of the US is less sustainable than in the cold areas," Michael Sivak of the University of Michigan, US, told environmentalresearchweb. "The present findings suggest a re-examination. Living in cold climates such as Minneapolis, Milwaukee, Rochester, Buffalo and Chicago is more energy demanding, and therefore less sustainable from this point of view, than living in warm climates such as in Miami, Phoenix, Tampa, Orlando and Las Vegas."

Sivak found that Minneapolis deviates downwards from the desired indoor temperature – set in this case at 21° – more than Miami deviates upwards. He calculated the number of heating degree days over a year by subtracting the mean daily temperature from 18°C and adding up positive values. Buildings typically require heating to maintain 21° indoors when outdoor temperatures drop below 18°. Similarly, adding up the number of degrees by which days were hotter than 18 °C gave the number of cooling degree days for Miami. Minneapolis had 4,376 heating degree days while Miami had 2,423 cooling degree days, a factor of 1.8 times less.

What's more, air conditioners can be four times more energy efficient than furnaces, Sivak calculated. Since the energy efficiency of heating and cooling appliances is rated differently, he calculated a common measure of energy output, the coefficient of performance (COP) – the energy generated for heating or energy transferred for cooling, divided by the energy input. New furnaces and boilers powered by natural gas or heating oil had a COP of 0.8–0.98 while new central air conditioners had a COP of 3.1–4.3; air conditioners can transfer a larger amount of heat energy than they consume in electrical energy.

"In simple terms, it takes less energy to cool a room down by one degree than it does to heat it up by one degree," said Sivak. "In the US, the energy consumption for air conditioning is of general concern but the required energy to heat is often taken for granted."

Cooling appliances in the US typically use electricity, compared to just 7% of heating appliances. The weighted efficiency of US power plants, taking fuel source into account, is 0.43. So the effective power-plant efficiency for cooling is 0.43, said Sivak, while the energy used for heating has an effective power plant efficiency of 0.96, allowing for the 7% of electricity-powered heating appliances.

To calculate energy demand, Sivak took into account climatological demand, appliance efficiency and power-plant efficiency. When also allowing for Miami's demand for heating (83 heating degree days) and the demand for cooling in Minneapolis (388 cooling degree days), Minneapolis is 3.5 times as energy demanding as Miami.

People are generally more tolerant of heat than of cold, added Sivak, so his calculations are likely to underestimate Miami's advantage over Minneapolis.

"The present study focused on energy demand only," he said. "Natural extensions would be analyses of actual energy used and costs involved."

Sivak reported his findings in Environmental Research Letters (ERL).

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