Dear Pablo,

It’s too damn hot to sleep at night! To cool our bedroom on these summer nights, my husband and I can run either three fans or one air conditioner. Which is better for the environment?

Let me first answer the question and then I will discuss some alternatives. Based on additional information from the reader, I learned that each fan uses 50 watts of 120 volt electricity (150 total), while the air conditioner uses 530 watts of 120 volt electricity. Already we can see that the three fans use 28 percent as much energy. If the three fans are truly comparable in the level of comfort they provide, by all means, use the fans!

Fans work in three beneficial ways:

  • Once the sun has gone down, and things begin to cool off, they bring in the outside cool air. They can also vent a hot attic space that might otherwise radiate heat into your living space throughout the night.
  • Stagnant (nonmoving) air creates a boundary layer around your skin. Because air is an excellent insulator (in fact, wall insulation is mostly air), this layer acts to impede the loss of excess heat. Even a light air current from a fan can significantly lessen the thickness of this boundary layer, allowing excess body heat to escape.
  • Sweat creates an evaporative cooling effect that is limited both by the skin’s ability to produce sweat and by the air’s ability to take up excess moisture. Creating airflow allows more air to contact the skin and facilitates evaporation.
  • Air conditioners work in a different way. Inside an air conditioner is essentially a heat pump. This device uses the expansion of compressed gases to absorb heat energy from the indoor air (similar to how a compressed air canister or can of whipped cream gets colder as you use it), and then compresses the gases again to release that heat energy outside. Because a compressor is involved, some of the electricity used is lost to friction, heat and other inefficiencies.