Energy Consumption of Air Conditioning Systems

Why They Consume Energy

• Air conditioning systems work by pumping air through coils facing the inside of rooms, then compressing this to raise the temperature of the air in the coils. This then passes through a coil facing the outside of the building, and loses its heat. When it goes back through the inside coil, it is decompressed, resulting in a temperature drop. Electricity is therefore required to continually compress and pump air through the coils, as well as to run fans inside the unit.

Coefficient of Performance

• The coefficient of performance (CoP) for a cooling unit is a measure of the amount of heat energy removed for each unit of electrical energy used. The higher the CoP, the more efficient the unit. It is given by the following formula:

  CoP = Q/W

  Q is the heat energy removed from the building, and W is the electrical energy consumed by the system. Both are given in watts. Typically, air conditioning units have a CoP of between 2.5 and 4, but can be as high as 5.

Seasonal Energy Efficiency Ratio

• Another measure of the efficiency of air conditioning systems is the seasonal energy efficiency ratio (SEER.) Again, it divides the energy output by the energy input, and the higher the SEER, the more efficient the system. It is given by the following formula:

  SEER = BTU / (W*h)

  BTU is the typical heat removed by a cooling unit in a given amount of time, measured in British thermal units, W is the typical power consumption of the unit measured in watts, and h is the time in hours.

Energy Consumption and Climate Change

• Because air conditioning systems are powered by electricity, this must be taken into account when calculating their carbon footprint. Power plants are typically around 40 percent efficient at converting coal or gas to electricity. This means that it has a considerably larger carbon footprint than gas when used in the home.