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Efficiency of Smaller Units

Air Conditioner Efficiency

Also See: Top Rated Energy Star Air Conditioners

Heating and cooling costs the average homeowner more than $1,000 a year — nearly half the home’s total energy bill. If your air conditioning unit is more than 12 years old, replacing it with an efficient model could cut your cooling costs by 30%. Air conditioner efficiency express the ratio between heat removed to watt of power used.

In the United States, the efficiency of air conditioners is often (but not always) rated by the seasonal energy efficiency ratio (SEER). The higher the SEER rating, the more energy efficient is the air conditioner. The SEER rating is the BTU of cooling output during its normal annual usage divided by the total electric energy input in watt hours (W·h) during the same period

SEER = BTU ÷ (W·h)
this can also be rewritten as:

SEER = (BTU / h) ÷ W, where “W” is the average electrical power in Watts, and (BTU/h) is the rated cooling power.
For example, a 5000 BTU/h air-conditioning unit, with a SEER of 10, would consume 5000/10 = 500 Watts of power on average.

The electrical energy consumed per year can be calculated as the average power multiplied by the annual operating time:

500 W × 1000 h = 500,000 W·h = 500 kWh
Assuming 1000 hours of operation during a typical cooling season (i.e., 8 hours per day for 125 days per year).

Another method that yields the same result, is to calculate the total annual cooling output:

5000 BTU/h × 1000 h = 5,000,000 BTU
Then, for a SEER of 10, the annual electrical energy usage would be:

5,000,000 BTU ÷ 10 = 500,000 W·h = 500 kWh

Today, it is rare to see systems rated below SEER 9 in the United States, since older units are being replaced with higher-efficiency units. The United States now requires that residential systems manufactured in 2006 have a minimum SEER rating of 13 (although window-box systems are exempt from this law, so their SEER is still around 10). Substantial energy savings can be obtained from more efficient systems. For example by upgrading from SEER 9 to SEER 13, the power consumption is reduced by 30% (equal to 1 – 9/13). It is claimed that this can result in an energy savings valued at up to US$300 per year (depending on the usage rate and the cost of electricity). In many cases, the lifetime energy savings are likely to surpass the higher initial cost of a high-efficiency unit.

As an example, the annual cost of electric power consumed by a 72,000 BTU/h air conditioning unit operating for 1000 hours per year with a SEER rating of 10 and a power cost of $0.08 per kilowatt hour (kW·h) may be calculated as follows:

unit size, BTU/h × hours per year, h × power cost, $/kW·h ÷ (SEER, BTU/W·h × 1000 W/kW)
(72,000 BTU/h) × (1000 h) × ($0.08/kW·h) ÷ [(10 BTU/W·h) × (1000 W/kW)] = $576.00 annual cost
A common misconception is that the SEER rating system also applies to heating systems. However, SEER ratings only apply to air conditioning.

Air conditioners (for cooling) and heat pumps (for heating) both work similarly in that heat is transferred or “pumped” from a cooler heat source to a warmer “heat sink”. Air conditioners and heat pumps usually operate most effectively at temperatures around 10 to 13 degrees Celsius (°C) (50 to 55 degrees Fahrenheit (°F)). A balance point is reached when the heat source temperature falls below about 4 °C (40 °F), and the system is not able to pull any more heat from the heat source (this point varies from heat pump to heat pump). Similarly, when the heat sink temperature rises to about 49 °C (120 °F), the system will operate less effectively, and will not be able to “push” out any more heat. Geothermal heat pumps do not have this problem of reaching a balance point because they use the ground as a heat source/heat sink and the ground’s thermal inertia prevents it from becoming too cold or too warm when moving heat from or to it. The ground’s temperature does not vary nearly as much over a year as that of the air above it.