Seasonal energy efficiency ratio
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The efficiency of air conditioners are 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. [Definition of SEER] (scroll down to "Seasonal energy efficiency ratio")
- SEER = BTU ÷ W·h
- 5000 Btu/h × 1000 h = 5,000,000 Btu
- 5,000,000 Btu ÷ 10 = 500,000 W·h
- 500,000 W·h ÷ 1000 h = 500 W
Relationship of SEER to EER and COP
SEER is related to the Energy Efficiency Ratio (EER) and also to the coefficient of performance (COP) commonly used in thermodynamics. The EER is the efficiency rating for the equipment at a particular pair of external and internal temperatures, while SEER is calculated over a whole range of external temperatures (i.e., the temperature distribution for the geographical location of the SEER test). Formulas for the approximate conversion between SEER and EER or COP are available from the Pacific Gas and Electric company in California:[SEER conversion formulas from Pacific Gas and Electric]
- (1) SEER = EER ÷ 0.9
- (2) SEER = COP x 3.792
- (3) EER = COP x 3.413
US Government SEER Standards
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 residental 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).[Minimum SEER ratings required in the US] 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 is likely to surpass the higher initial cost of a high-efficiency unit.
Calculating the annual cost of power for an air conditioner
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
Heat pumps
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 50 to 55 degrees Fahrenheit. Typically when the heat source temperature falls below 40 degrees Fahrenheit, the system begins to reach a point called the "balance point", where the system is not able to "pull" any more heat out of the heat-source (this point varies from heat pump to heat pump). Similarly, when the heat-sink temperature rises to about 120 degrees Fahrenheit, the system will operate less effectively, and will not be able to "push" out any more heat. Ground-source (geothermal) heat pumps don't 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 the air above it does.
References
See also
External links
- [Buying an energy efficient air conditioner]
- [U.S. Department of Energy's SEER Standards]
- [Understanding Energy Efficient Ratings]
- [Facts about SEER ratings]
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