SEER (Seasonal Energy Efficiency Ratio)
The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio (SEER) which is defined by the Air Conditioning, Heating and Refrigeration Institute in its standard ARI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.
The SEER rating of a unit is the cooling output in British thermal unit (BTU) during a typical cooling-season divided by the total electric energy input in watt-hours during the same period. The higher the unit’s SEER rating the more energy efficient it is.
For example, consider a 5,000-British-thermal-unit-per-hour (1,500 W) air-conditioning unit, with a SEER of 10 BTU/W·h, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days).
The annual total cooling output would be:
- 5000 BTU/h × 8 h/day × 125 days/year = 5,000,000 BTU/year
With a SEER of 10, the annual electrical energy usage would be about:
- 5,000,000 BTU/year / 10 BTU/W·h = 500,000 W·h/year
The average power usage may also be calculated more simply by:
- Average power = (BTU/h) / (SEER) = 5000 / 10 = 500 W
If your electricity cost is 20¢/kW·h, then your cost per operating hour is:
- 0.5 kW * 20¢/kW·h = 10¢/h
Relationship of SEER to EER and COP
The Energy Efficiency Ratio (EER) of a particular cooling device is the ratio of output cooling (in Btu/hr) to input electrical power (in watts) at a given operating point. EER is generally calculated using a 95F outside temp and an inside (actually return air) temp of 80F and 50% relative humidity.
The EER is related to the coefficient of performance (COP) commonly used in thermodynamics, with the primary difference being that the COP of a cooling device is unit-less: the cooling load and the electrical power needed to run the device are both measured using the same units, e.g. watts. Therefore a COP is universal and can be used in any system of units. EER = COP * 3.412 .
The Seasonal Energy Efficiency Ratio (SEER) has the same units of Btu/W·hr, but instead of being evaluated at a single operating condition, it represents the expected overall performance for a typical year’s weather in a given location. The SEER is thus calculated with the same indoor temperature, but over a range of outside temperatures from 65 to 104 degrees F, with a certain specified percentage of time in each of 8 bins each spanning 5 degrees F. There is no allowance for different climates in this rating. It just gives an indication of how the actual EER is typically affected by different outside temperatures over the course of a cooling season.
Typical EER for residential central cooling units = 0.875 × SEER. SEER is a higher value than EER for the same equipment.
A more detailed method for converting SEER to EER uses this formula:
- EER = -0.02 × SEER² + 1.12 × SEER Note that this method is used for benchmark modeling only and is not appropriate for all climate conditions.
A SEER of 13 is approximately equivalent to an EER of 11, and a COP of 3.43, which means that 3.43 units of heat energy are removed from indoors per unit of work energy used to run the air conditioner or heat pump.
The SEER and EER of an air conditioner are limited by the laws of thermodynamics. The refrigeration process with the maximum possible efficiency is the Carnot cycle.
Assuming an outdoor temperature of 95 °F (35 °C) and an indoor temperature of 80 °F (27 °C), the above equation gives (when temperatures are converted to Kelvin or Rankine) a COP of 35.98, or an EER of 122.69. This is about 10 times more efficient than a typical home air conditioner available today.
The maximum EER decreases as the difference between the inside and outside air temperature increases, and vice versa. In desert climates, where the temperature may be as high as 120 °F (49 °C), the maximum COP drops to 13.49, or an EER of 46.00 (assuming an outdoor temperature of 120 °F (49 °C) and an indoor temperature of 80 °F (27 °C)).
The maximum SEER can be calculated by averaging the maximum EER over the range of expected temperatures for the season.
US Government SEER Standards
SEER rating more accurately reflects overall system efficiency on a seasonal basis and EER reflects the system’s energy efficiency at one specific operating condition. Both ratings are useful when choosing products, but the same rating must be used for comparisons. As of January 2006, all residential air conditioners sold in the United States must have a SEER of at least 13. ENERGY STAR qualified Central Air Conditioners must have a SEER of at least 14.
Today, it is rare to see systems rated below SEER 9 in the United States because aging, existing units are being replaced with new, higher efficiency units. The United States now requires that residential systems manufactured after 2005 have a minimum SEER rating of 13, although window units are exempt from this law so their SEERs are 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.
With existing units that are still functional and well-maintained, when the time value of money is considered, retaining existing units rather than proactively replacing them may be the most cost effective. However, the efficiency of air conditioners can degrade significantly over time. Therefore, maintenance (such as cleaning the coils) should be performed regularly to keep their efficiencies as high as possible.
But when either replacing equipment, or specifying new installations, a variety of SEERs are available. For most applications, the minimum or near-minimum SEER units are most cost effective, but the longer the cooling seasons, the higher the electricity costs, and the longer the purchasers will own the systems, the more that incrementally higher SEER units are justified. Residential split-system AC units of SEER 20 or more are now available, but at substantial cost premiums over the standard SEER 13 units. The higher SEER units typically have larger coils and multiple compressors, the cost reflects the additional complexity.
Calculating The Annual Cost Of Electric Energy For An Air Conditioner
Electric power is usually measured in kilowatt (kW). Electric energy is usually measured in kilowatt-hour (kW·h). For example, if an electric load that draws 1.5 kW of electric power is operated for 8 hours, it uses 12 kW·h of electric energy. In the United States, a residential electric customer is charged based on the amount of electric energy he or she uses. On the customer bill, the electric utility states the amount of electric energy, in kilowatt-hour (kW·h), that the customer used since the last bill; and the cost of the electric energy per kilowatt-hour (kW·h).
Air conditioner sizes are often given as “tons” of cooling, where 1 ton of cooling equals 12,000 BTU/h (3.5 kW). 1 ton of cooling equals the amount of power that needs to be applied continuously, over a 24 hour period, to melt 1 ton of ice. The annual cost of electric energy consumed by a 72,000 BTU/h (21 kW) (6 ton) air conditioning unit, operating for 1000 hours per year, with a SEER rating of 10, and an electric energy cost of $0.12 per kilowatt-hour (kW·h) is:
- unit size, BTU/h × hours per year, h × energy cost, $/kW·h ÷ SEER, BTU/W·h ÷ 1000 kW/W
- (72,000 BTU/h) × (1000 h) × ($0.12/kW·h) ÷ (10 BTU/W·h) ÷ (1000 kW/W) = $864
Example 2. A residence near Chicago has an air conditioner with a cooling capacity of 4 tons and an SEER rating of 10. The unit is operated 120 days each year for 8 hours per day, and the electric energy cost is $0.10 per kilowatt-hour. What is the annual cost of the electric energy required to operate the air conditioner? First, we convert tons of cooling to BTU/h:
- (4 tons) × (12,000 BTU/h/ton) = 48,000 Btu/h.
The annual cost of the electric energy is:
- (48,000 Btu/h) × (960 h/year) × ($0.10/kW·h) ÷ (10 BTU/W·h) ÷ (1000 kW/W) = $461
Maximum SEER Ratings
When operated in heating mode, a heat pump is more efficient than operating resistance heaters in most cases. This is because a space heater can convert only the input electrical energy directly to output heat energy with none of the efficiency or conversion advantages of a heat pump. Likewise, when a heat pump operates near its most inefficient outside temperature, typically 0 °F (−18 °C), the heat pump will perform close to the same as a resistance heater.
A resistance heater, converting 100% of its input electricity to output heat would have a 3.41214 EER (COP = 1), because it takes about 0.293071 watt hours to equal one BTU.
Lower temperatures may even make a heat pump operate below this threshold, which is why conventional heat pumps include heater coils or auxiliary heating from LP or natural gas to prevent low efficiency operation of the refrigeration cycle. To compensate for this inefficiency, “cold climate” heat pumps are designed to operate efficiently below 0 °F for heating in cold climates.
In the case of cold climates, water or ground source heat pumps are the most efficient solution. They use the relatively constant temperature of ground water or of water in a large buried loop to moderate the temperature differences in summer and winter and improve performance year round. The heat pump cycle is reversed in the summer to act as an air conditioner.