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Air Conditioning > Energy & Capacity
  Power Sources
  Energy Efficiency
  Cooling Capacity
Whether you are in the market for a central air conditioning system or a portable unit, you will find that there is generally a trade-off between the cooling capacity of the air conditioner and the amount of energy required to run it. Consequently, it is important to identify the appropriate capacity for your needs and to determine which units are the most energy efficient, as both actions will result in reduced utility bills.
Power Sources
Every type of air conditioner has to perform work in order to transfer heat from interior spaces to the outside. In each case, this work performed by the air conditioning system must rely on some type of power. In virtually every case, this power is provided by electricity. However, although not common, other options have become available in recent years.

Those of you who are familiar with heat pumps may know that heat pumps may also be powered by natural gas. These models are known as “absorption heat pumps”. As air conditioners work on the same principle as heat pumps – the refrigeration cycle – they can also be built based on absorption technology.

Interestingly, gas-powered central air conditioning systems were popular in the 1960s, but they fell out of favor because, at the time, they were characterized by lower efficiencies and higher purchase costs. However, there have been three important changes since that time which may have an impact on how consumers view gas-powered central air conditioning.

First, technology has advanced, meaning that today’s gas-powered appliances are far more efficient. Second, the typical sizes of homes have grown, making conventional electric-powered central AC solutions much more expensive to operate on a day-to-day and month-to-month basis. Third, there is a much greater environmental consciousness today and the opportunities to reduce the environmental footprint offered by natural gas relative to electricity play a far larger role in many consumers’ decision-making.

Gas-powered cooling has been used in commercial applications for many decades and a number of the technology advances from those applications have been transferred to residential systems. As a result, for 2,000 square foot and larger homes, natural gas air conditioning is a real alternative that can lead to increased cooling efficiency and economy. However, because this has not yet become a mainstream option, it can be expensive and it may also prove difficult to find service contractors with the necessary experience and expertise.

An even more environmentally friendly and energy efficient option is solar powered air conditioning. As you probably know already, solar heat is energy that is captured from the sun’s rays. Since air conditioning is only needed when the temperature is hot, there is plenty of sunlight during those periods. Moreover, collector plates can store up enough energy during the day to allow the air conditioner to be run at night. Unlike every other energy source, the sun’s radiant energy is free, abundant, and requires neither fuel nor electricity.

Solar powered air conditioning systems can either be complete systems, or a separate solar energy source that connects to a conventional electric-powered air conditioning system. In both cases, specialized collector plates are installed outside or on the roof of the house. These plates collect sunlight and capture the energy, converting them to usable form. This energy is then used to power the air conditioning system.
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Energy Efficiency
As described earlier, the cooling process requires the use of energy, whether in the form of electricity, fuel, or solar power. Energy efficiency refers to how effectively that energy is used to provide cooling by the system, whether that system is a central air conditioner or a portable air conditioner.

There are two arguments in favor of greater energy efficiency: monetary savings and environmental impact. A more energy efficient system requires either less electricity or less fuel in order to generate the same amount of air conditioning. In turn, this translates to a lower utility bill. In addition, the government provides tax credits and rebate programs which can put additional money into your pocket. Additionally, the consumption of both electricity and fuel has a negative impact on the environment because the production of those commodities is generally an environmentally destructive process. A more energy efficient system uses up a smaller quantity of these commodities, thereby decreasing your environmental footprint.

The issue of energy efficiency is particularly important when it comes to central air conditioning, as central AC systems generate far more power and, by extension, use much greater resources to maintain their operation. The energy efficiency for a central air conditioner is commonly measured using a metric called the seasonal energy efficiency ratio (SEER) and defined as the total amount of heat removed, expressed in British Thermal Units (BTU), divided by the total amount of electricity used by the air conditioner, expressed in watt-hours. In case of a room air conditioner, the metric used is simply called the energy efficiency ratio (EER) and is also expressed as BTUs of heat removed divided by watt-hours expended. The main difference between SEER and EER is that SEER provides the average cooling efficiency across a range of temperatures, while EER provides cooling efficiency for a specific temperature, most commonly 95 degrees Fahrenheit.

The government mandates that new central AC systems have a SEER of at least 13, but systems go as high as 23. Generally, between 13 and 15 is considered to be the lower range, between 15 and 17 is considered to be the middle range, and between 18 and 23 is considered to be the high range. Systems are priced accordingly, so an 18 SEER central AC system will cost more than a 15 SEER central AC system.

With respect to room air conditioners, government regulations mandate an EER of at least 8. Most products fall in the range between 8 and 15. Generally, it is recommended that consumers look for an EER of 10 or higher for optimal efficiency. Units with EERs in the 13 to 15 range are typically much pricier than those in the 10 to 12 range.

Many older air conditioning systems have efficiencies that are lower than current mandated minimum levels. By replacing an older, inefficient system with a new high SEER unit, the consumer is able to reduce utility bills by 20% or more and shrink his or her environmental footprint while at the same time improving cooling operation.
In addition to the SEER and EER metrics, the federal government has introduced a program to help consumers identify the most efficient systems. This program is called ENERGY STAR and it has a two-pronged purpose: “(1) to reduce greenhouse gas emissions and other pollutants caused by the inefficient use of energy; and (2) to make it easy for consumers to identify and purchase energy-efficient products that offer savings on energy bills without sacrificing performance, features, and comfort.”

The ENERGY STAR program is run by the Environmental Protection Agency (EPA). The agency identifies specific products that it deems to be energy efficient and then assigns an ENERGY STAR label to those products. In order to qualify for the ENERGY STAR label, a product must be cost-effective, provide significant energy savings, and deliver the features and performance demanded by consumers. Before the label can be assigned, each product is tested to ensure that it not only works as stipulated, but also delivers the requisite level of energy efficiency.

In addition to identifying products which meet ENERGY STAR’s guidelines, the program also provides tax rebates for consumers who purchase energy efficient cooling systems. These rebates allow consumers to recoup a portion of the purchase costs of a new or upgraded air conditioner. In order to determine whether a particular product qualifies and also to calculate the amount of the possible rebate, consumers should visit the ENERGY STAR web site at www.energystar.gov. 
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Cooling Capacity
It is important to size an air conditioner appropriately to the needs of the residence. In the case of room air conditioners, sizing is not as critical because the cooling capacity will not vary as dramatically from one unit to another. However, for central AC systems, incorrect sizing can lead to a range of problems, including ineffective operation, unnecessary wear and tear, and high utility bills.

For an air conditioner, cooling capacity is measured in either BTUs, which are British Thermal Units, or tons, where one ton is equivalent to 12,000 BTUs. As mentioned, identifying the appropriate air conditioner size is important. An oversized air conditioner will be constantly turning on and off, which leads to efficiency losses and increased likelihood of compressor breakdown. On the other hand, an undersized air conditioner will be running too often and may not provide sufficient cooling during the hottest days of the summer season. 

Properly sizing an air conditioning system requires understanding the space configurations and requirements of the residence. Detailed sizing calculators may be found online, free of charge. Service providers who will be assisting with the purchase decision and doing the system installation can also perform the necessary calculations. Finally, a general approximation can be used based on the climate zone and square footage of the residence, although it is not a substitute for a detailed calculation.

For residences located in the southern-most states, which are those that border on Mexico or the Gulf of Mexico, the estimate is 1 ton of additional cooling capacity for every 500 square feet of the home. For residences located in the middle of the country, stretching from Nevada, through Kansas, and onto Maryland, the estimate is 1 ton of additional cooling capacity for every 550 square feet of the home. Finally, for residences in the northern-most states, which are those that border on Canada or the Great Lakes, the estimate is 1 ton of additional cooling capacity for every 600 square feet of the home. Since most air conditioning systems are manufactured using increments of 0.5 tons, a ballpark estimate can be a relatively accurate predictor of the necessary cooling capacity.

A similar approach can be used for room air conditioners, although these smaller units are usually rated in terms of BTUs. Thus, for cooling a single 500 square foot room, a 12,000 BTU air conditioner would be most appropriate. However, for a 250 square foot room, a 6,000 BTU air conditioner would be the most efficient solution.

You may think that buying an air conditioner with a larger capacity than you need is the way to go, using a sort of “better safe than sorry” logic. However, this type of logic does not apply to air conditioning. A room air conditioner that is too powerful for the space will lead to additional expenditures of energy, resulting in higher utility bills and a larger environmental footprint. Like the bowl of porridge in the story of Goldilocks, the right air conditioner is neither too hot nor too cold for the space, but just right.
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