About coefficient of performance

The efficiency of a refrigeration system is measured by its coefficient of performance (COP), and is normally given as the ratio of the refrigeration effect to the compressor power: COP = (Cooling output) / (Electrical input)
For most packaged commercial cooling units the COP varies between 2.5 and 4.0. However, some modern commercial cooling units have COPs of about 5.0. As the external temperature increases the COP is progressively reduced to about 2.0 at 40.

How to save Energy when using air conditioners

There are some simple things you can do to save energy when using an air conditioner:
Install the air conditioner (or outdoor unit of a split system) on the shady side of the building (or shade the air conditioner itself); make sure the air flow around it isn't obstructed.
The temperature of a heated room in winter should be between 18-21 while the temperature of a cooled room in summer should be about 23-26  (remember the humidity indoors will be low, so it will feel cooler). The temperature should be checked after the air conditioner has been operating for 30 minutes.
When a hot day is expected, turn on the air conditioner early rather than wait till the building becomes hot (it operates more efficiently when the outside air temperature is cooler).
Keep windows and doors closed when using a refrigerative air conditioners (evaporative air conditioners require some air flow). Close curtains on hot summer and days and cold winter nights. Outdoor shading of windows in summer is most effective.
If the machine has adjustable louvres, adjust them towards the ceiling when cooling, and towards the floor when heating (as cool air falls, hot air rises). Follow the manufacturer's instructions for filter cleaning.
Energy Used by Percentage Air conditioning systems use roughly 11% of the energy used in US buildings, and are the main contributors to peak demand in summer months leading to brown-outs and black-outs of the electric grid.

About Greenhouse Gases and air conditioners

The air-conditioning and refrigeration industry is the largest user of hydroflurocarbons (HFCs), a group of potent synthetic greenhouse gases, and is a significant user of energy. Refrigeration and air conditioning equipment that is not maintained in good order consumes more energy than necessary thus increasing the amount of CO2 released to the atmosphere.

How does an air conditioner work

Air conditioners and refrigerators work the same way. Instead of cooling just the small, insulated space inside of a refrigerator, an air conditioner cools a room, a whole house, or an entire business.
Air conditioners use chemicals that easily convert from a gas to a liquid and back again. This chemical is used to transfer heat from the air inside of a home to the outside air.
The machine has three main parts. They are a compressor, a condenser and an evaporator. The compressor and condenser are usually located on the outside air portion of the air conditioner. The evaporator is located on the inside the house, sometimes as part of a furnace. That's the part that heats your house.
The working fluid arrives at the compressor as a cool, low-pressure gas. The compressor squeezes the fluid. This packs the molecule of the fluid closer together. The closer the molecules are together, the high its energy and its temperature.
The working fluid leaves the compressor as a hot, high pressure gas and flows into the condenser. If you looked at the air conditioner part outside a house, look for the part that has metal fins all around. The fins act just like a radiator in a car and helps the heat go away, or dissipate, more quickly.
When the working fluid leaves the condenser, its temperature is much cooler and it has changed from a gas to a liquid under high pressure. The liquid goes into the evaporator through a very tiny, narrow hole. On the other side, the liquid's pressure drops. When it does it begins to evaporate into a gas.
As the liquid changes to gas and evaporates, it extracts heat from the air around it. The heat in the air is needed to separate the molecules of the fluid from a liquid to a gas.The evaporator also has metal fins to help in exchange the thermal energy with the surrounding air.
By the time the working fluid leaves the evaporator, it is a cool, low pressure gas. It then returns to the compressor to begin its trip all over again.Connected to the evaporator is a fan that circulates the air inside the house to blow across the evaporator fins. Hot air is lighter than cold air, so the hot air in the room rises to the top of a room.
There is a vent there where air is sucked into the air conditioner and goes down ducts. The hot air is used to cool the gas in the evaporator. As the heat is removed from the air, the air is cooled. It is then blown into the house through other ducts usually at the floor level.
This continues over and over and over until the room reaches the temperature you want the room cooled to. The thermostat senses that the temperature has reached the right setting and turns off the air conditioner. As the room warms up, the thermostat turns the air conditioner back on until the room reaches the temperature.

Inventer of Air Conditioning

In 1902, only one year after Willis Haviland Carrier graduated from Cornell University with a Masters in Engineering, the first air (temperature and humidity) conditioning was in operation, making one Brooklyn printing plant owner very happy.
Fluctuations in heat and humidity in his plant had caused the dimensions of the printing paper to keep altering slightly, enough to ensure a misalignment of the colored inks. The new air conditioning machine created a stable environment and aligned four-color printing became possible. All thanks to the new employee at the Buffalo Forge Company, who started on a salary of only $10.00 per week. 
Cooling for human comfort, rather than industrial need, began in 1924, noted by the three Carrier centrifugal chillers installed in the J.L. Hudson Department Store in Detroit, Michigan. Shoppers flocked to the 'air conditioned' store. The boom in human cooling spread from the department stores to the movie theaters, most notably the Rivoli theater in New York, whose summer film business skyrocketed when it heavily advertised the cool comfort. Demand increased for smaller units and the Carrier Company obliged. 

Inverter

Through new, advanced technology, Inverter air conditioners are more economical to operate and quieter to run than conventional units. They can handle greater extremes in temperature, are smoother and more stable in operation, and reach the desired temperature more quickly than conventional air conditioners.

Split Systems

Split systems are where the compressor and outdoor heat exchanger are located outside, some distance from the indoor air-handling unit. They are joined together by refrigerant lines. You need to get the Systems installed by a professional installer.
The indoor units can be floor mounted, wall mounting, ceiling mounting or as cassette units. Outdoor units are usually located externally ?on the roof, on a balcony or at ground level.

 Temperature for efficient use of air conditioner

The Queensland government (Australia) promotes 24 degrees celcius as being the recommended temperature for the efficient use of an air condiioner when cooling.This is supposedly a temperature that keeps you comfortable and does not use undue power to the unit.