How to Maintain Air Conditioner?

Dear VK Gupta

The cooling capacity of room air conditioners is measured in BTUs or British Thermal Units, per hour. The detail is as under.
 
To cool this area Look for this size air conditioner
100 to 150 square feet 5,000 BTUs
150 to 250 square feet 6,000 BTUs
250 to 300 square feet 7,000 BTUs
300 to 350 square feet 8,000 BTUs
350 to 400 square feet 9,000 BTUs
400 to 450 square feet 10,000 BTUs
450 to 550 square feet 12,000 BTUs
550 to 700 square feet 14,000 BTUs
700 to 1,000 square feet 18,000 BTUs
1,000 to 1 400 square feet 24,000 BTUs

When looking for the correct size air conditioner, keep these suggestions in mind, as well:
If the room is heavily shaded, you can reduce the BTU capacity by 10%
If the room is very sunny, you should increase the BTU capacity by 10%
If more than 2 people regularly occupy the room, add 600 Btu for each additional person
If the unit is for a kitchen, increase the capacity by 4,000 Btu/Hr.

Additional Recommendations
In addition to size, consider an air conditioner's Energy Efficiency Ratio. The Energy Efficiency Ratio approximates the unit's operating cost as compared to similar models.  If the Energy Efficiency Ratio is high than it’s operation cost will be less.
Room air conditioners are one of the appliances given ENERGY STAR® ratings. ENERGY STAR® Room Air Conditioners feature high-efficiency compressors, fan motors and heat transfer surfaces.
 
ENERGY STAR® air conditioners use advanced heat transfer technologies that transfer more of the heat from the air into the coils. This saves on the amount of energy required to compress the refrigerant. ENERGY STAR® Room Air Conditioners exceed minimum federal standards for energy consumption by at least 15%.
A new energy-saver feature cycles the compressor off and on so that it doesn't operate continuously, reducing your operating costs. Other features to look for are three-speed fan controls, which allow you to adjust the amount of cool air (and noise!) the unit produces, and digital thermostats that are more precise and easy to use. Some room air conditioners also feature times that allow you to set the time you want the unit to come on, so it isn't governed by the temperature of the room alone. For maintaining air conditioner you can see the web site gharexpert.com at the link given below
http://www.gharexpert.com/articles/Home-Improvement-2051/How-to-Maintain-Air-Conditioner-_0.aspx

Dear Sir,

            Keeping in view the orientation of house and the room and room is to be used by more than 2 people 0.75 ton AC is suitable for your 130sqft house. The relation of BTU and Ton is as under.

1 Ton of cooling will eliminate 12,000 BTU/hr of heat.

Dear Sir,

 You should go for 1.5 Ton 3 Star Window Model (Carrier Estrella or Voltas Vertis Plus)

or 3 Star/4 Star Split(Wall mount) Voltas or Carrier Durakool/Durakool Plus

If Weather conditions are normal(less humidity) in your area as well as another room has been constructed over the same roof then buy 1 ton AC(Carrier Estrella/Ogeneral/Voltas Vertis).

BTU output are different as per different brands & Models.