The cooling capacity of an air conditioner is measured using a unit called a British Thermal Unit or a BTU/hour. Needless to say that if the BTU of a unit is high, its cooling capacity will be higher. However, this is not always the answer to your cooling requirements. A unit with a very high BTU often requires more maintenance because it gets worn out faster. So choose the one that is moderate and serves the purpose.
Most air conditioners these days come with thermostats and even timers that can be modified as per the requirements of the person. So, you can program the air conditioner to a certain temperature for a particular time of the day. While buying an air conditioner, you need to check if it has such features, and if it does, how accurate it is to the actual setting that it has been programmed to be in. How close is it to the temperature that it is expected to be at? Keep these points in mind while buying it.