Air Conditioning Facts & Information
1 ton of cooling is 12,000 btu's.  (British Thermal Units).
A British Thermal Unit is the amount of energy required to raise 1 lb of water 1 degree Farenheit.
One ton of cooling will take care of 400 sq ft. residential
One ton of cooling will take care of 300 sq ft. commercial not including heat load from equipment
and additional heat gaining areas such as windows, doors, etc.
1000 sq ft - 2.5 ton residential
1200 sq ft - 3 ton residential
1400 sq ft - 3.5 ton residential
1600 - 4 ton residential
2000 - 5 ton residential

Here in Arizona, it is wise to size your equipment a 1/2 ton larger than the rule to accomodate 
the high temperature days when we are above 100 deg F.

A good way for you to check the performance of your unit is to measure the 'split'.
This is how many degrees the unit is changing the temperature of the air.

Take a thermometer and place it in the grill where your filter goes and record the 'return air' temperature.
Then move it to a nearby 'supply register' or 'vent', as some call it, and read the temperature there.
The minimum difference should be 16 deg F.

Return Air Temp - 80
Supply Air Temp - 63
Split - 17 degrees

During the drier months of the year your split can get up to 20 - 23 degrees.

When the humidity comes up during the monsoons it will be less but should be 16 deg minimum.

Remember to change your filters every time you pay your electric bill if not more often.

If you're in the city and not out of town where it's particularly dusty you can use thin fiberglass filters that breathe better than the heavy pleated filters and  so called 'lifetime' or '3 month filters'.

Spray furniture polish on each side of your fiberglass filters prior to replacing.  This will help catch the light dust better.

If you are out of town or in a particularly dusty area, you will probably need to use the heavier pleated filters to pull all of the material out of the air.  It's not uncommon to need to change a pleated filter every week or two if you're in a dusty area.