Here in north Florida we heard 500 sq ft per ton tossed around for years. Then in the 90s, we were all told you could figure 600 sq ft per ton if the house had good insulation & if you used a "high efficency" unit.

Pretty vague huh.?

The only "rule of thumb" I've seen that I liked (just a little) was cubic ft X 3 = the BTU requirement.

In other words:

sq ft X ceiling height = cubic ft. So, cubic ft X 3 = BTUs needed. 

I only like it because it has a little bit of science to it & believe it or not, it's been close the the load calc more than once over the years.

One more factor you can add for commercial sites: Humans = 600 BTUs per head & it can 900 BTUs per head if they are active, soulful church, dance hall, gym, etc.

I don't want to hi jack the thread but I think it would more interesting to discuss where the term "rule of thumb" came from.

Air conditioning is no longer a luxury, but has become a necessity to Indians. Planning to buy an air conditioner? Well, there are some things which can help you out in determining the right product.

This machine is a power hungry device, and buying an inexpensive model will make you shed more on the power bills. As a rule of thumb, one can expect 990W/TR. Means, a good model (star rated) would approximately consume 990W /hour for a 1TR model split A/c. Consider Rs.3/kWh* (BSES Delhi).This means, 0.99KW X 3Rs X 10 hours/day = 29.7 Rs/day x 30 days = Rs 891/month.

Consider running the unit for 10 hours. It has to be noted that the air conditioner doest consume full power all the time, but only when the compressor runs. Buying an under sized air conditioner makes the compressor running 24 X 7, causing huge power bills, in efficient cooling and quick wear and tear to the compressor. So, choose the right capacity . Models are available ranging from 0.8TR to 3.0TR Split units.So, if the right capacity is selected, the compressor runs 8 hrs(approx), and save 162 Rs/month. (Do the math). So how to determine the right capacity?

As a rule of thumb, consider 1 TR for 120sq.ft bed room.Again, keep in mind if any of the walls of the room is having a West exposure, consider the next higher capacity, as sun gain is high in this side. (The dealer will help you in this, else go to the website of any brand, and you have the option to determine this).

Now that you know what capacity is needed, the next thing which you need to know is the technology. The heart of any air conditioner is the compressor, and this is where the

technology is. For a typical airconditioner, once the desired temp is reached, the compressor stops, while the blower is still on. As the temp of room rises, the compressor kick starts. The power drawn can be seen if the lights of the room are on, and there’s this slightdimming of lights for a fraction of second. This process repeats, and compressor comes On and Off as required at a constant speed. 

Inverter technogy / DC Inverter

This is the latest technology, where in the air conditioner has a variable speed compressor. The term “DCInverter” is because ac current is converted into dc, and the frequency is varied, according to the speed required. (Typical ac power is at 50Hz, 230V). Unlike the traditional method, the compressor regulates the speed according to the load, and there’s no frequent On and Off of compressor cycle saving energy, and wear and tear of the machine. As a matter of fact, a 1 TR machine will work for 300-400W/TR for minimum. So, see the savings for yourself. Technology comes for a price, and so these units are priced accordingly. Also, the temp remains constant in the room, as the compressor varies its speed as required. 

BEE/Star rating

Cooling capacity is defined in TR. 1 TR = 12000 BTU/hr1 TR = 3512 W (This is not the power consumption, but cooling capacity in Watts)1 TR = 3024 calories/hour

Even though unit expressed in TR is common, different companies express capacities in different units as mentioned above. To find the star rating, covert the capacity in terms of Watts (cooling capacity)

For a 1TR machine, this is 3512 Watts. (Cooling capacity). Now determine the power consumption. Check the unit, (outdoor unit) on which this is written. Consider 990W. 

Divide cooling capacity by power consumtion, this gives the EER (Energy Efficieny Ratio)

In our case, 3152 / 990 = 3.5 Star rating is determined from this value. 3.5 is 5 star rating. More can be found on the website of Energy Efficiency.

Best Quality Brands suggested are:-1. Carrier2. Daikin3. Blue Star4. Voltas