Summer is approaching and I rediscovered a small guide I had created for myself some time back. So here it goes...

Whenever we go to the showroom, the salesman always asks 'Room Size'. Generally, rule of thumb they apply is :

If the room size is upto 100 sq ft (or 120 sqft sometimes), then they recommend AC Upto 1 ton.

More than 100 sqft but less than 150 sq ft -> 1.5 ton

More than 150 sqft -> 2 ton.

This is very simplistic and has room for errors.

**The first error comes from not accounting for height**.

The thing is that the AC has to cool a complete room and the room is a cuboid. So, total volume of the Room matters. As some have 10ft, some have 12ft and some old homes have 14ft ceilings, the same AC might not be sufficient. (Or overcool).

So, a more accurate calculation is to take the complete volume into account. This works for all kinds of rooms. Here is the calculation for it:

a) Calculate its volume in cubic feet - height x length x breadth.

b) Multiply this volume by 6.

c) Now add (no. of people x 500) to this value. As having more people warms up the room. So for 2 people, we add 2 x 500 = 1000.

So for a 12x12x12 room which will stuff 3 interns, we get a value of 12x12x12x6+1500=11,868.

This is the 'Cooling Capacity' you need in terms of BTUs

For watts divide it by 3.412. So this room needs an AC whose Cooling Capacity is 3478 watts. (You can get this in a 1 Ton AC)

Here, the salesman would have told you to get a 1.5 Ton AC as 12x12=144sqft is the room area.

Thus, calculating the Cooling Capacity not only gives you an exact calculation for purchasing AC, when you know this, you can get away with a smaller AC in many cases.

Also, if you are feeling like saving money or are short on cash, you can also multiply volume by 5.5 above. But dont go below that for the room may not cool sufficiently.

**The second error comes from not accounting for sunlight**.

If your ceiling has no room above it, then there is optimization to be applied in terms of reflective paint (you can get these from paint companies like Nerolac etc.), or simply a reflective white plastic sheet. This will reflect away the sunlight and thus room will stay cooler. This will have to applied on the roof.

The plastic sheet can also be removed in winters when you might want the sunlight to warm the room.

Although there are better alternatives like having a row of plants on the roof directly above the room where you can grow your own vegetables or fruits and get organic chemical free pure food while getting the room cooled by quite a bit.

Best wishes