When you consider the efficiency of an A/C unit, you'll likely see numbers or ratings for the air conditioner BTU. You may or may not know what this number represents, and further, how to use it to properly size units for your space.
We can first define what "BTUs" really are and why we need them. There actually are some key benefits related to a A/C unit's efficiency, its up-front price and the cost to run it over time. Then, we can learn some useful calculations that will help us choose our best BTU range when buying an A/C. All this adds up to a machine you can depend on to do its job and have minimal maintenance and repair needs.
A BTU's literal definition stands for British Thermal Unit. A scientific principle, BTU is just a unit of energy, measured in joules. It's value is derived from how much energy is needed to heat a pound of water (about a 1/10th of a gallon) from 39F to 40F.
This definition doesn't really matter to you, but if you think about how an A/C pulls heat from a given space to cool it, you might care just how much it can pull to make a space cooler like a living room. How much heat an air conditioner can pull is measured in, you guessed it, BTUs. This is considered the unit's capacity to remove heat from an area. This capacity, or quantity, is measured in a standard per hour basis.
First, you don't want to have a unit that is too small for your space. This happens when a model is designed to pull only so much heat (limited amount of BTUs) per hour to cool the atmosphere, but the volume of the space it's working in is bigger and demands more. You'll never get your air cool enough despite your system running constantly.
If you go too big, you're wasting energy and money. Worse yet, is that you still won't feel cool. You might think that you can be aggressive and go for super-high BTUs to get super cool, right? At first, it seems you could just go overkill and get an air conditioner with the most BTUs. After all, it'll just cool you down that much faster and then run less. Not so easy. What happens is that an A/C with too many BTUs will chill air so rapidly that it doesn't adequately remove enough moisture vapor from the air. The result is that you'll still sense the humidity in the air, and it will feel a lot less comfortable.
Now that you know why it's so important to match the BTU rating of an air conditioner to the space you want to cool, let's figure out how to calculate what you need. There are some rough calculations to do the job. Note, that the bigger your space (e.g. a house with many rooms and levels) the more rough these calculations become. Also, other factors such as the amount and quality of your home's insulation, sun exposure and air leaks to the outside should be considered. It's fine to err on the side of more if you feel these factors have an impact, as you'll see below. If you're in doubt, a professional installer can better determine your BTU needs when sizing your home's air conditioner.
Let's take the scientific stuff we learned about above and apply it to the air conditioner capacity you'll need. For example, working backwards, if we have a unit that's rated at 10,000 BTUs, we know that it will be able to remove that amount of energy (heat) in one hour's time. The size of space that an air conditioner BTU is based on to be efficient is about 20-30 square feet of living space. So that means a 5,000 BTU air conditioner would be required for a 150 sq. ft. room according to the Department of Energy.
First, find the volume of the space to cool by multiplying the length (L) x width (W) x height (H). This will give you your cubic feet.
Second, multiply your total cubic feet by 5 BTUs to get your approximate BTU rating.
Room is 16′ x 18′ with an 8′ ceiling. Multiple 16 x 18 x 8 to get 2,304 cubic feet. Then multiply by 5, and you get 11,520. For this room, you'd want an air conditioner BTU 12,000 rating. You would probably be fine for a higher one, especially if the room has lots of sunny windows. In this case, look for a 12,000 BTU window air conditioner or portable model. Do not try to save money by buying a 10,000 BTU model for the reasons described earlier.
For entire homes, obviously you will need to add up all the cubic feet of your individual rooms and hallways. Again, this can get complicated so it's best to have a professional do it. But you can use this rough calculation to get an idea of how big of unit you'll need for the entire house and start pricing things out.