Archives for British Thermal Unit

Understanding BTU’s

So, as you’re reading about air conditioners, you see a lot of talk of BTU’s.  What is a BTU?

BTU stands for “British Thermal Unit,” and it is basically a measure of energy.  Specifically, one BTU is the amount of energy required to heat one pound of water one degree Fahrenheit, as you no doubt remember from the last time you and your buddy tossed couple of pounds of water on the grill.  Seriously, the scientific definition of the BTU is not that important; what is important to understand is exactly how much cooling power each one has.

How many BTU’s do you need?

Your particular needs will depend on a number of things, like the climate you live in and how well insulated your home is.  But you can use the following as a general guide:

  • a 5,000 BTU air conditioner will cool 100-150 square feet
  • an 8,000 BTU air conditioner will cool 300-350 square feet
  • a 10,000 BTU air conditioner will cool 400-450 square feet
  • a 12,000 BTU air conditioner will cool 450-550 square feet

Those are the most common air conditioner sizes.  For other sizes, see the helpful chart here.

These are conservative estimates, but it’s good to be conservative, because the last thing you want to do is to go to all of the trouble of buying and installing an air conditioner only to find that it’s not really cooling the area you intended it for.

Doing the math, we can see that, on average, it takes about 24 or 25 BTU to cool one square foot of space.

These sizes are for small air conditioners, like portable air conditioners, and vertical/casement window air conditioners.  Of course, a central air conditioning system would have a much, much higher rating.  An air conditioning system intended for an entire house, for example, might have 60,000 BTU.

When deciding what size air conditioner to get, you should also be aware that manufacturers will give their own rating of the area that they think a particular unit is capable of cooling, and this may be larger than the area that would be indicated based on the BTU rating of the unit.  In those cases, you should consider the manufacturer’s rating along with the BTU rating to determine what size air conditioner you need.  But, as we said previously, it’s probably better to err on the side of having slightly more than you need rather than not quite enough.

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