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Most air conditioners have their capacity rated in BTUs, or British
Thermal Units. A BTU is, generally, the amount of heat required
to raise the temperature of one pound of water one degree F. Specifically,
a BTU is 1,055 joules, but the first definition is easier to understand
in real-life terms. One "ton", in heating and cooling terms, is
12,000 BTUs. A typical window air conditioner that you find at
K-mart might be rated at 10,000 BTUs. What that means is that
the air conditioner has the ability to cool 10,000 pounds of water
(about 1,200 gallons) one degree in one hour. Or it could cool
5,000 pounds 2 degrees in one hour. Or 2,500 pounds 4 degrees
in one hour, and so on.
Not many of us live in aquariums, so knowing how much water an
air conditioner can cool is not much use. To get a very rough
idea of how much air can be cooled, take the fact that a cubic
foot of water weighs about 63 pounds. Water is about 6,300 times
denser than air. So 100 cubic feet of air weighs about a pound.
A typical bedroom contains about 1,000 cubic feet of air, or 10
pounds of air. That means (ignoring differences in heat capacity)
that a 10,000 BTU air conditioner can lower the temperature of
a bedroom, if it is perfectly insulated, by 10 degrees in just
a couple of minutes. It is not the case that the room is perfectly
insulated (in fact many rooms have little or no insulation) but
what that tells you is that you probably do not need a 10,000
BTU air conditioner for a typical 10' x 12' bedroom. For comparison,
you can happily cool an insulated 2,000 square foot house with
a 5 ton (60,000 BTU) or so system, implying that you might need
perhaps 30 BTU per square foot. Keep in mind that these are al
l rough estimates and you should not rely on any of this information
to size your home's air conditioner - ask a HVAC contractor.
The EER (Energy Efficiency Rating) of an air conditioner is its
BTU rating over its wattage. For example, if a 10,000 BTU air
conditioner consumes 1,200 watts, its EER is 10,000/1,200 = 8.3.
Obviously you would like the EER to be as high as possible, but
normally a higher EER is accompanied by a higher price. How do
you decide if the higher EER is worth it?
Let's say that you have a choice between two 10,000 BTU units.
One has an EER of 8.3 and consumes 1,200 watts and the other has
an EER of 10 and consumes 1,000 watts. Let's also say the price
difference is $100. To understand what the payback period is on
the more expensive unit you need to know:
- Approximately how many hours per year you will be operating
the unit
- How much a kilowatt-hour (KWH) costs in your neighborhood
Let's say that you plan to use the air conditioner in the summer
(4 months a year) and it will be operating about 6 hours a day.
Let's also imagine that a kilowatt-hour costs 10 cents in your
neighborhood. The difference in energy consumption between the
two units is 200 watts, which means that every 5 hours the less
expensive unit will consume one more KWH (and therefore one more
dime) than the more expensive unit. Assuming there are 30 days
in a month, you find that during the summer you are operating
the air conditioner 4 months * 30 days/month * 6 hours per day
= 720 hours. 720 hours * 200 watts/hour / 1000 watts/KW * 0.10
cents/KW = $14.40. Since the more expensive unit costs $100 more,
that means that it will take about 7 years for the more expensive
unit to break even.
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