The capacity in most air conditioners is rated in BTUs or British Thermal Units. Generally, the amount of heat required to raise the temperature of 1 pound of water 1ᵒ F is called 1 BTU. Specifically, 1 BTU is measured as 1055 Joules but, in real-life conditions, the first definition is easier to understand. 1 “ton” in heating and cooling terms is measured as 12,000 BTUs. That means that the air conditioner can cool 10,000 pounds of water (about 1,200 gallons) up to 1 degree in 1 hour. Or it can cool 5000 pounds 2 degrees in 1 hour or 2500 pounds 4 degrees in 1 hour and so on.
Naturally, we don’t live in aquariums so, knowing the water cooling capacity of an air conditioner is not of much use to us. The fact that a cubic foot of water weighs about 63 pounds can give us a very rough idea of how much air can be cooled. The density of water is 6300 times of air. So, the weight of 100 cubic feet of air is 1 pound. The air-containing capacity of a typical bedroom is about 1000 cubic feet or 10 pounds. That means if we ignore the differences in heat capacity, a 10000 BTU air conditioner can lower the temperature of a perfectly insulated bedroom by 10 degrees in just a couple of minutes. It is not to state that the room has to be perfectly insulated (in fact commonly most rooms have very little or no insulation) but, it tells you that you probably do not need a 10000 BTU air conditioner for a typical 10’x12’ bedroom. For comparison, a 5 ton (60000 BTU) air conditioner can easily cool an insulated 2000 square feet house. That means you perhaps need 30 BTU per square foot. Keep in mind that these all are estimated roughly and you should not blindly rely on this information to decide the size of your home’s air conditioner. Rather you should consult an HVAC contractor.
The EER (Energy Efficiency Rating) of an air conditioner is determined by its BTU rating over its wattage. For example, the EER of a 10000 BTU air conditioner that consumes 1200 watts is 10000/1200 = 8.3. You would obviously like a system of an EER as high as possible but, you should keep in mind that higher EER normally comes at a higher price.
Let’s assume that you have a choice between two 10000 BTU units having EER rating and power consumption of 8.3 & 1200 watts and 10 & 1000 watts respectively. Let’s also assume the price difference to be $100. For a better understanding of the payback period on the more expensive unit, you need to know:
Let’s assume that you have planned to use the air conditioner in the summer only (4 months in a year) and it will be operating about 6 hours a day. Let’s also assume that the cost of 1 kilowatt-hour is 10 cents in your neighborhood. The energy consumptions of the two units differ by 200 watts. That means that every 5 hours, the less expensive/less efficient unit will consume 1 kWh (and therefore one more dime) more than the more expensive/more efficient unit. Taking consideration of 30 days in a month, you will 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 per hour / 1000 watts per KW*0.10 cents per KW = $14.40. As we see that the more expensive unit costs $100 more, it means that about 7 years will be required for it to break even.