A British Thermal Unit (BTU) is the amount of heat required to raise the temperature of 0.45 kg (1 pound) of water by 1 degree Fahrenheit (0.56 degrees Celsius). To be precise, 1 BTU is equal to 1,055 Joules and in heating or cooling terms, 1 ton equals 12000 BTU.

A 2000 square-foot house may require a 5-ton (60,000 BTU) air conditioning system. A typical window-type air conditioner has a rating of 10, 000 BTU. Therefore, you might need 30 BTU per square foot, but these are all rough estimates.

The Energy Efficiency Rating (EER) 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 will be 10000/1200 which will equal to 8.3. Normally a higher EER comes with a higher price.

Suppose that you had to chose between these two 10,000 BTU units.

**Unit 1 having:**

- EER of 8.3

- Consumption of 1200 watts

**Unit 2 having:**

- EER of 10

- Consumption of 1000 watts

And the price difference is $100. To find out which unit is more economical, you will have to find out how many hours per year you will be using the unit, and how much a kilowatt-hour will cost in your area of living.

Lets say you decide to use the air conditioner for four months in a year, i.e. in the summer season only and supposing that we use it for 6 hours daily. Taking the cost in your area as $0.10 kWh, the difference in energy consumption for the two units will be as follows:

**For unit 1:**

- number of hours in 4 months

4 x 30 days x 6 hrs/day = 720 hrs

- 720 x 1200/ 1000 x $0.10 = $ 86.4

**For unit 2:**

- number of hours remains the same as above i.e. 720 hours

- 720 x 1000/1000 x $0.10 = $ 72

Therefore, the difference between the two becomes:

- $86.4 – $72 = $14.4

what you have to understand is that as the difference between the two is of 200 watts, i.e. 1200- 1000 = 200 watts, for every 5 hours the less expensive unit will consume an additional kWh and therefore $0.10 more than the more expensive unit.

And as the more expensive unit costs $100 more, it will take about seven years for the more expensive unit to break even.