A 25,000 BTU unit should comfortably cool down a 1,300-square-foot house. BTU stands for British thermal unit and is a traditional unit to measure energy. It is defined as the amount of heat required to raise by 1 degree Fahrenheit the temperature of 1 pound of water.
The capacity of an air-conditioning unit is sometimes measured in tons, and there are 12,000 BTU in a ton. Air-conditioning units also have an energy efficiency rating separate from their cooling capacity. The energy efficiency rating of an AC is equal to its BTU rating over wattage. The higher the rating is, the less energy a unit consumes.