What size A/C unit do you need to cool a 1,300-square-foot house? 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. What size air conditioner do I need for my house? How do you calculate the proper air conditioner size for a house? 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. How many BTUs does a furnace need to heat a large home? The size of a home does not dictate the furnace British thermal unit output needed to heat it properly or efficiently. Several factors must be considered w...