Home Guides Home Home Improvement Sustainability How to Figure What Size of Air Conditioning Unit Is Needed for a Sunroom Email Reddit Sunrooms should be bright, not hot. While a sunroom brings a lot of sunlight into your house, it also can affect the comfort of your living space. Exposure to excessive heat decreases the efficiency of your home air conditioning system. Installing a room air conditioning unit in the sunroom supplements the work of your whole house system, and provides a more comfortable space. Proper sizing is key to purchasing the best unit for your needs. Measure the length and width of the sunroom using a tape measure. For larger rooms, a laser measuring tool provides accurate measurements without the help of a second person. Multiply the length and width to establish the square footage of the sunroom. For example, a room 25-feet long by 10-feet wide occupies 250 square feet of space. Multiply the square footage of the room by 20 to establish the correct BTU rating for your sunroom.