The amount of square footage that 15000 BTU will cool depends on several factors, including the size, insulation type and layout of the space being cooled. Generally speaking, however, a 15000 BTU air conditioner can cool up to 500-550 square feet with an 8-foot ceiling. If you have higher ceilings or large windows in the room that is being cooled, it may be necessary to use a larger unit for effective cooling. Additionally, if you are using an older model air conditioning unit with poor insulation or inadequate ductwork this number could be lower. To accurately determine how much square footage your 15000 BTU AC unit can effectively cool it’s best to consult with a qualified HVAC technician who understands your specific needs and equipment requirements.