Window air conditioner and portable air conditioner designs work well to cool a small space. While both house all the necessary components in a single unit, packaged HVAC systems feature a much larger design and can heat or cool entire homes. Additionally, packaged units typically install outside on a roof or near the building and connect to ductwork to distribute conditioned air. Window units do not require ductwork. Fun fact: the BTUs of a product act as a unit of measure for energy. BTU stands for British Thermal Unit. It measures the amount of energy required to heat one pound of water by one degree Fahrenheit.