Energy cost allocation systems apportion cooling and/or heating costs among the individual apartments in centrally metered buildings based on various methods of energy metering. The most commonly used systems measure one or more parameters related to the thermal output of the terminal element but do not measure enough parameters to provide an accurate energy measurement because of many reasons, such as investment, installation, maintenance cost, etc. They typically assume that cooling or heating output is linearly proportional to a quantity, which may be a single temperature, a difference between two temperatures, or an average of two temperatures. Most of them only perform well over a certain limited range of application conditions (Hewett 1994). To solve this problem, this paper puts forward a new method called “cooling/heating metering on the air side,” which uses five parameters, including indoor air temperature and humidity, entering water temperature, airflow rate, and water flow rate, to calculate the thermal output of the terminal elements. These needed parameters can be obtained by a direct or indirect way, which is discussed at length in this paper. Theoretic analysis shows that the metering errors of the method are lower than 25% in most cases and will be further reduced when some measurements are made in the actual situation. The new method is comparatively economical and efficient in air-conditioning thermal metering.