Law, Normative and technical literature concerning building calculation of costs and energy performance, are usually reported to winter regime. During winter regime buildings have higher energy consumption to keep the inside temperature at a comfort range. Energy performance building (EPB) is related with outside temperature and with inside comfort temperature, therefore EPB depends on external weather trend. The “measure unit” of weather trends, used in some calculation models, is the Day Degree, that represents the hours when outside temperature is below the inside temperature standard. The development and diffusion of HVAC system, especially single units system (split system) introduced the concept of comfort including summer regime, and weather derivates (Hot Day Degree HDD and Cool Day Degree CDD) used to establish the weather risks for insurances. HDD and CDD could be used to evaluate energy audit ante and post project, or in order to plan and to check the efficiency of energy projects, or to have different building solutions related with weather variations. This method will enable to know in advance the cost of energy saved and the cost of energy efficient project. To evaluate energy performance in building during summer regime we could use the studies on comfort perceptions to translate a Day Degree concept in a “measure units” to evaluate with respect to a normalized cooling regime. In the paper we propose Summer Day Degree according to outside temperature trend, inside fixed temperature value (comfort range), humidity, wind velocity, and incidence radiation. Moreover, an example of application of Summer Day Degree is presented.