Building energy modeling (sometimes abbreviated as BEM) is a dynamic, physics-based calculation of building energy consumption. It is typically a computerized simulation focusing on utility bill data and life-cycle costs of various energy-related items such as air conditioning, lights, and hot water. The simulation involves reproducing the weather conditions of an entire year on the building model. The result can be used for many different purposes including design of new buildings, deep retrofit projects, and development of whole-building energy codes and standards (such as ASHRAE 90.1). It is also used to evaluate the payback of renewable energy solutions like solar panels and photovoltaics, wind turbines, and high-efficiency appliances.