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The energy efficiency of switchable (variable transmittance) windows having a control system based on occupancy, temperature and impinging solar radiation is evaluated. The heating and cooling performance of the switchable windows are compared with the performance of several conventional glazings. The study indicates that the energy efficiency of switchable windows depends highly on several different parameters, such as location, type of building, orientation, occupancy, etc., but that there is a potential to use a “intelligent” control system for the switchable window in order to reduce the need for external energy and potentially also improve the comfort. The energy efficiency of switchable windows increases with internal heat production and is more beneficial the sunnier and the warmer the location is.