This paper proposes a behavioral model of DC-DC converters losses dedicated to photovoltaic applications. The objective is to introduce accurate DC-DC converter's efficiency into long range system simulations. Our approach has been validated on a four switch buck boost converter (FSBB). A large number of tests has been performed on a FSBB converter, for different input voltages, duty-cycles and output currents. Power losses have been fitted with multivariable polynomial functions for the different operating modes of each cell of the converter (i.e. when the FSBB operates like a Buck converter, the Buck cell is active and the Boost cell is passive). A VHDL-AMS model of each cell has been developed. Thanks to its conservative energy considerations, this language is very well adapted to the proposed approach. Today, our model enables us to get fast and accurate simulations regarding the converter efficiency.