For long-term simulation of renewable power systems with a large number of switch-mode power converters, both academia and industry require a highly efficient simulation model. This paper proposes an averaging model, which demonstrates significantly fast speed, while capturing important dynamics to ensure simulation accuracy. The modelling technique is applicable to the entire conduction state, including both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The proposed averaged modelling approach has been tested for a typical PV-battery system by Simulink as shown in the case study. It can also be generalized for further extension to other distributed power systems since this kind of simulation method is purely based on the mathematical calculation representation of the system dynamics rather than any specific simulation tool.