Volumetric expanders are used for low to medium power output ORC applications. For low power output ORCs (< 10 kW), rotary vane expanders represent a suitable choice. Their isentropic efficiency is often reported as the most important or even the only metrics for comparison. Such approach however neglects the effect of leakages within the expander on the rest of the cycle, especially on the evaporator pressure. Filling factor of rotary vane expanders may be affected, among other leakages, by delayed closure of working chamber.This work describes a semi-empirical model with two different leakages – lumped leakage area between inlet and outlet and leakage between working chambers due to delayed contact of vane and stator. Primary purpose of the model is to demonstrate the effect of the delayed chamber closure. This is identified as a necessary parameter for modeling of the rotary vane expanders, compared to only single lumped leakage area for the other expander types. Results of the model for several case scenarios are presented, showing an impact of these leakages on an overall cycle performance, isentropic efficiency and filling factor. It is demonstrated that isentropic efficiency of the rotary vane expander might not be always sufficient to compare vane expanders or their modifications even within a same ORC. Lastly the model conclusions are preliminarily checked with our experimental data.