Vibration energy harvesters allow the extraction of the ambient mechanical energy. Our analysis focuses on a bistable generator with Duffing nonlinearity. Bistable generators exhibit larger frequency bandwidth than linear monostable generators, but they exhibit complex behaviors (multiple periodic orbits and chaos), making their evaluation challenging. To assess the quality of a vibration energy harvester, it is necessary to evaluate its energy performance in a generic way. In this presentation, we describe a numerical approach to predict the probability of occurrence of each orbit regarding the initial conditions of the dynamical system. The estimation of the orbit probabilities and powers allowed us to calculate an expected power for each vibration frequency. This approach has been used to demonstrate that our generator has a constant expected harvested power for a driving frequency between 23 Hz and 55 Hz. Moreover, the choice of the initial conditions grid is fundamental since it affects the capture rate of the orbits and thus their probability of occurrence. Thanks to numerical simulations, we show that the initial conditions grid must be precise enough to ensure the reliability of the orbit probabilities derived with the proposed method.