In this literature, an electret-based porous microcantilever capacitive energy harvester is theoretically modeled. The current and power generation of the energy harvester is the primary focus of our investigation. Porous silicon-based Micro Electromechanical Systems (MEMS) have a wide range of applications, including energy harvesters, signal processing, microbiology, microengineering, and ultrasonic. Here, a material called electret, which has a quasi-permanent charge, is used to generate the induced voltage required in capacitive systems to extract energy from external vibration. The system receives the external vibrations as a harmonic excitation. The equations of motion of the electret-based solid microcantilever and the related boundary conditions can be obtained through the principle of Hamilton, utilizing Euler–Bernoulli beam theory. Kirchhoff’s voltage law is then applied to link the electrical and mechanical domains. The Galerkin method is employed to discretize the coupled electro-mechanical governing equations, which are then numerically integrated over time. The developed model is validated with existing literature on solid cantilever energy harvesters. Porosity levels of 15 and 30% are then introduced into the cantilever material. This porosity reduces the effective stiffness of the material and significantly influences the power generation of the energy harvester under ambient vibrations. As a result, the performance of the capacitive energy harvesting system is remarkably affected. Naturally, by modifying the physical characteristics, the device can be customized to suit the specific requirements of the intended application. This theoretical investigation and results may be helpful in designing efficient porous MEMS energy harvesters.