Building integrated renewable energy sources e.g. photovoltaic system is one of the promised solution for improving energy efficiency in building. However such kind of the system is restrained by irregular power supplied and necessity to convert current from direct to altering form. Therefore, very often the electrical energy generated by photovoltaic system cannot be effectively utilised to supply building devices, e.g. components of HVAC or lighting system. The other problem is energy storage during the day to be successfully used during the night. The paper is devoted to analysed numerically overall effectiveness of two types of ventilation system: low-power, direct current mechanical ventilation and natural ventilation. Both systems were applied for office building, generally occupied during a day. A comparison of both system were conducted for the whole calendar year. Mechanical system considered in the paper is fully supplied from the photovoltaic panels integrated with façade cladding system. Panels covers a total opaque area of external wall, transferring solar energy into electricity with constant performance 15%. The proposed, low-power, mechanical ventilation system can be treated as an alternative for natural ventilation, because both are free from exploitation costs. On the other hand the effectiveness in providing proper indoor air quality of both systems is expected to be different. Efficacy of natural ventilation strongly depends on environmental conditions, while efficiency of proposed mechanical system can productively work during the day only. Taking into account the changeably character of both ventilation systems work a dynamic simulation methods were proposed to analysed their effectiveness. The model of one office zone, occupied during a day were modelled in ESP-r system. The geometry of the room was discretized by control volume elements. The air flow inside the room was defined by a nodal network method. In the case of natural ventilation the window cracks and openings characterized the inlet, while outlet was defined as a ventilation chimney. On the other hand, while mechanical system is considered the inlet and outlet were defined as a mechanical ventilation devices. Presented results shows the effect of both system on indoor air quality and thermal comfort. The results are limited to the period of occupancy only, during the whole calendar year.