A reduction of demand for electrical power in peak periods, commonly called peak shaving, is beneficial for customers from the economic point of view. However, it is also of considerable importance for the National Power System operation in terms of grid stability. This research focuses on the issue of nationwide potential of peak shaving by PV installations in office buildings in Poland. It is assumed that this sector can play a significant role in decreasing the demand for peak power on a large scale. The modelling of office building load was conducted with the use of an Artificial Neural Network. The study took into consideration different tilt and orientation configurations of PV modules as well as their degradation. The impact of peak shaving on the National Power System was assessed on the basis of the monthly peak load balancing factor. The influence of PV installations on grid infrastructure was estimated based on changes in transformer lifetime. The research showed that the optimum PV capacity for an average office building was approximately 600-800 kW(p) with a south orientation and tilt angle of 30 degrees. The highest peak shaving capacity on the country level is observed for PV systems with a south-east orientation. 273.75 MWp in PV capacity reduces the maximum national load by 200 MW in May. The analysed PV system producing electricity for self-consumption, can prolong the lifetime of the overloaded transformer connected to the office building by up to 36%.