The transition to electric mobility has become an unstoppable reality, whose speed of implementation in society, is generating specific needs to address the incorporation of electric vehicle charging systems in buildings. This paper presents a study that evaluates how the electrical systems of three residential buildings, and specifically, the electrical transformer that powers them, will be affected when all the neighbors have an electric vehicle that they need to charge in the garage of their houses when they return from work. The objective of the study is to address the minimum size (capacity) of this electrical transformer, carrying out different simulations in five use cases, which include demand response strategies and the use of a photovoltaic facility to be installed on the roof of the buildings. It is concluded that, in this type of scenarios, electricity production and consumption are decoupled, so while renewable production systems may be helpful, they have little effect unless a relevant percentage of vehicles can be charged during the central hours of the day. Additionally, to be able to use efficient transformers sizes in buildings, which suppose reduced costs for neighbors, it is needed to manage three concepts: demand response strategies, adequate charging battery levels in electric vehicles, and a proficient stewardship of photovoltaic energy in buildings.