The integration of solar photovoltaic-thermal (PV/T) systems within building energy systems offers a multifaceted approach to enhancing energy sustainability and reducing carbon footprints. The PV/T systems can generate a surplus of energy (i.e., more than a building’s energy demand), improve the power network reliability (i.e., improve energy flexibility), and lower the buildings’ energy costs (i.e., independent from fluctuations of energy prices). In this context, this study aims to introduce an innovative system design that focuses on leveraging PV/T panels, thermal storage units, and local grids to reduce energy costs and ensure building energy efficiency. This paper evaluates a smart building system with the deployment of PV/T panels and a heat storage tank in Oslo, and Trondheim (Norway). TRNSYS software was used to model and analyze the system’s functionality across diverse operational scenarios. Findings reveal distinct seasonal energy demand patterns in a Norwegian building, emphasizing peak heating needs of 6 kW in winter and 3.5 kW in summer. Trondheim had a slightly greater PV/T power, while Oslo showed better PV/T efficiency. This identifies an optimal setup for PV/T panels and demonstrates their adaptability in various climates. This approach marks a significant stride toward achieving nearly zero energy expenses in smart building energy systems.