All kinds of energy are one of the basic requirements in life to sustain it. Therefore, the majority of research and applications have been mostly focused on self-sufficient energy or zero-energy buildings in developed countries. Most of these countries encounter energy demand and suffer from carbon dioxide emissions from existing buildings. Energy-efficient applications must be also spread across developing and undeveloped countries especially in the Mesopotamia region due to the hot climate. Hence, the efficiency of a common practice, implementation of photovoltaic panels (PVs), in Iraq is investigated in this study. PVs were designed and empirically implemented in an existing conventional primary school building in Baghdad, Iraq as a case study for the Mesopotamia region. The sustainability and energy analysis of a school building was conducted in diverse scenarios to explore the potential of energy-saving and payback period of the PVs. The slope and number of PV panels located on the school building are the parameters of the current study. Results showed that PV panels with a 30° inclination angle reduced the energy cost of the building by 50%. The payback for the implementation of PVs can be obtained approximately in 8 years. Consequently, the PVs have major potential in energy efficiency and can be implemented in governmental buildings such as public-school buildings in the Mesopotamia region. Besides, society and private institutions may become aware of the importance of sustainable energy with this study. With this case study, the importance of taking advantage of solar energy has been emphasized for undeveloped countries.