Driven by economic, political, and environmental dynamics, the path towards achieving sustainability in energy supply on a global scale is one of the key challenges for the twenty-first century. Within this long-term transition process, the transformation of the current energy sector has been identified as a key challenge for a sustainable energy future, which implies that incumbent firms in the two major energy industries (oil-gas and electricity) have a vital role in the development and commercialization process of renewable energy technologies. Solar photovoltaic (PV) technology has seen the most extensive growth over the last decade compared to other renewable energy technologies, and has been identified as a key technology for a sustainable energy future. This study analyzes the role of incumbent firms in the energy industries in the development and commercialization of solar PV technology, as their powerful position in the energy industry gives them a crucial position in the diffusion process of renewable energy technologies. It provides a comparative analysis of strategies regarding solar PV investments, based on in-depth case studies of Europe's leading energy companies.