This contribution deals with approach of Constitutional Court to assessing payback period of investment to solar power plant projects in Czech Republic. The main aim of the article is to confirm the hypothesis that Constitutional Court of Czech Republic failed when judging payback period of investment, because it based his verdict on incorrect evidence provided by Energy Regulatory Office. This contribution will explain the circumstances of implementing solar tax and emphasize individual mistakes in Constitutional Court's and Energy Regulatory Office's actions. To confirm the hypothesis stated above methods of comparison, synthesis and analysis and literature review were used.