The planned and already observed increase in the use of photovoltaic power plants is partially constrained by the uncontrolled variability of energy generation and the need to allocate significant areas for the placement of photovoltaic panels. The weakening of the influence of these factors is possible through the construction of a floating photovoltaic plant on the reservoirs of a large-scale pumped storage hydropower plant. This paper is devoted to the operation of large-scale photovoltaic and pumped storage power plants under market conditions as well as the problem of taxation. We are determining an option that is acceptable to both the transmission network operator and the power plant. An optimization problem has been posed and solved, including the maximization of the profit of the plants and the transmission grid operator. The rationality of changing the taxes for transmission grid services is demonstrated. The methodology is based on forecasts of photovoltaic generation, changes in market prices and application of linear programming. The example of maximizing one week’s cash flow proves the importance and significance of the tax system as well as the need to change it.