Increasing capacity of intermittent generation brings new challenges to balance demand and supply in power systems. With retirement of conventional fossil generation, the role of energy storage is increasing. One of the most competitive storage technologies is pumped storage hydropower plant (PSHP). Usually, such PSHPs are constructed as green field solutions, but in some cases conversion of a hydropower plant into a pump storage hydropower plant by building a pump station is possible. To evaluate the feasibility of such modernisation it is necessary to estimate the benefits of PSHP operation. The simplified model was developed for simulation of charging and discharging cycles of PSHP in Latvian power system and trading electricity in Nord Pool power exchange. The nature of this task is stochastic as the price volatility has a trend to increase with expansion of wind and solar power plant capacity. Results of PSHP operation simulation were then used in the economic model to evaluate the feasibility of the proposed conversion.