A method for using hydroelectric power as a power reserve of an electricity grid, comprising: - running a plurality of hydroelectric power generators, each at a respective output power level, to supply electric energy at a total output power to said electricity grid, - monitoring an AC frequency of said electricity grid to detect deviations (302, 304) from a default frequency range, - as a response to detecting a deviation (302, 304) from said default frequency range, making electric energy flow (303, 305) between an electric energy storage and said electricity grid, - initiating a change in said total output power of said plurality of hydroelectric power generators from a first total output power level to a second total output power level, and - continuing to make electric energy flow between said electric energy storage and said electricity grid for the duration of time it takes for the total output power of said plurality of hydroelectric power generators to reach said second total output power level; characterized in that: - said plurality of hydroelectric power generators draw their operating power from water flows of a common drainage basin while being located at different parts of said drainage basin, - at least one of said first and second total output power levels is selected on the basis of executing a drainage system optimization algorithm that optimizes at least one of: amount of electric energy obtainable from hydroelectric power plants within said drainage basin; a total obtainable price of electric energy produced by hydroelectric power plants within said drainage basin; a total efficiency of the hydroelectric power generators; minimal need of mechanical movements of gates; reactions to icing and other causes of screen losses; distribution in time of produced electric energy, - after the total output power of said plurality of hydroelectric power generators has reached said second total output power level at a first moment of time, the method comprises making said total output power of said plurality of hydroelectric power generators continue changing in the same direction until a second, later moment of time, - the method comprises returning said total output power of said plurality of hydroelectric power generators to said second total output power level at a third, even later moment of time, and - between said first and third moments of time, the method comprises recharging said electric energy storage if it was discharging before said first moment of time, or discharging said electric energy storage if it was charging before said first moment of time. - running a plurality of hydroelectric power generators, each at a respective output power level, to supply electric energy at a total output power to said electricity grid, - monitoring an AC frequency of said electricity grid to detect deviations (302, 304) from a default frequency range, - as a response to detecting a deviation (302, 304) from said default frequency range, making electric energy flow (303, 305) between an electric energy storage and said electricity grid, - initiating a change in said total output power of said plurality of hydroelectric power generators from a first total output power level to a second total output power level, and - continuing to make electric energy flow between said electric energy storage and said electricity grid for the duration of time it takes for the total output power of said plurality of hydroelectric power generators to reach said second total output power level; - said plurality of hydroelectric power generators draw their operating power from water flows of a common drainage basin while being located at different parts of said drainage basin, - at least one of said first and second total output power levels is selected on the basis of executing a drainage system optimization algorithm that optimizes at least one of: amount of electric energy obtainable from hydroelectric power plants within said drainage basin; a total obtainable price of electric energy produced by hydroelectric power plants within said drainage basin; a total efficiency of the hydroelectric power generators; minimal need of mechanical movements of gates; reactions to icing and other causes of screen losses; distribution in time of produced electric energy, - after the total output power of said plurality of hydroelectric power generators has reached said second total output power level at a first moment of time, the method comprises making said total output power of said plurality of hydroelectric power generators continue changing in the same direction until a second, later moment of time, - the method comprises returning said total output power of said plurality of hydroelectric power generators to said second total output power level at a third, even later moment of time, and - between said first and third moments of time, the method comprises recharging said electric energy storage if it was discharging before said first moment of time, or discharging said electric energy storage if it was charging before said first moment of time.