A mathematical theory has been developed and a numerical calculation has been performed for the non-steady-state conditions of operation of a solar energy conversion plant in which the solar array is loaded simultaneously by an electrolyser (for hydrogen production) and the storage battery. A method is suggested to determine the system optimum design parameters, i.e. the numbers of the series-connected modules: solar arrays, electrolysers and batteries affecting the solar array operation at the maximum power during the whole light day and at any moment in the battery charging. The regime of successive switching of the battery sets for charging, allowing arbitrary presetting of the distribution of electrical energy between the electrolyser and the battery has been established.