Storage system and power control method thereof with mode selection defined by timeout period and power supply

In the process of controlling a plurality of storage devices in any one of a normal mode and a plurality of power-saving modes (such as standby and LRPM), a controller selects the storage devices whose mode should be shifted from the normal mode to any of the power-saving modes; and before shifting to control of the selected storage devices in any of the power-saving modes, the controller calculates time and electric power required to make the selected storage devices return from each power-saving mode to the normal mode; and based on each calculated value, the controller sets a combination of power-saving modes and the number of storage devices that satisfy the return conditions defined by the host timeout period and the supplied power; and then, the controller controls the selected number of storage devices in the set combination of power-saving modes.