A storage device operating in a plurality of power modes including an active mode and a low power mode having a lower power consumption than the active mode, the storage device comprising: a nonvolatile memory including a plurality of nonvolatile memory cells; a storage controller configured to process commands inputted from a host device in the active mode; and a power mode manager configured to adjust the plurality of power modes, wherein the power mode manager is further configured to predict an input prediction time for a second command to be inputted from the host device after a first command when the first command is inputted, change from the active mode to the low power mode when a processing operation of the first command is completed, and return to the active mode from the low power mode when a return time elapses according to the input prediction time, and wherein the power mode manager is configured to compare the input prediction time with a mode switching time taken to return from the low power mode to the active mode, compare electric energy saved by performing the low power mode with electric energy consumed by changing from the active mode to the low power mode and set the return time according to the input prediction time and the mode switching time when the electric energy saved by performing the low power mode is equal to or greater than the electric energy consumed by changing from the active mode to the low power mode, and wherein the power mode manager is configured to change from the active mode to the low power mode based on comparison results of the comparing operations and change to the active mode from the low power mode when the set return time elapses. a nonvolatile memory including a plurality of nonvolatile memory cells; a storage controller configured to process commands inputted from a host device in the active mode; and a power mode manager configured to adjust the plurality of power modes, wherein the power mode manager is further configured to predict an input prediction time for a second command to be inputted from the host device after a first command when the first command is inputted, change from the active mode to the low power mode when a processing operation of the first command is completed, and return to the active mode from the low power mode when a return time elapses according to the input prediction time, and wherein the power mode manager is configured to compare the input prediction time with a mode switching time taken to return from the low power mode to the active mode, compare electric energy saved by performing the low power mode with electric energy consumed by changing from the active mode to the low power mode and set the return time according to the input prediction time and the mode switching time when the electric energy saved by performing the low power mode is equal to or greater than the electric energy consumed by changing from the active mode to the low power mode, and wherein the power mode manager is configured to change from the active mode to the low power mode based on comparison results of the comparing operations and change to the active mode from the low power mode when the set return time elapses. <EOS>