An electricity storage system includes a main body and a portable unit. The main body includes a casing, a first storage battery module that stores power, and a first terminal unit to supply the power. The portable unit includes a second terminal unit and a second storage battery module, and is attachable to and detachable from the first terminal unit. The second terminal unit is electrically connectable to the first terminal unit and receives the power supplied from the main body. The second storage battery module has a storage capacity less than the first storage battery module. The portable unit further includes a charging controller, upon determination of power supply from the system power supply has been stopped, the charging controller controls a charging rate for the second storage battery module to be charged at least one of a normal mode and a high-speed mode.