There is provided a power storage system including: a DC bus; a plurality of batteries which are connected to the DC bus, and which are connected to each other in parallel; a plurality of diagnosis units each of which is provided for each of the plurality of batteries and measures a degradation state of a battery that is included in the plurality of batteries, by causing the battery to be discharged at a constant current or constant power; and a power conditioner which is connected to the DC bus, and which performs conversion between DC power and AC power, in which at least a part of power discharged from a battery under test among the plurality of batteries, the degradation state of which is being measured by the diagnosis unit, is charged into another at least one battery among the plurality of batteries, or is supplied to the power conditioner.