The invention relates to an energy storage system and a method and a device for controlling power balance of the energy storage system. The energy storage system comprises p battery modules and n energy storage current transformer modules. M battery clusters are formed by the p battery modules, the m is larger than or equal to 2, the n is larger than or equal to the m, and at least one battery module is arranged in each battery cluster. The method includes acquiring power ratios of the various battery clusters; computing given power of the corresponding energy storage current transformer modules according to total given power and the power ratios of the various battery clusters so as to control the power balance. The energy storage system, the method and the device have the advantages that the various battery clusters can work without interfering with one another, only current transformer equipment of a certain battery cluster needs to be shut down on the assumption that batteries of the certain battery cluster are damaged, the other battery clusters still can normally work, great change of working of systems can be prevented, and the fault tolerance and the operation stability of the systems can be enhanced; the stability and the fault tolerance of the systems can be improved by the energy storage system, and influence of damage of the single batteries on the integral systems can be lowered to a great extent.