According to an aspect of the invention there is provided a method of controlling an energy storage system to provide a frequency regulation service for a power network, to which the energy storage system is connected. The energy storage system is controlled according to one or more regulations of the power network to counteract frequency deviations within a frequency contingency deadband of the power network to provide the frequency regulation service for a service period. The energy storage system is one of a plurality of energy storage systems connected to the power network, each energy storage system being operated to counteract the frequency deviations according to the one or more regulations of the power network.