A method of suppressing frequency deviations in a power grid relative to a desired frequency behaviour, in particular at hour crossings, wherein an energy storage is connected to the grid, includes: monitoring frequency in the grid relative to the desired frequency behaviour, to determine detected frequency deviations; comparing detected frequency deviations with a threshold; if detected frequency deviations exceed the threshold, deploying the storage to charge therein energy from the grid or to discharge energy therefrom into the grid; and setting a variable threshold. A corresponding system includes: a connection to a power grid; an energy storage selectively connectable to the connection; a frequency monitor connected to the connection; and a control to configured to selectively deploy the energy storage in accordance with the method.