Method of operating a local AC power grid comprising a genset which includes a combustion engine and a generator, and an uninterruptable power supply which includes an energy storage, - wherein, by means of the genset, it is responded to deviations of a frequency (f) of an AC voltage of the AC power grid from a desired frequency (f Soll) in one direction by an increase of power and in the other direction by a reduction of power, and - wherein, in case of an increase of the power demand of the AC power grid, missing power is temporarily fed out of the energy storage into the AC power grid, characterised in - that the frequency (f) of the AC voltage of the AC power grid present at the genset is defined by means of the UPS, - wherein the frequency (f) of the AC voltage defined by means of the UPS is altered in the one direction away from the desired frequency (f Soll), if a present power demand increases beyond a present power supply in the AC power grid, and altered in the other direction, if the present power demand falls below the present power supply in the AC power grid, - wherein with increasing power demand of the AC power grid the alteration of the frequency (f) of the AC voltage defined by means of the UPS is limited to a maximum value in the one direction in that the missing power is temporarily fed out of the energy storage into the AC power grid, - wherein the frequency (f) of the AC voltage defined by means of the UPS remains shifted in the one direction with regard to the desired frequency (f Soll) at least until no more power flows out of the energy storage into the AC power grid. - wherein, by means of the genset, it is responded to deviations of a frequency (f) of an AC voltage of the AC power grid from a desired frequency (f Soll) in one direction by an increase of power and in the other direction by a reduction of power, and - wherein, in case of an increase of the power demand of the AC power grid, missing power is temporarily fed out of the energy storage into the AC power grid, - that the frequency (f) of the AC voltage of the AC power grid present at the genset is defined by means of the UPS, - wherein the frequency (f) of the AC voltage defined by means of the UPS is altered in the one direction away from the desired frequency (f Soll), if a present power demand increases beyond a present power supply in the AC power grid, and altered in the other direction, if the present power demand falls below the present power supply in the AC power grid, - wherein with increasing power demand of the AC power grid the alteration of the frequency (f) of the AC voltage defined by means of the UPS is limited to a maximum value in the one direction in that the missing power is temporarily fed out of the energy storage into the AC power grid, - wherein the frequency (f) of the AC voltage defined by means of the UPS remains shifted in the one direction with regard to the desired frequency (f Soll) at least until no more power flows out of the energy storage into the AC power grid. - wherein with increasing power demand of the AC power grid the alteration of the frequency (f) of the AC voltage defined by means of the UPS is limited to a maximum value in the one direction in that the missing power is temporarily fed out of the energy storage into the AC power grid, - wherein the frequency (f) of the AC voltage defined by means of the UPS remains shifted in the one direction with regard to the desired frequency (f Soll) at least until no more power flows out of the energy storage into the AC power grid. - wherein the frequency (f) of the AC voltage defined by means of the UPS remains shifted in the one direction with regard to the desired frequency (f Soll) at least until no more power flows out of the energy storage into the AC power grid.