Method for saving energy in operating a first and second mobile communication networks (11, 12) by invoking an energy saving operational mode of the first and second mobile communication networks (11, 12), wherein the first mobile communication network comprises a first radio access network for covering a geographic area and a first core network, wherein the geographic area is also covered by the second radio access network of a second mobile communication network, the second mobile communication network comprising a second core network, characterized in that -- the first radio access network is able to be operated in a first operational mode for peak time usage of the first mobile communication network, and in a second operational mode for off-peak time usage of the first mobile communication network, wherein at least partly during off-peak time usage of the first and second mobile communication networks (11, 12), the energy saving operational mode of the combination of the first and second mobile communication networks (11, 12) is invoked such that the first radio access network is operated in the second operational mode while the second radio access network is operated such that coverage and capacity requirements in the geographic area are ensured by the combination of the first and second mobile communication networks (11, 12), or wherein -- the second radio access network is able to be operated in a first operational mode for peak time usage of the second mobile communication network, and in a second operational mode for off-peak time usage of the second mobile communication network, wherein at least partly during off-peak time usage of the combination of the first and second mobile communication networks (11, 12), the energy saving operational mode of the first and second mobile communication networks (11, 12) is invoked such that the second radio access network is operated in the second operational mode while the first radio access network is operated such that coverage and capacity requirements in the geographic area are ensured by the combination of the first and second mobile communication networks (11, 12) wherein with respect to the geographic area, at least the first radio access network and the second radio access network as well as at least parts of the first and second core networks (21, 22) are shared network components of a network sharing operation of the first and second mobile communication network (11, 12), wherein the network sharing operation is applied to the energy saving operational mode of the first and second mobile communication networks (11, 12) wherein with respect to the geographic area and during the energy saving operational mode of the first and second mobile communication networks (11, 12), the first radio access network and the second radio access network are commonly operated to form a logically different mobile communication network in the geographic area, wherein for a subscriber of the first or the second mobile communication network (11, 12) using the logically different mobile communication network within the geographic area, a roaming procedure is invoked. -- the first radio access network is able to be operated in a first operational mode for peak time usage of the first mobile communication network, and in a second operational mode for off-peak time usage of the first mobile communication network, wherein at least partly during off-peak time usage of the first and second mobile communication networks (11, 12), the energy saving operational mode of the combination of the first and second mobile communication networks (11, 12) is invoked such that the first radio access network is operated in the second operational mode while the second radio access network is operated such that coverage and capacity requirements in the geographic area are ensured by the combination of the first and second mobile communication networks (11, 12), or wherein -- the second radio access network is able to be operated in a first operational mode for peak time usage of the second mobile communication network, and in a second operational mode for off-peak time usage of the second mobile communication network, wherein at least partly during off-peak time usage of the combination of the first and second mobile communication networks (11, 12), the energy saving operational mode of the first and second mobile communication networks (11, 12) is invoked such that the second radio access network is operated in the second operational mode while the first radio access network is operated such that coverage and capacity requirements in the geographic area are ensured by the combination of the first and second mobile communication networks (11, 12)