A system, comprising: an electrical power generator for generating an electrical power, the electrical power generator comprising a first element and a second element configured to have a state of charging and configured to be moveable with respect to each other for generating the electrical power when the elements comprise the state of charging, the first element and second element being further configured such that they can be brought into and out of contact with each other; a drive mechanism for bringing the first element and the second element into and out of contact, wherein the system is configured to be operated in: - a charging mode in which the first element and the second element are in contact for altering the state of charging, and - a power generation mode in which the first element and the second element are out of contact; characterized in that the system further comprises: a device for determining the state of charging; and a controller for: - controlling the timing and/or duration of the charging mode in dependence on the state of charging; or - outputting information for use in controlling the timing and/or duration of the charging mode in dependence on the state of charging. an electrical power generator for generating an electrical power, the electrical power generator comprising a first element and a second element configured to have a state of charging and configured to be moveable with respect to each other for generating the electrical power when the elements comprise the state of charging, the first element and second element being further configured such that they can be brought into and out of contact with each other; a drive mechanism for bringing the first element and the second element into and out of contact, wherein the system is configured to be operated in: - a charging mode in which the first element and the second element are in contact for altering the state of charging, and - a power generation mode in which the first element and the second element are out of contact; characterized in that the system further comprises: a device for determining the state of charging; and a controller for: - controlling the timing and/or duration of the charging mode in dependence on the state of charging; or - outputting information for use in controlling the timing and/or duration of the charging mode in dependence on the state of charging. - a charging mode in which the first element and the second element are in contact for altering the state of charging, and - a power generation mode in which the first element and the second element are out of contact; characterized in that the system further comprises: a device for determining the state of charging; and a controller for: - controlling the timing and/or duration of the charging mode in dependence on the state of charging; or - outputting information for use in controlling the timing and/or duration of the charging mode in dependence on the state of charging. a device for determining the state of charging; and a controller for: - controlling the timing and/or duration of the charging mode in dependence on the state of charging; or - outputting information for use in controlling the timing and/or duration of the charging mode in dependence on the state of charging. - controlling the timing and/or duration of the charging mode in dependence on the state of charging; or - outputting information for use in controlling the timing and/or duration of the charging mode in dependence on the state of charging.