An electric power module system for use in an electrically operated vehicle, comprising: - a base module comprising a cooling passage for fluid coolant and coolant connections (6, 7) for supplying fluid coolant to the cooling passage, the base module further comprising at least one external electrical connection for power input to and/or power output from the electric power module system, - an add-on module configured to be mounted to and electrically connected to the base module, so that the add-on module, when mounted and electrically connected to the base module, can be cooled and powered via the base module, wherein the base module and the add-on module are power conversion modules of the same type, and wherein the add-on module is configured to, when mounted to and electrically connected to the base module, increase a power conversion capacity of the electric power module system, so that a maximum amount of converted power available via the at least one external electrical connection of the base module is increased, wherein, when the add-on module is mounted to the base module, the cooling passage is delimited by at least a first delimiting surface provided on the base module and a second delimiting surface provided on the add-on module, so that fluid coolant supplied via the coolant connections (6, 7) of the base module comes into contact with both of said delimiting surfaces (10, 11) characterized in that the electric power module system further comprises - power connection means for electrically connecting the modules (2, 3) to each other via mutually facing and connected sides (16, 17) of the modules (2, 3), and - signal connection means for communicating signals between the modules via mutually facing and connected sides (16, 17) of the modules (2, 3), wherein the base module comprises at least one external signal connection. - a base module comprising a cooling passage for fluid coolant and coolant connections (6, 7) for supplying fluid coolant to the cooling passage, the base module further comprising at least one external electrical connection for power input to and/or power output from the electric power module system, - an add-on module configured to be mounted to and electrically connected to the base module, so that the add-on module, when mounted and electrically connected to the base module, can be cooled and powered via the base module, wherein the base module and the add-on module are power conversion modules of the same type, and wherein the add-on module is configured to, when mounted to and electrically connected to the base module, increase a power conversion capacity of the electric power module system, so that a maximum amount of converted power available via the at least one external electrical connection of the base module is increased, wherein, when the add-on module is mounted to the base module, the cooling passage is delimited by at least a first delimiting surface provided on the base module and a second delimiting surface provided on the add-on module, so that fluid coolant supplied via the coolant connections (6, 7) of the base module comes into contact with both of said delimiting surfaces (10, 11) characterized in that the electric power module system further comprises - power connection means for electrically connecting the modules (2, 3) to each other via mutually facing and connected sides (16, 17) of the modules (2, 3), and - signal connection means for communicating signals between the modules via mutually facing and connected sides (16, 17) of the modules (2, 3), wherein the base module comprises at least one external signal connection. - power connection means for electrically connecting the modules (2, 3) to each other via mutually facing and connected sides (16, 17) of the modules (2, 3), and - signal connection means for communicating signals between the modules via mutually facing and connected sides (16, 17) of the modules (2, 3), wherein the base module comprises at least one external signal connection.