An uninterruptible power supply, UPS, system (100; 200), comprising: an AC input configured to be coupled to an AC power source; an AC output configured to be coupled to a load; a first power conversion module (110; 210) comprising a first converter circuit (112; 212) configured to receive power from a variably available power source, a second converter circuit (114; 214) coupled to the AC output, and a first DC link (115; 215) coupling the first and second converter circuits (112, 114; 212, 214); a second power conversion module (120; 220) comprising an AC/DC converter circuit (122; 212) coupled to the AC input, a DC/AC converter circuit (124; 214) coupled to the AC output and a second DC link coupling the AC/DC converter circuit (122; 212) to the DC/AC converter circuit (124; 214), isolated from the first DC link (115; 215); a bypass circuit (150; 230) configured to selectively couple and decouple the AC input and the AC output; and a control circuit (160; 240) operatively associated with the first power conversion module (110; 210), the second power conversion module (120; 220) and the bypass circuit (150; 230) and configured to support the following modes of operation: concurrent transfer of power to the load at the AC output from the AC power source and the variably available power source via the bypass circuit and the first power conversion module (110; 210), respectively; concurrent transfer of power to the load at the AC output from the variably available power source and a first auxiliary power source coupled to the DC link of the first power conversion module (110; 210); concurrent transfer of power to the load at the AC output from the AC power source and the variably available power source via the second power conversion module (120; 220) and the first power conversion module (110; 210), respectively. an AC input configured to be coupled to an AC power source; an AC output configured to be coupled to a load; a first power conversion module (110; 210) comprising a first converter circuit (112; 212) configured to receive power from a variably available power source, a second converter circuit (114; 214) coupled to the AC output, and a first DC link (115; 215) coupling the first and second converter circuits (112, 114; 212, 214); a second power conversion module (120; 220) comprising an AC/DC converter circuit (122; 212) coupled to the AC input, a DC/AC converter circuit (124; 214) coupled to the AC output and a second DC link coupling the AC/DC converter circuit (122; 212) to the DC/AC converter circuit (124; 214), isolated from the first DC link (115; 215); a bypass circuit (150; 230) configured to selectively couple and decouple the AC input and the AC output; and a control circuit (160; 240) operatively associated with the first power conversion module (110; 210), the second power conversion module (120; 220) and the bypass circuit (150; 230) and configured to support the following modes of operation: concurrent transfer of power to the load at the AC output from the AC power source and the variably available power source via the bypass circuit and the first power conversion module (110; 210), respectively; concurrent transfer of power to the load at the AC output from the variably available power source and a first auxiliary power source coupled to the DC link of the first power conversion module (110; 210); concurrent transfer of power to the load at the AC output from the AC power source and the variably available power source via the second power conversion module (120; 220) and the first power conversion module (110; 210), respectively. concurrent transfer of power to the load at the AC output from the AC power source and the variably available power source via the bypass circuit and the first power conversion module (110; 210), respectively; concurrent transfer of power to the load at the AC output from the variably available power source and a first auxiliary power source coupled to the DC link of the first power conversion module (110; 210); concurrent transfer of power to the load at the AC output from the AC power source and the variably available power source via the second power conversion module (120; 220) and the first power conversion module (110; 210), respectively.