An uninterruptible power supply, in short UPS, system (400; 600), 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 comprising a first converter circuit coupled to the AC input, a second converter circuit coupled to the AC output and a first DC bus coupling the first converter circuit to the second converter circuit; the UPS system (400; 600) characterised by further comprising: a second power conversion module comprising a third converter circuit having an output coupled to the AC output, a fourth converter circuit coupled to the third converter circuit, and a second DC bus coupling the third converter circuit to the fourth converter circuit and configured to receive power from a photovoltaic power source; and a control circuit operatively associated with the first power conversion module and the second power conversion module and configured to control the first converter circuit, the second converter circuit and the third converter circuit to concurrently transfer power to the load from the AC power source via the first converter circuit and the second converter circuit and from the photovoltaic power source to the load via the third converter circuit. 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 comprising a first converter circuit coupled to the AC input, a second converter circuit coupled to the AC output and a first DC bus coupling the first converter circuit to the second converter circuit; the UPS system (400; 600) characterised by further comprising: a second power conversion module comprising a third converter circuit having an output coupled to the AC output, a fourth converter circuit coupled to the third converter circuit, and a second DC bus coupling the third converter circuit to the fourth converter circuit and configured to receive power from a photovoltaic power source; and a control circuit operatively associated with the first power conversion module and the second power conversion module and configured to control the first converter circuit, the second converter circuit and the third converter circuit to concurrently transfer power to the load from the AC power source via the first converter circuit and the second converter circuit and from the photovoltaic power source to the load via the third converter circuit. a second power conversion module comprising a third converter circuit having an output coupled to the AC output, a fourth converter circuit coupled to the third converter circuit, and a second DC bus coupling the third converter circuit to the fourth converter circuit and configured to receive power from a photovoltaic power source; and a control circuit operatively associated with the first power conversion module and the second power conversion module and configured to control the first converter circuit, the second converter circuit and the third converter circuit to concurrently transfer power to the load from the AC power source via the first converter circuit and the second converter circuit and from the photovoltaic power source to the load via the third converter circuit.