A method of modifying a power system comprising an uninterruptible power supply, UPS, comprising: an AC input coupled to an AC source by a static switch having a first terminal coupled to the AC source ; an AC output coupled to a load and coupled to the AC source by the static switch ; a first converter circuit having a first port configured to be coupled to the AC input; a second converter circuit having a first port configured to be coupled to the AC output and a DC bus coupling second ports of the first and second converter circuits ; and an auxiliary power source coupled to the DC bus ; wherein the method comprises: disconnecting the AC input and the AC output of the UPS from the static switch ; connecting a first terminal of a first switch external to the UPS to the AC input of the UPS , a first terminal of a second switch external to the UPS to a second terminal of the static switch , a second terminal of the second switch to the AC input of the UPS , a first terminal of a third switch external to the UPS to the AC output of the UPS , and a second terminal of the third switch to a second terminal of the first switch and the load ; and controlling the UPS , the first switch , the second switch , and the third switch to provide a first mode of operation in which the first and second converter circuits respectively operate as a rectifier and inverter coupled between the AC input and the AC output, and a second mode of operation in which the first and second converter circuits operate as parallel inverters coupled between the DC bus and the AC output. an AC input coupled to an AC source by a static switch having a first terminal coupled to the AC source ; an AC output coupled to a load and coupled to the AC source by the static switch ; a first converter circuit having a first port configured to be coupled to the AC input; a second converter circuit having a first port configured to be coupled to the AC output and a DC bus coupling second ports of the first and second converter circuits ; and an auxiliary power source coupled to the DC bus ; disconnecting the AC input and the AC output of the UPS from the static switch ; connecting a first terminal of a first switch external to the UPS to the AC input of the UPS , a first terminal of a second switch external to the UPS to a second terminal of the static switch , a second terminal of the second switch to the AC input of the UPS , a first terminal of a third switch external to the UPS to the AC output of the UPS , and a second terminal of the third switch to a second terminal of the first switch and the load ; and a first terminal of a first switch external to the UPS to the AC input of the UPS , a first terminal of a second switch external to the UPS to a second terminal of the static switch , a second terminal of the second switch to the AC input of the UPS , a first terminal of a third switch external to the UPS to the AC output of the UPS , and a second terminal of the third switch to a second terminal of the first switch and the load ; and controlling the UPS , the first switch , the second switch , and the third switch to provide a first mode of operation in which the first and second converter circuits respectively operate as a rectifier and inverter coupled between the AC input and the AC output, and a second mode of operation in which the first and second converter circuits operate as parallel inverters coupled between the DC bus and the AC output. a first mode of operation in which the first and second converter circuits respectively operate as a rectifier and inverter coupled between the AC input and the AC output, and a second mode of operation in which the first and second converter circuits operate as parallel inverters coupled between the DC bus and the AC output. <EOS>