Uninterruptible power supplies (UPS) form a major element in modern standby power systems. A wide range of UPSs with various designs are now available, and it is important to be aware of the relative merits of their electrical performance and how their operational characteristics reflect on the overall design and performance of standby systems. Optimising the standby system design is conditional on a complete ‘system’ approach being adopted. Various design aspects are discussed and a hybrid UPS scheme is proposed