This paper investigates the application of the Boolean Truth Table modeling method in estimating the reliability parameters such as the system’s failure rate and Mean Time Between Failures (MTBF), for the uninterruptible Power Supply (UPS) systems. The term “uninterruptible” in the UPS implies that the system never fails. However, like any electrical system, failure will occur at some point in time. The reliability of the system is a function of the component selection, the environment of the UPS and the proper operation and maintenance of the UPS. The reliability estimation of the UPS enables the system designers, manufacturers and lastly the end-user to ensure that the UPS will be able to support the critical loads for a specified time during unforeseen circumstances. This paper proposed the Boolean Truth Table method in predicting the failure rates, and mean time between failures (MTBF), of the three types of UPS configurations.