This project is on Design and Construction of a 2.5kw uninterruptible power supply (UPS) system. Power supplies from the mains supply in Nigeria are neither steady nor clean. Many abnormalities such as spikes, sags (brownouts) surges, and noise can occur. Under the best conditions, power interruptions can be an inconvenience. At their worst, they can cause loss of data in computer systems, damage to vulnerable electronic devices, break in transmission of information and even loss of human life. As a result of these inconveniences, Uninterruptible Power Supply (UPS) systems were constructed to minimize interruptions from Power Supply Units (PSUs). The UPS acts as a buffer and provides clean, reliable power to vulnerable electronic equipment for a certain (short) period of time. At least a time that would be enough to save data and properly shut down the protected machine or effect a power change over. Finally, this project “Uninterruptible power supply (UPS) system” after construction and testing were able to give us an output of 2.5KW which after charged was used to power a computer system and we got a dwell-time of 7-10 minutes. However, after a blackout has occurred, this UPS can provide power to the connected equipment for a short period of time.