A system, device and method of determining an efficiency of a distribution transformer that receives power from a medium voltage power line and that provides power to a plurality of customer premises via a low voltage subnet is provided. In one embodiment, the method includes determining an input power to the transformer from the medium voltage power line, determining an output power from the transformer, determining a transformer efficiency based on the input power and the output power, comparing the transformer efficiency with a threshold efficiency, and providing a notification when the transformer efficiency is below a threshold efficiency. Determining the input power may include measuring the current input and estimating the voltage input based on a voltage measure of the low voltage subnet and the turns ratio of the transformer.