When referring to transformer efficiency, the common measurement is often expressed in:

Transformer efficiency is typically expressed as a percentage. This percentage represents the ratio of the output power to the input power, indicating how effectively the transformer converts electrical energy from its primary side to its secondary side. Essentially, it provides a measure of how much of the input power is successfully transformed into usable output power, with losses (e.g., due to resistance and heat) being accounted for.

Expressing efficiency in percent allows for a straightforward comparison of different transformers and their performance. For example, a transformer with an efficiency of 95% means that 95% of the input electrical energy is converted to output energy, while 5% is lost.

The other options—Watts, Volts, and Amperes—are measurements of power, voltage, and current, respectively, but they do not provide a direct measure of how well a transformer operates in terms of energy conversion efficiency.