Transformer efficiency measures what aspect of a transformer?

Transformer efficiency is fundamentally concerned with how effectively a transformer converts input electrical power to output electrical power, which is precisely captured by the output power to input power ratio. This ratio helps in determining how much of the input power is successfully transformed into usable output power, and it accounts for any losses that may occur during the transformation process, such as heat loss due to resistance in the windings or losses in the core.

In practical terms, a highly efficient transformer will have a high ratio of output power to input power, indicating that it is effectively minimizing energy losses. It's measured in percentage, where a higher percentage indicates better efficiency. This concept is crucial for understanding the operational performance of transformers, particularly in applications where energy conservation and low operational costs are important.

The other options, while related to various qualities and characteristics of transformers, do not directly define transformer efficiency. They focus on aspects like current relationships, physical characteristics, or cost considerations, rather than the core efficiency metric itself. Thus, the focus on the output to input power ratio is what establishes the measure of a transformer's efficiency accurately.