The windings in a transformer consist of typically a few 100 to a few 1000 meters of conductor. The current flowing through those conductors sees their resistance and produces ohmic losses. The windings also produce a magnetic stray field inside the transformer tank. All conducting parts exposed to this field will produce some eddy current losses. Even with out a load current the transformer core, which is needed to maintain the voltage, produces some magnetic hysteresis and eddy losses.
More information on transformer losses and energy efficiency in transformers here.
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