Induced eddy currents and the power grid frequency
There is no 100% efficiency. There is a input power, output power and loss. A loss is further split into a copper loss and iron loss. The copper loss is caused by the resistance of the wires/windings, while the iron loss is caused by magnetizing the iron core. Eddy currents are one another type of loss in the iron core. The core made of steel laminations reduces eddy currents.
The answer of your question is: There is a decrease of the output power or there is an increase of input power, to maintain the energy balance: \$P_{in}=P_{out}+P_{loss}\$