P=I X E
Power =current times voltage

If current is reduced due to LEDs instead of incandescent bulbs, voltage still remains constant, therefore, power consumption is reduced.

You can use an Ammeter to measure this.

Without getting into the weeds discussing losses, the windings determine the voltage on the secondary. The physical size of the windings determine how much current the secondary ( and therefore, power) Can handle.

As mentioned before, weeds. Don’t forget about AC impedances, flux affects, etc.