James,
Please bear in mind that a 50W lamp will never dissipate 100W of energy, not matter what it is connected to.
The current drawn by a given size of lamp, is based on the resistance of the lamp filament, the higher the lamp resistance, the lower the current drain on the supply.
Also, with regard to transformers, you can never draw the full nameplate wattage (VxI) from a transformer, because they have a certain "loss factor", caused by Iron losses, Copper losses and Hysteresis losses.
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