"J. Clarke" wrote:
If you have visions of becoming an electrical engineer, don't quit
your day
job.

For a resistive load P=E^2/R. If the lamp is rated for 100 watts at
130
volts then it will dissipate (120^2/130^2)*100 watts at 120v or
about 85
watts if its resistance remains constant.
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Review lamp data found in any lamp catalog.

The proof is left to the student.

Lew