J. Clarke wrote:
Lew Hodgett wrote:
"J. Clarke" wrote:

If he's using 130v bulbs on 120v then he's using about 15% less KWH
than he
would be using 120v of the same nominal wattage.
-------------------------------------
NOT!

100 watts is 100 watts regardless the voltage rating of the lamp.

The current flowing thru the lamp is reduced which reduces lumen
output when the voltage rating of the lamp is increased.

(E = I*R for a resistance load.)

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.

The assumption that the resistance will remain constant is a bad one. As has
already been pointed out elsewhere in this thread, the resistance of a light
bulb varies with the temperature of the filament. A colder filament will have
a lower resistance. A lower resistance will result in a higher current and
a higher power. The actual power at 120 volts will be somewhere between the
85 watts that you calculated and the 100 watts that it would dissipate at
130 volts.