Underdriven lamp. Even if 120V is more efficient for incandescent lamps then 225V or higher, and the lamp can use better filament shapes which last longesr and gets slightly brighter, the difference in life shound be at least +500 hrs and so 1500 hrs. All regular incandescent that can last longer then 1500 hrs are underdriving their filaments to do this or use thick filaments which is may also result in an underdriven lamps.
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