Long-Life Bulbs

In North America, a typical "long life" incandescent lamp is actually a 125V or 130V lamp; when operated on 120V, it can live a very long life. The lifetime of an incandescent lamp is approximately inversely proportional to the sixteenth power of the voltage. Approximately 95% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light. An incandescent light bulb, with this ~5% efficiency, is about one quarter as efficient as a fluorescent lamp (about 20% efficiency), and produces about six times as much heat for the same amount of light from both sources.

Since it is impossible (and in fact against electrical codes) to get 130 volts from any normal mains, these typically run at a more realistic 115 volts in North America. By dropping the voltage by 12%, the current also drops (non-linearly) by approximately 7%, reducing the actual wattage by about 18%. This in turn reduces the light output by 34%, but also increases the bulb's service life by a factor of 7. This is the concept of the "long-life bulb".

A 5% reduction in operating voltage will double the life of the bulb, at the expense of reducing its light output by 20%.

Offered by Mike.