Incandescent light bulbs transform electrical energy into light and heat energy. With a normal incandescent bulb, what percentage of the energy winds up as light?
Most incandescent bulbs convert about 95% of the electrical energy into heat, giving you about 5% of the energy as light. Fluorescent bulbs are much more efficient, and LEDs are even more energy efficient. That is why incandescent bulbs are being replaced by fluorescents and LEDs.