HOw efficient are light bulbs?

Only 2% of the energy used in an incandescent bulb comes out as visible light and the other 98% is converted to heat. 1
 
Digging deeper:
Incandescent light bulbs work by sending a lot of electricity through a thin wire until friction causes it to heat up so much that it glows and gives off light.  The definition of incandescence is “the emission of light  as a consequence of raising its temperature”. 2  When electricity passes thru a tungsten wire filament in a light bulb it is heats up to about 1500°F.  At this temperature most of the electric energy (about 98%) is released as infrared radiation which we can feel as heat, the rest of energy, only 2%, is released as visible light.
Here is a close-up of the Tungsten wire filament: