Verified answer

Light bulbs don't produce energy, they use it.

IF you mean how much light energy does it emit, then 20% of 15 watts is 3 watts.

3 watts is 3 joules per second.

8 hours is 28800 seconds

3 joules per second x 28800 seconds = 86400 joules.

The bulb converts 80% of the 15 watts electrical energy to heat energy = 12 watts of heat energy; 3 watts of light energy. The light bounces around the room for a few nanoseconds until it is absorbed and becomes heat energy, which is a waste, unless you want the room warmer.

3 times 8 = 24 watt-hours of light are produced, but nearly 15 times 8 = 120 watt hours of heat energy are produced in the room in 8 hours. Neil

I don't know if you are the same person who asked a similar question yesterday or not, but I assume you are. First of all, light bulbs do not produce any energy, they consume energy. Actually, power applied to a bulb is converted to heat and light. In your case 20% becomes light and the rest is heat.

15 watts X 8 hours = 120 watt-hours of energy, 20% of which produced visible light.

Hope this helps,

Newton1Law