Question

If a 75 W lightbulb is 15% efficient, how many joules of light energy does the bulb produce every minute?

Answer 1

1 W = 1 J / sec Definition of watt is 1 joule / sec

So if a bulb uses 75 J / sec it must use

75 J/s * 60 sec / min = 4500 J/min energy used by bulb

If bulb is 15% efficient then the light delivered is

P = 4500 J / min * .15 = 675 J / min