Efficient Lighting Example

It's definitely time for a worked example!

The author replaced a 60 W filament bulb with an energy efficient alternative rated at 11 W (shown right). Both bulbs give out 9 W of light. Find their efficiency.

Firstly, remember that power is just energy per second, so we can use the power numbers instead of energy.

The efficiency of the filament lamp will be: useful/total = 9/60 = 0.15 (15%).

Whereas the CFL efficiency will be: useful/total = 9/11 = 0.82 (82%).

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