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%).