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Efficiency

A filament lamp is a good example of an everyday device that does not do its job very well. Of the 60 J of energy input to the lamp each second, only 3 J is output as light. It has a very low efficiency!

Fluorescent lamps are much more efficient at doing the same job, but until recently they were only available in the form of long tubes. Modern energy efficient lamps are fluorescent lamps that are designed as direct replacements for filament lamps. Although their running costs are much lower and they last much longer, the initial cost is higher. To achieve the same light output, an energy efficient lamp needs only 20% of the energy input from electricity that a filament lamp needs. It is more efficient at doing the same job.

Efficiency is defined as:

useful energy output ÷ total energy input

It is usually expressed as a fraction or decimal, which can be multiplied by 100 to give a percentage efficiency.