efficacy & efficiency

1. efficacy (uncountable) : Ability to produce a desired amount of a desired effect.

2. efficiency (countable and uncountable, plural efficiencies) 

The extent to which time is well used for the intended task.  

The efficiency of the planning department is deplorable.

(dated) The quality of producing an effect or effects.  

The extent to which a resource, such as electricity, is used for the intended purpose; the ratio of useful work to energy expended.

The efficiency of this loudspeaker is 40%.

(United States) A one-room apartment.

I have an efficiency available June through July.

fully-furnished efficiencies

1. Efficacy means effectiveness, i.e. the ability of something, e.g. a medicine, to produce the results you want. 

2. Efficiency means ability to do something well without a waste of time or money,

 e.g. I was impressed by her efficiency in this project. 

For a more technical context, I found the following additional definitions of efficiency: 

a)The ratio of the effective or useful output to the total input in any system. 

b) The ratio of the energy delivered by a machine to the energy supplied for its operation. 

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