I’ve got troubles perceiving the difference in meaning between these terms. They have different Latin roots which means they have to be different (efficacy—efficacia, efficiency—efficientia). These nouns also have different derived adjectives (efficacy—efficacious, efficiency—efficient).
I will try to swap these terms and you say whether the sentences sound correct?
There is little information on the efficiency of this treatment.
Greater energy efficacy.
Efficacy refers to the effectiveness of a body or a system. Whether is has the capacity or power to produce the desired result.
Efficiency on the contrary is the ratio of output to the input of any system. In other terms how well has something been done.
Source : Link , Question Author : Graduate , Answer Author : Pradit