"his efficacy" is correct and usable in written English.
You can use it to describe the effectiveness or efficiency of something someone has done. For example, "The speaker praised his efficacy in solving the problem."
It was his efficacy, not his money, that Cameron valued.
If she is unsure of his efficacy, she'd do well to check with her teachers.
The only concern should not be with Berbatov's effort, but his efficacy.
On Earth the possibility of his efficacy is likewise limited by the return of the Lord.
That interaction is part of the cure; it is why a physician's bedside manner can have such an enormous impact on his efficacy.
On the back of Magic Mike and 21 Jump Street, he was looking like a bona fide star in 2012, but the twin failures of XXL and Jupiter Ascending this year cast serious doubt over his efficacy as a leading man.
Still, this book confronts us with Freud's virtuosity as a cannibalizing theoretician, his savage efficacy as a tactician — and his irreducible brilliance as a bestower of names: Oedipus complex, dream-work.
However, Milk's assassination has become entwined with his political efficacy, partly because he was killed at the zenith of his popularity.
In 1986 Bandura developed his self efficacy theory, which states that belief is the basis for success.
Ballmer's tenure as CEO was controversial, but it was rare to find someone who doubted his conviction, whatever the efficacy of his decisions.
The Oxford-educated economics professor has won plaudits for his low-key efficacy, in sharp contrast to his forerunner, Yanis Varoufakis.
