Efficacy: n. having ability to produce a desired and/or intended goal(s)
Efficiency: n. a minimum of matter and/or energy per unit time used to achieve a goal(s)
Efficacy is the ability to produce a desired and/or intended goal(s) or result(s) and efficiency is a measure of how well you can do so. Hopefully you have reached your goal(s) in the least amount of time with the least amount of materials and lifeform(s) effort which determines how efficient you have been.
Even if you are not very efficient you can be efficacious and produce a desired goal(s) or result(s) being a little wasteful with time and/or materials and/or lifeform(s) effort.
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