Efficiency > the state of being efficient , and efficient is the ability to do or make something well. You can rely on him , he is well known for his efficiencies in doing anything we ask of him/ We sack all those workers who are lazy to increase the company's production efficiency. Efficacy > effectiveness . He gave me some medication , but I doubt its efficacy. ( because the efficacy of this medicine is not yet proven. The efficacy of having more supervisors in the factory is there for all to see : production efficiency has increased by 20 %.

Efficiency is used to describe how much effort or resources are required to produce a result. If one man takes two days to build a wall to a particular standard and a second man takes one day to build a wall to the same standard, then the second man is twice as efficient as the first man. Efficacy is used to describe how much influence something has to cause a change in something. A single bucket of water would have no efficacy on a forest fire but great efficacy on a burning coal. In many cases effect is more common in normal speech.

efficiency is how fast you can do it. example He cleaned the house with an efficiency that would make the rabbit in "Tortoise and the Hare" jealous. efficacy definition http://beta.merriam-webster.com/dictionary/efficacy Example He had a lot efficacy to make 100 toys in a day. (he had the power to get that done in a day)