Most people are not aware of the difference between efficacy and efficiency. The truth is that this two are not similar and here is the difference between the two.
Efficacy is usually expressed in lumens per watt while efficiency is expressed in terms of percentage of the LED efficiency. The efficiency of the light fixture is the ratio of the total lumens exiting to the total number of lumens that were initially produced by the light source. An example is that if a bare 100 watts incandescent bulb produced 1,000 lumens. If it is put into a fixture that gives 700 lumens, then its efficiency would be 70 percent.
Another difference is that efficacy is a term that is used where the input and output units differ. When it comes to lighting, one is interested with the amount of light in lumens produced by a particular amount of electricity in watts. Thus, efficacy is equal to the lumens per watt. Therefore, efficacy and efficiency are not similar.