Power Conversion Efficiency in general is a term that tells us how much energy is converted from one form to another. In the case of lasers, power conversion efficiency tells us the amount of electrical input power that is converted into output optical power.

Power Conversion Efficiency is expressed in percentages as,

If one form of energy is completely converted to the other form, then it is said to have 100% power conversion efficiency. However, in the real world, achieving 100% efficiency is not possible because some of the input power is lost in the cavity either due to minute imperfections or the presence of any impurities. So efficiency is always less than 100%.