Do you want power transmission across the power lines to be 70% efficient? Then stepping it down to mains voltage levels to be 70% efficient? Then your wall adapter to be 70% efficient? Then your phone's charging circuit to be 70% efficient? You get the point. You end up with only .7^4 = 25% efficiency. When you do things 70%, they accumulate and you may end up with much less. Often there may be more demanding situations where you have to put in more effort and time to achieve something decent. You must always do 100% of what is required.

 

It is funny more than one person has tried to give me a hard time over my signature, what they don't understand is it is good enough.