PukiDuiki

You've got it, for the most part. The system only draws what it needs. The PSU will draw what is necessary to deliver that. The worse the efficiency of the PSU, the more the PSU will draw from the wall (above and beyond what the system needs) and waste in heat output. 

Efficiency is the factor of converting one form of energy in other forms you want to use.
the law of energy conservation says that energy can not be lost, only converted in other forms.
 
The other forms are, in this case, one Voltage into other voltages. That is the form of energy we want.
Another example is a motor that converts some form of Energy into movement energy.
Or take for example a lightbulb, wich converts electrical energy in light-engy (and the old incadescent mostly heat)
 
What is normally said to be the loss is the form of energy that are annoying and you do not want - and often cause you trouble. 
In general its mostly heat.
 
You don't loose it, but it is mostly heat, yes.
You also need a bit of energy for the magnetics in the PSU as well.
 
But the 60-70% Efficiency is something we haven't really seen in somewhat OKish PSU for like 15 years or so. In 2005 or 6 those were mostly gone. 
You only see them in low cost d
Correct, 
With 90% of efficiency, only 10% of the Wattage consumed gets converted to things you don't want.
 
Correct.
If it is in that 60-70% area, wich should be rather unlikely.
 
In theory, that is possible.
But Unlikely. With a decent "Named Brand" PSU of the last couple of years, we're talking usually about like 80% or so minimum under normal loads (20-100%).

For example with 80% Efficiency and 280W Load on the PC Side, we're around 350W Primary.
With 90% Efficiency that shrinks to 311,1111111111111111111111111111111111111111111111111W

i had gtx 660 that died with psu. Whole system drained 380w from wall
And now im looking at gtx 1050ti or 1060, but i don't know what wattage psu sould i get..