WHAT DOES FILL POWER ACTUALLY MEAN?
Roughly speaking fill power is a measure of how much volume
is taken up by a given quantity of down. The higher the fill power the
loftier the material, loft meaning the amount of air held within the
material, which translates into insulation (still air is a great insulator
remember).
So if we were to compare the fill power of say one pound of potato
peelings with one pound of cotton wool balls, the cotton wool would
win as it would take up a much larger volume, (something worth remembering
if you're forced to make your own sleeping bag out of household items).