Possession & Apostrophes

You will know that possession can also be indicated by use of 's.
e.g. Jane's hat was bright pink.

One common error is to use the 's with words that are not nouns.
For example:

Who's hat is it?
It's power output is 60 watts.

Unfortunately, the two examples above actually mean:

Who is hat is it?
It is power output is 60 watts.

Neither sentence making much sense!