A. What happens to the current (I) when they add another solar panel (V)?

The importance is particulalrly acute if your career consists of serving
those clients, i.e., if that's how you make your living. If that's the case,
then client records must be completed correctly, because if you don't,
then it's only a matter of time before one of your mistakes is discovered
and you find yourself in deep doo doo. 

Depending on exactly what kind of business you are in, correcting your
mistake might cost you big bucks.  But whether or not it costs you anything,
the news that you don't always complete your clients' records correctly will
spread like wildfire throughout your market of potential clients, and you will
soon be out of business because you have no more clients. (They have all
taken their business to somebody else.) 

You'll then be out on the street looking for another way to make a living, and
wherever you go, your reputation for sloppy work and inaccuracy will ride into
the employment interview on your back.  You may have to start at the bottom
again, settling for a job where you have no clients ... and I can't even think of
a job like that just now.