Right now the trend must be +100 or better to turn a fabber on.
You can be sitting at full energy, steady +99, with a t2 fabber that will refuse to turn on, even though you're throwing energy away.
I really, really want to suggest more complex logic for getting a better, more useful result, but Jip is entirely correct in wanting to keep it simple.
As far as the trend for turning a fabber off, let's assume you just Overcharged something and you're down to 60% storage, but you're at +5000/second. Turning off fabbers makes no sense in that situation.