It only comes out if you produce less than you use in a month. I've had it happen on cloudy months, once or twice.
Individuals producing power have to abide by anti-islanding laws (you can't be feeding power back into the grid when there's no power out there) in order to protect the workers who go out to fix the outages (feeding power back into the grid while the grid's down = electrocuted workers).
So, with that in mind, if the grid's down, my system goes offline and stops producing electricity, so I'm neither producing nor consuming electricity besides what's in my UPS batteries.
When the grid comes back up, if it's cloudy and I'm underproducing, I might draw a little bit off of the grid, but the system's purely based on reading the meter. If KWH reading on the meter at the end of the month < KWH reading on the meter at the beginning of the month, then I've overproduced and it goes 1:1 into my net metering account. If I under produce, it comes 1:1 out of my net metering account.
I'm usually between 400KWH and 600KWH consumption. It goes up in the summer with my swamp cooler. In August, it was nearly 1000KWH.