Here is the situation. If you are the original owner and designer of the system and are still receiving a large electrical bill because the system was undersized, you may be able to go through channels to have your system enlarged to an appropriate size and keep NEM 1 or NEM 2 if you are not on NEM 3.
Otherwise, enlargement is limited to 10% more watts by faceplate of the panel. So an 8 kW system could have two 400 watt panels added.
If you need a lot more power, say like 20 kW a day or more, I would just install a new second system using a hybrid inverter, set up a sub panel and route some of the circuits to that sub panel and put in place a good battery back up. You could install a transfer switch to the panel if the inverter fails or use a generator. You may possibly be be able to grid tie, but not sell back too. Getting new equipment gets you the tax break to and you can tailor this second system to your needs. And it needs to be permitted, but nothing to involve the utility company about.