There are basically two configurations of your solar system with Powerwalls.
Backup Mode is just that. Your Powerwalls will not be used unless there is a grid outage. At that point, they will kick in and supply power to your home. When solar power is available, it is used to refill the backup as your home is powered.
Self Power Mode
Self Power Mode uses solar to power your home first. When solar power is not available or not enough, you draw from your backup system. If your solar power and battery system are not enough, you then draw from the grid.
Self Power Mode allows us to become truly independent of the grid sooner. You use solar during the day and run off your backup at night. During the day, your backup is replenished as extra solar power is generated. Any excess power that is generated once your backup is full, is exported to the grid.
Self Power mode is usually used with multiple Powerwalls. Three Powerwalls will give most homes plenty of reserve power to use all night or during periods of outages.