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Mobile solar power installs are not putting out full power all of the time, even in direct sunlight. This can happen when the batteries are charged and there are no loads on the system. When power is needed the controller jumps into action and lets the panels make power again; it's almost instantaneous.
This means there can be a large gap between:
If you run your discretionary loads (charging devices, running heavy loads) when the system would otherwise be twiddling its thumbs you are effectively running the load off the panels. Battery state of charge is not affected at all.