All these calculations are above my head and I feel no motivation to become an expert here...

... BUT:

My main use case for a solar panel would not be to use it instead of regular charging. Instead, I would want it to produce just enough juice that I can use the phone a little longer while outdoors. In other words: I would be perfectly happy if such a panel could make the phone "discharge at a slower rate" (if this is technically possible at all)

Is this a realistic expectation?