Question about Digitec Fast Charger (100W)

Yes, it does 20V USB-C output.
The label states: USB-C output 5V 3A or 9V 3A or 12V 3A or 15V 3A or 20V 5A.

I have never tried that. Typically they are tolerant, plus minus 2V or so, because the battery/alternator can supply almost anything between 11 and 14V, but 20V is probably too high. There are DC-DC voltage converters, but cars usually have 12V or 24V.

Why 20V? If this is a solar panel, then it is essential to use a charge controller. PWM charge controllers are available cheaply. MPPT charge controllers are better, e.g. Victron is very good. Many have a 12V output. Note the volt and ampere range, there are only small and large models!
Victron Energy SmartSolar MPPT 75/10