Going to leave my car in an outdoor parking lot for a week, and it made me curious. Can I attach a small solar panel (<100W) to the inside of any modern EV such that there is a higher state of charge when I come back?
A Tesla supercharger maxes out at about 250kW.
Your 100W solar panel is a 250th of the power, so assuming you can work the adapters required (unsure if this is possible - probably need to create an inverter circuit) the Tesla-quoted 15 minutes for 200 miles of charge would be just under 3 days for 200 miles of charge.
Realistically, it isn't going to happen. You need more current! You need more power!
In theory this could be done. But in practice there is no way to plug in a PV panel to any currently made EV. I suppose if you were creative enough you could design and build an interface but it's not going to be trivial.
So the answer is effectively NO, you cannot do this.
Even if you could do it, the contribution of your 100W panel is going to be insignificant. The smallest Tesla battery is a 50 kWH unit and I'm not sure they actually make any of those. So assuming you are someplace very sunny and you get 12 hours of sunlight that maxes out your panel at 100W. That will give you 1200 WH of energy every day.
That amounts to a paltry 2.4% of the total charge in the battery. Now in reality there is nowhere on earth that gets 12 hours of "noon" sunlight so you are more likely going to be in the range of less than 1% charge per day.
Honestly the answers above are quite good but I mean to answer this in a more general matter and say that there are hybrid cars specifically designed with this idea in mind. You can see one example here https://www.eproinn.com/en/life-save-en/ but you have to realize that the batteries of a hybrid vehicle are significantly smaller than an ev and there a lot of circuits and design specifications under the hood to enable a charge boost with lets say 8 hours of sunlight.