Since they’ve declared Martial Law, partially in the area where they are supposedly starting the wall, and in other areas not anywhere near the fake wall construction staging, not sure if they can access the staged site to keep an eye on the supposed “progress.” I think one reason for the Martial Law is so this can’t be proven because no one can get anywhere near the first phase of the supposed wall construction to check it out with their own eyes. But make no mistake, in the coming weeks Trump will post photos of the early/first phase of the wall. Don’t fall for it. It’s a lie.
But, let’s assume your 20,000 TWh, just to be nice. That’s a TW yearly average rate (divide 20,000 by the number of hours in a year). Your 300 GW is peak only. If we assume (generously) that power is proportional to the cosine of the Sun angle off the panel, you will get about 300 GW / pi = 95 GW average over the day. That average would hold over the year as well, if every day were sunny. But, probably at least 1/3 are not, so you get more like 60 GW yearly average. 60/ = %. So, we’re right back where we started. It is a pittance of power.