Those 'thru-glass' antennas work because the two parts act like the two plates of a capacitor. RF is AC and AC can 'pass' through a capacitor rather easily. The 'size' of those two plates, along with what separates them, determine the value of the capacitor.
Never having 'played' with them to determine the required size of those plates, or the dielectic insulation between those plate, I won't even guess as it. I can tell you that the frequency is also important in all that because 'higher' frequencies require 'less' capacitance than 'lower' frequencies.
Power ratings for the things? I don't know, but I certainly wouldn't expect as much capability as with a "real-live" antenna. That automotive glass insulated capacitor just isn't going to have a huge power rating.
Do I like the 'thru-glass' antennas? No, and I wouldn't have one unless absolutely necessary. Do they work? Yes, they do. Very well? No. They are adequate at VHF/UHF
and higher, but forget it for HF
. Just want one, or have to have one for some reason? Fine, do it. You can have the one reserved for me, tell'em I said so.