I frequently hear people bring up transmission losses as a concern, and genuinely curious where this idea comes from? Was this taught in schools or part of some disinformation campaign?
My understanding is that it is "simple" resistance heating of the transmission lines (P = I^2 R). Which is why high voltage lines are good ideas ( V = I R ) -- they lower the current.
Heating can be quite significant. I guess whether or not it’s economically significant depends on your cost of generation. There was a mega-outage which cut off Italy when overheated, sagging cables, struck the treetops in the middle of either another random outage or some scheduled maintenance. It’s been many years since I read the full report on the incident (which was excellent) but there was some great data in there about degrees of heating vs degree of sagging.