There's a perfectly cogent definition: it's the technological level where you can scale out solar photovoltaics (or equivalents) to within an order of magnitude of the solar energy incident on a planet. It's a useful framework of thinking. Human civilization in particular does not possess this technology level. We're four orders of magnitude short of this energy output, and we don't have the technical ability to reach it even if we really wanted to—i.e. today's manufacturing methods are too primitive, not automated, rely too heavily on manual labor.
It's a framework to measure and then think about this technology gap. What's the difference between that imaginary civilization, and ours? We don't have advanced AI automation: we simply can't print O(100 trillion meter^2) of solar PV at any price. (The automation fraction is too low: human labor is the finite, limiting resource). Nor can we cover oceans with solar panels: we don't have the raw infrastructure power to build megastructures on that scale (again, some sort of an "automation" gap). And another: our (cheap) photovoltaics are an order of magnitude short of theoretical efficiency limits, waste most of the light that's incident on them. This one's a technology gap, and there's presumably a giant amount of physics understanding, computational power, and engineering ability that separates us from that end-game technology level. That separates our civilization from a hypothetical reference one.
It's a framework to contemplate these kinds of questions.
I believe this is your only error: the cheap ones are about 20% efficient, the best are about 45%, and the theoretical maximum is about 69%.
https://en.wikipedia.org/wiki/Solar-cell_efficiency#Thermody...
That's like saying that building a house and building a metropolis is the same level of technology because cities are made of houses.
It does, however, give us a common point of reference for conversation about what may yet come.
Eg. capturing all energy output of a star, includes the ability to move (vehicles, robots etc) through space, build star-sized infrastructure, and so on.
Personally I prefer the magnitude of energy production/consumption. MWatt → GW → TW etc.
Or how advanced (or energy-dense) its source is:
Burning stuff → electric power → nuclear fission → nuclear fusion → antimatter
Galactus would like a word.
There is no point in hating such nonsense, but it is worth fearing.