Detectability of Solar Panels as a Technosignature
arxiv.org
arxiv.org
Microbial or other primitive life (carbon-based or otherwise).
Or remains of some type of civilization. Like Maya ruins, or a Voyager-style probe that was launched 100k+ years ago.
Both very difficult to detect from a distance.
Or they'd have to come to us. Not sure that would be a good thing...
(That is, unless they have Earth-like politicians granting tax incentives, or if alien hipsters install them because they are “retro”.)
(Or, we have no reason to believe other intelligent life is any more or any less flawed as we may be)
I guess the answer to your question is, "why not both?" Look for solar panels, look for fusion reactors, look for whatever you can think of. It's sort of like diversification, the wider your net the less you are speculating. Taken to the extreme where we cover all possible technologies (clearly impossible but bear with me), our speculation becomes "aliens exist and use technology."
The only other games in town, if we're talking huge amounts of power, and ignoring the speculative "beyond", are fission and chemical combustion. Unlike PVs these can be expected to alter the planet's atmosphere distinctively, and that is the thing we can actually observe during transit events, in contrast to panels (and dams, wind turbines, geothermal etc) down on the surface.
Apparently it's worth bothering just to remain inconspicuous. Aliens are clearly doing everything possible just to hide themselves from Earth.
And for longer distances you would not be wasting EM power in all directions, but using highly directional beam of EM energy towards target or relay satellite.