Drones aren't limited to quadcopters.
> Not unless it sits still for a week in a sunny spot.
So do that?
Humans are persistance predators, or so I'm told. We didn't domesticate horses by running faster than them, but by because they had to rest, and when they stopped we'd catch up, and then they'd run again, a cycle that repeated until they couldn't run any more.
But these days, we humans are no longer nomadic: we live in predictable homes, and most of us who work do so at predictable locations.
A drone that takes a year to get to one of us? We could outrace it, or shoot it down… but only if we know it's there.
As someone in the IRA once told Margaret Thatcher, "We only need to be lucky once. You need to be lucky every time".
Why would that be an issue? Planners have to think further ahead, but the threat is only marginally decreased.
Isn’t that the answer to your question, then?
In any case, the solar cells should still be visible.
Such cells would be half as efficient I suppose, so would need to be twice as large and probably at least twice as heavy. I wonder how that might affect design and payload constraints as it scales.