That was more Fermi estimation than using hard numbers. But there are lots of non-numeric reasons this is a bad idea, some of which have already been mentioned in this thread.
- electricity company almost certainly won't buy back a half-acre field's generation
- municipal approval is unlikely to happen
- creating a giant heat island will not be popular with neighbors
- creating a giant field of glare will be extremely unpopular with neighbors and depending on placement in relationship to a road may actually be dangerous
- nobody is successfully running a small solar power plant on their personal property
Maybe I'm wrong about the last one, but I really doubt it.
ok...
1) you don't need half-an-acre of solar panels. You could install 30 panel grid on about 25' x 25' of space, with room between panels for solar actuators. That would be in line with other people's solar installations.
that's < 5% of a half-acre lot, leaving the rest of it available for other purposes, like an epic garden, an open-space buffer, whatever.
2) muni codes vary place to place. In many less developed places I've lived there's no problem installing PV arrays in a field.
3) No idea how it would be dangerous to a road, given how PV arrays are already popping up alongside roads throughout California. As long as it's a safe and legal distance from the road why woudl this be a problem (see #1) Whether or not it would be unpopular with the neighbors depends on where and who the neighbors are.
4) Not a 'small solar power plant' - an array of panels to power a home with some surplus.
I'm guessing you are coming at this from a strictly urban/suburban mindset where there's a house on every lot and distances between houses is very small. That's fine. Perhaps that's where the OP is too (but since it was an 'empty lot' - maybe not).
But in less developed parts of the US it's already common to see a bunch of PVs in a field powering a nearby home. It's cheaper and often more efficient than mounting on a roof. The big question is 'what's the land cost' and (in the case of the OP) whether they would get additional benefit(s) from owning that extra half-acre.