Earlier quoted context omitted.
> Solid state parts can last decades though. But every year we seem to get better silicon. Better CPUs, GPUS, SSDs, etc. Same holds for photovoltaics. To run with the investing analogy, it's like claiming you bought a bond with a 5 percent return, when six months later interest rates shift in the bond market. Now the rest of society is getting 7 percent returns, and you have to sell your to sell your bond at a discou…
To run further with that same analogy, what if you expected bond rates to keep increasing for 50 years? It would not seem wise to keep your savings in a checkings account for these same 50 years, to then get the highest rate. Yet that's what most of us keep doing with solar, missing out on a good deal because it gets better every year.
Assuming these are real, not nominal rates, who cares? I'd rather maybe miss out on the extra 2% than definitely miss out on the original 5%. Why be jealous?
You can also hedge your bets by gradually increasing your investment (maybe even "dollar cost averaging" or some other strategy)
For solar, that could mean getting a small system like the OP describes, trying it for a year or two, then expanding it a little more every year or two with the latest cheapest most effecient panels. This should work even better than dollar cost averaging does for securities, since you know it's unlikely the cost of panels will go up.