I've been thinking about this a lot with the current congressional action. I need a new roof ~now. But I have a lingering fear that huge product improvement will happen in the next 18 months due to added government money. I don't know enough about the industry to weight in that possibility.

Keep in mind, with the new law you are talking about, more money will be dumped into the market with no increase in supply.

That is what is called inflation. If people have an extra 7500 to buy solar, then solar becomes up to 7500 more expensive.

This assumes supply is perfectly inelastic. Even assuming, it also ignores the fact that the money is only dumped in one nation.

I'd absolutely do it now. The price of solar has been beneath that of racks, mounts, wiring and installation for a while now. Even assuming a major breakthrough your costs probably wont change that much because panel cost isnt a big % of what your costs will be anyway.

There's also a risk that supply chain bottlenecks/cold war with china will drive up the cost & your electric bill is only gonna go up.

Agreed, do it now. The cost of panels in Australia is already going up due to supply issues thanks to covid, and I don't think you'll see prices come down again for a few years even with that investment.