When is the right time to buy a new/emerging technology?
When it's trading at a discount that allows you to make your money back and then some.
The way you presented it, it is of course impossible to say. You have to look at exactly how much cheaper and efficient it will be, and how long that will take. Time too has a cost, and being invested has opportunity cost, so the returns must not only arrive in expected quantity but also arrive on time.
Since you tagged this
investing, you should look at the financial forecasts of the business, likely future price trajectories, growth opportunity and so on, and buy if you expect a return commensurate with the risk, and if the risk is tolerable to you. If you are new to investment, I would say avoid Musk, there's too much hype and speculation and their valuations are off the charts. You can't make any sensible analysis with so much emotion running wild. Find a more obscure, boring company that has a sound business plan and a good product you think is worth a try. If you read about it on mainstream news every day you can be sure it's sucker bait.
Also, my impression that these panels are actually really expensive and have a snowball's chance in Arizona (heh) in a free market. Recently the market has been manipulated through green energy subsidies of a government with a strong environmentalist voter base. This has recently changed, in case you haven't heard. So the future of solar panels is looking a bit uncertain.
I am thinking about buying solar panels for my roof.
That's not an investment question, it's a shopping question.
Do you actually need a new roof? If no, I'd say don't bother. Last I checked the payoff is very small and it takes over a decade to break even, unless you live in a desert next to the Mexican border. Many places never break even. Electricity is cheap in the United States.
If you need a new roof anyway, I suppose look at the difference. If it's about the same you might as well, although it's guaranteed to be more hassle for you with the panels. Waiting makes no sense if you need a new roof, because who knows how long that will take and you need a roof now.
If a solar roof appeals to you and you would enjoy having one for the price available, go ahead and get one. Don't do it for the money because there's just too much uncertainty there, and it doesn't scale at all. If you do end up making money, good for you, but that's just a small, unexpected bonus on top of the utility of the product itself.