What happens if you buy this battery and a year or two down the road someone comes out with a battery that is twice as efficient as the one you have?
Then the whole world changes, whole corporations go out of business overnight while others swell, and there is widespread financial chaos.
This is the exact question I asked Solar City when I was considering solar panels for my house.
That's because you don't understand the solar industry even a little bit. When new, more efficient panels come out, not only is their price per watt higher but the price per watt on the old panels comes down. The primary benefit is not reduction of cost, at least not at first, but in reduction of panel area needed. That reduces the size of an installation which can reduce its cost — but in the case of a residential solar system, that is rarely the case. Since they're usually fixed and roof-mounted, the amount of materials used to mount them is fairly small and there are no property cost considerations whatsoever. The homeowner doesn't care if they have three or six panels on their roof, because they're on their roof and they're not taking up any space they were using before.
The truth is that improvements in batteries and solar panels do not come in 100% increments. They come in small increments delivered over long periods of time, just like the savings on energy costs delivered by a solar installation. Not installing solar now because you're worried that solar is going to get better is just depriving yourself of the benefits that you enjoy by doing it sooner. Meanwhile, your system can be upgraded piecemeal, so you can replace your batteries in 15 years and your panels in 30, maybe add some more batteries then. You can mix and match different kinds of panels to a certain extent; sure, you need different charge controllers for old and new style panels, but you can have both kinds of charge controllers right next to one another, connected to the same battery bank. So really, there is no basis whatsoever for your concern that a 100% efficiency improvement will come along tomorrow and eliminate the value of your investment. And frankly, if such a leap in efficiency were realized in a commercial product, then some government would probably buy up 100% of it and you wouldn't be able to get any anyway. Kind of like what happened with nanosolar, which was then driven out of existence by the chinese dumping panels on our market so none of us got to buy any of it. That stuff had the potential to be disruptive, but now we have to wait for someone to conceive of the idea again with some new and even cheaper technology because we're okay with goods produced with slave labor so long as it doesn't happen within our borders.