On the prior article before the announcement, I posted about the economics not being that compelling, using my own actual numbers for consumption and peak vs off-peak pricing. I also noted that in some states, CA in particular, when peak pricing is ~60% higher than where I live, that it could start to be somewhat attractive.
That was all based on the pre-announcement rumored price of $13,000 for 10KWh. At $3,500 for 10KWh, I'd be looking at a 4-year payback, or, in other words, about a 25%/year ROI. To be clear, that's without solar PV panels to generate electricity, that's strictly charging the battery during off-peak hours and then running the house on it during peak hours. (Quick calculation based on battery price alone; total installed system more likely to see 15%-20% ROI, but still, not bad.)
I had wondered what Musk was up to and if the rumors were correct. Because you can already buy a 10KWh nickel-iron battery system for $13,000, so it did raise the question of what was the point? Well, now we know the point--1/4 the cost of existing competitive systems.
One big question not answered by the linked article, is what technology is used and what's the depth of discharge without damaging the battery. With nickel-iron, you can discharge most of the charge safely. With lead-acid technologies, you can't go below about 70% without shortening the lifespan. So 10KWh can actually mean anything between 3KWh and 8KWh of usable power--a huge range. (Hey, maybe Tesla's going to be consumer-friendly here--maybe 10KWh means 10KWh of usable power... As this kind of thing becomes more common in the home, it would make sense to rate battery systems that way, to make direct comparisons easier...)