Tl;dr:
Battery-backing your solar installation gives you easy, clean outage resilience that is hard to achieve any other way. If you live in a particularly outage-prone area, it may be very useful to you. As with rooftop solar, it takes a few months longer than it should to get started, but once started it’s pretty reliable.
I’m not in a particularly outage-prone area, but I like fancy earthquake prep and doing my part to balance the grid, so I got a Powerwall once it became easy-ish. It was probably too expensive for the power cost savings to pencil out, but it’s a cool gadget to have.
There are now a lot more backup battery options out there, and there will probably soon be a better kind of option: using your car battery. If you haven’t bought an EV yet, consider getting one that supports this option.
When I first got rooftop solar in 2013, battery backup was still closer to idea than commercial reality. SolarCity sent us an invite to be beta testers for what turned out to be the prototype Powerwall at the end of 2013. They would have charged us a $1500 installation fee and $15/mo for a leased battery. We didn’t take them up on it— we weren’t that early-adopterish— and overall I'm glad we waited; the beta product looks really clunky in retrospect.
Several years later, after Tesla bought SolarCity, they finally rolled out the production Powerwall. At first these weren’t available to customers with our 2013 inverter model, only to new customers with a different inverter. But finally in October 2017, they opened up the option to us and we put in an application. The initial design process was quite smooth and done mostly by emailing them photos of relevant equipment and areas of our house. By early November they had everything ready but the permits.
The actual installation happened in early March 2018. As far as I can tell, the delay was mostly caused by the same bureaucratic process I ranted about in my last post on rooftop solar. At least the installation itself was very smooth: they hung the thing on the outside wall near the breaker panel and hooked it up, and that’s it. There was an early glitch a few months in where the discharge control stopped working and the unit needed to be reset, and since then it’s been trouble-free. The Tesla app integration is pretty neat: you can see how power is flowing at any given point among house, panels, battery, and grid, get statistics about how much total flows each way over time, and even test off-grid “islanded” operation so you know what it’ll be like if there’s a real outage. I haven’t worked up the courage to do that last one yet.
Tesla lets you configure how much to discharge the battery to help power your house and at what times. By default it discharges during the utility’s peak power demand window, which in our case is 4-9 PM, and then recharges first thing in the morning when demand is lower, having kept a reserve overnight in case of an outage. I feel a pleasant sense of earning prosociality brownie points whenever there’s a news story about high power demand in California due to a heat wave, and I look at the app and it says we’re running the house off the battery and sending the power from the panels to the grid so somebody else can run their A/C. Of course, one of our San Francisco privileges is that we rarely need A/C ourselves at those times— though a warming climate is steadily changing that.
I paid $7600 for those brownie points and for the security of knowing that if the next big quake takes out power for an extended period, our lights and fridge will stay on. Definitely closer to the “fun techie toy” end of the spectrum than the “sober financial investment” end, but not crazy expensive. This was an outright purchase, not a lease like the solar panels. I don’t really know what to expect the service life to be and I am not sure anyone else has a great idea either; it’s too new a thing.
If I were doing over our solar-and-storage installation today, Powerwalls would be one option among many; at a cursory glance it seems like a bunch of major vendors offer both purchase options and bundling of a battery into a solar lease. But if I didn’t already own an EV, I’d be really interested in making sure that when I did buy one it supported so-called V2G/V2H (vehicle-to-grid/vehicle-to-home) capability, where your car can discharge power from its battery back through the charging cable to power your house. Most EVs and most home chargers on the market in 2022 don’t yet support this, from what I can tell, but several are starting to do so (the F-150 Lightning supports it from the beginning and the VW ID.4 is supposed to get support added this year, for example). I hope this becomes standard quickly, because it’s an obvious improvement: car batteries are much larger-capacity than Powerwalls, they’re mobile so you can go back up someone else’s house if they need it, and consolidating more useful functions in one device is often really good generally, as anyone with a modern phone in their pocket knows.
Did you end up financing, or just cutting a check? Also curious about the capacity--how long can your home run on battery if the power goes out?