Well, here is some first hand experience. We have signed up to install Powerwalls in addition to our solar. We were definitely influenced by local incentives. Us installing batteries helps lower the cost of power on the grid, as our batteries will supply peak power that would otherwise have to come from expensive "peaker" generation units. We get outage reliability, and others get more reliable power at a lower cost. Win-win.
MossRoad is right on the money about charging solar vs. grid, although there is apparently one guy in Arizona who has managed to be able to charge from the grid and solar.
The Powerwalls run about $7,000/13.2kWh battery. Each group of up to ten batteries require a gateway controller (transfer switch, plus some smarts) for another $1,000. Installation costs are variable. Tesla will typically do it for several thousand, but they are very cookie cutter. If your situation doesn't match what they do, forget about it. Tesla won't do it. Third party contractors may charge $3-10,000 around here, depending on the complexity of the job. (Not really apples to apples, as they tend to do the ones Tesla won't.) Since each Powerwall can output up to 7kw, or 29+amps, many people need a main panel upgrade to handle the solar & Powerwall currents which may be enough to overload the existing bus bars. You need at least one Powerwall for every 3.3kW of solar power, or the Powerwalls will throttle the solar down to keep from exceeding the charge rate on the Powerwall. If you have household demand (ovens, AC/heat) that can absorb the excess solar energy, you can get away with more solar relative to the Powerwalls.
The devil is in the details, as different "authorities having jurisdiction" are all over the map on what is allowable on a battery install. As in crazy, crazy differences of opinion. I think that the US would benefit from a uniform national electric and national fire codes. As far as I can tell, this is an evolving, you might even say rapidly evolving, area of building code and code enforcement. Some of those requirements will greatly affect your cost. E.g. the next town over from here is worried about driving a fully loaded 1 ton pickup into your garage wall, so they want concrete filled steel bollards strong enough to stop said pickup from hitting a battery on a garage wall. Another local town requires a big disconnect next to every battery and at least one more next to the main panel. Mine requires batteries to be 36" apart, and at least 36" all directions from any door or window that goes into living spaces. It all adds up.
Unlike some of the competitors, these are lithium ion batteries, just like Tesla cars. Every non-Tesla battery alternative that I looked at was much more expensive, and none of the competitors have the long track record to support their MTBF numbers. I'm not a Tesla fan, nor do I rush to buy the latest just because it is new. Tesla's batteries literally have hundreds of millions of discharge cycles on them today, so you have a good idea of what you are buying. Nobody else comes close. I am not saying that the other brands aren't reliable, but I am pointing out that they don't have the data to support the MTBF reliability claims that Tesla has.
Tesla warrantees the batteries for 10 years, and an unlimited number of charge / discharge cycles if it is coupled with solar. The guarantee is that the battery will have 70% of its capacity at the end of ten years.
Whether installing batteries has a return on investment is dependent on lots of variables; reliability of your grid, availability of alternate sources of energy, cost of alternative energy sources, other risk factors, and how long you expect to be out of power. As MossRoad mentioned, incentives can make a huge difference on the costs and the ROI. I have watched lots of folks go through the various trade offs and come to very different decisions for what I think are very sensible reasons. This is truly YMMV.
I do know of someone locally who went to a small commercial Tesla PowerPack (3 phase, 232kWh battery system) and solar because it was cheaper than having the local utility trench the 350 feet from the nearest pole. He is totally off grid, within sight of a utility pole. In what can only be described as large lot suburbia. No incentives. Blows my mind. I am always amazed at what pencils out. One way to look at that is to see the coming end of centralized electrical utility grids. Oh, he charges his electric cars from the solar as well.
Our utility is in the process escalating the fees to be connected to the grid. On one of our meters, the non-power charges are 90% of the bill. As this utility pricing trend continues, I think more folks will find that these solar / battery systems make more sense than joining the grid. That meter is close to going off grid.
Not to pick on Texas, but I think that the failure of the natural gas supply during this winter storm due to electrical grid shutdowns highlights some of the challenges of relying on standby natural gas generators for an outage. Locally, when we moved in our propane supplier was very clear not to expect propane during a winter storm event if we had a generator. They told us that if we were to install a standby propane powered generator we would need to install a separate propane tank for however long we wanted to run the generator for during the outage. Locally, we have seen ten day plus outages due to storms, far longer due to fire and earthquakes. That can run to some rather large propane tanks.
I live in California and our electrical utility has gone from ok to bad to worse. We had four outages in December of at least four hours each, three of which just happened. No storm, nothing, just "poof", no power. With the utility's plan to shut power off in advance of any temperature/wind event for fire reasons, we are losing power for one to two weeks or more every year. Having to work from suddenly crystallizes what the hourly cost of not being able to work is, and those ROI calculations suddenly look a lot more favorable to a whole house standby generator or powerwalls. In a fire prone area, having batteries has certain obvious safety advantages over large propane/diesel/gasoline tanks, along with extended run times.
I firmly believe that these are very individual decisions, and where you come out is greatly influenced by your local details.
Sorry for the long post.
All the best,
Peter
P.S. FWIW: Tesla is currently running six months or more behind on Powerwall orders. Less, if you buy from them, longer if you buy from third parties. We signed a contract for Powerwalls in May of 2020, with a current delivery date mid to late 2021.
