There is a problem with the battery for the home off grid. If the whole system is off-grid, then the batteries will spend most of their time fully or nearly fully charged. The system has to be able to last over the longest time with little or no sun in winter. And battery packs tend to last longer when not deep cycled.
If that sysem is not connected to the grid, that means that most of the energy generated by the solar panels will be thrown away because the batteries are full.
In an optimum installation, solar panels will pay back their energy debt required to make them in 3-7 years. In a sub-optimum installation (but connected to the grid) they will pay it back slower. Add the batteries and you have a bigger energy debt for manufacture. The time to payoff of energy to manufacture and install rises to 5-10 years.
The question is not whether panels in an off-the-grid system will throw away energy. The question is how much energy that system will throw away. To have a good consumer experience, a designer would want to oversize both batteries and panels. I would expect at least half of the PV energy generated to be thrown away on average. I would expect that in the top-of-the-line installations, that the system could throw away 80% or more.
That extends the time to payoff by a factor of 2 – 5 times. So now, instead of a 5-10 year energy payoff when grid-connected, you are looking at a low of 10-20 years, and a high of 25-50 years.
Those figures get into never-will-pay-back territory. Why? Because while solar panels last a long time, batteries don’t. So you will have to add to your energy debt. Then, you won’t ever pay off the energy debt incurred by making it and installing it.
So the system becomes a sinkhole for energy.
Today, most solar panels and batteries are made using coal energy in China.
If we want to be carbon-neutral, we need to use nuclear power for that. So what this system really means is that China’s conversion to nuclear power will create the benefit for global warming.
I suspect that Musk and company are well aware of this. And I also think that the probability of a significant number of people going off-grid in our urban based world so that it really matters is very low. Consequently, I suspect that Elon and friends have just let this one go by because it will help generate buzz and sell systems. Yes, it will allow your house to have full power during outages – definitely that.
But stay on the grid unless you have an overriding reason not to. Going off grid will switch your system from an environmental benefit to an almost certain net carbon-polluting choice. When we see nuclear power taking over main-grid generation, then it won’t be a carbon-creation choice. But until then, yes.