One of the most important questions: What is your average kWh usage per month?
Yes, the maximum rated power for your appliances would be relevant if you want to add Powerwall battery storage, but for solar TCO/ROI calculations, knowing your overall energy consumption is key. We also have an all-electric kitchen, for example, but the overall amount of energy used in our kitchen isn't very high.
If you can go on a TOU (time of use) plan with your utility and charge your EV during the lowest rate period, at night, that could be ideal today. On the other hand, with the proliferation of solar in California and beyond, it will likely become cheapest to use electricity during the midday hours, which could negatively affect ROI for many homeowners with solar PV and EVs (such as myself). If I were calculating ROI for a new solar PV system today, I don't think I'd factor in any benefits from TOU. I'd keep things simple, and just look at my average cost per kWh. Then the question is, how much would that same energy cost with PV panels on my roof?
That said, installing PV plus batteries (Powerwalls) would come with an additional benefit, namely, a degree of energy independence. Being able to generate and store your own energy, even when the grid goes down, is a beautiful thing. Regardless of ROI, knowing that you're using clean, renewable energy is certainly a plus.
