In my previous Going off-grid post I reviewed the question of whether Tesla Powerwalls or overgeneration, considered separately, might allow a UK homeowner with a rooftop solar array to go off-grid. In this post I consider the two in combination. Once more using 10 Mossbank Way as an example I find that there are circumstances in which it might make marginal economic sense for Mossbank to install up to one Powerwall, but that again that there is no realistic combination of Powerwalls and overgeneration that would allow Mossbank to power itself year-round with solar alone. Going off-grid is again found to increase Mossbank’s electricity costs substantially no matter what combination of the two is adopted.
It’s becoming progressively more obvious that 100% renewables-powered grids will never work without adequate energy storage, and at present the storage system of choice is the Tesla battery. I therefore make no apologies for presenting the third post in a row on Tesla Powerwalls. This post, however, evaluates their performance in more detail than did the previous two by reviewing the impacts of varying the number of Powerwalls and the amount of solar overgeneration at the same time. To do this I had to construct a rather complex spreadsheet algorithm which after checking appears to give the right answers but which I can’t guarantee to be 100% correct in all cases. Having made this necessary disclaimer, on to the results.
First a brief recap. Mossbank’s rooftop solar array has an installed capacity of 4kW(p). I have again used Mossbank’s hourly solar generation data for 2016 (total 3,809kWh) and have again assumed that this was equal to Mossbank’s 2016 demand. The hourly demand curves are the same as those used in the previous post. Sixteen cases are considered – zero to eight Powerwalls and either 4kW or 8kW of installed capacity.
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In my 
