abasile wrote:In the "Powerwall 2 waiting list" thread on TMC, the user jeeps1979 (in Crestline, by the way) posted a system diagram that seems to show a "gateway" between the main panel and the service meter for this purpose.
But there is some question as to how that could work, particularly with main panels that integrate the service meter. Also, if the grid is down, the battery is fully charged, and there is surplus PV generation, I presume that the PV would need to be automatically disconnected. I hope to learn more.
No the PV would not need to be disconnected, at least from a technical standpoint. It is a common misconception that all the solar power generated by the PV panels and associated inverter(s) "must go somewhere."
In the normal situation with an active grid, the grid acts as an infinite load that will absorb any excess power, so the inverter has the simple goal of maximizing output by operating the panels feeding it at their MPPT (Maximum Power Point Tracking) point on their I vs V curve. However, the inverter is free to operate the panels anywhere on their I-V curve from Short Circuit (maximum current, zero voltage) to Open Circuit (zero current, maximum voltage). At both of these extremes, SC and OC, the power generated is zero. OC is equivalent to no inverter being connected at all. The illuminated panels are still generating their full photocurrent, but this current is exactly canceled by the forward bias current in the opposite direction.
Even when the grid is up, the inverter has to handle the common case of more DC power coming in from the panels than the inverter is rated to output. The inverter simply moves off the MPPT point far enough to bring the output back to its rated maximum value. For example, if the inverter reduces the current draw by 20%, the panels will be underloaded and their voltage may rise about 10%, for a net 10% loss in power. I have sized my new system so that this situation of "power plateau" occurs for an hour or so on the best solar days of the year. I can see the voltage rise above its MPPT point and the current drop below its MPPT point. Buying the next larger size inverter was not cost effective.
So in the case you consider, the surplus PV generation could be reduced to zero. The inverter would have to track the dynamic load of the house and adjust its output accordingly, rather than just optimize the output of the solar array.