Running Tesla Powerwall 2 Off-Grid - Excess Solar Question
Answer collated from comments...
Looks like there are three separate things going on here, with an interesting interaction...

a Grid Tie inverter

This is supposed to shut down when disconnected from the grid. That's a fundamental safety requirement of grid tie inverters called &quot;anti-islanding&quot;.
When the grid fails, does your inverter power the local grid segment and electrocute the poor lineman sent out to fix it?
No, because the inverter senses the incoming mains voltage, and shuts itself down when the voltage disappears or the frequency is out of range. This is the fundamental feature of a grid tie inverter as opposed to an off-grid inverter.

A battery storage system, the Tesla Powerwall. This charges itself when there is excess power available, and discharges when there is a power shortage. The question is, how does it know the state of the grid?

The power grid consists grid mainly of rotating machinery, which speeds up a little when lightly loaded (like a car rolling downhill). This means there is a good correlation between high demand and low frequency (and vice versa) which can be observed here
So we can speculate the Powerwall monitors frequency (as well as voltage) to supply power when frequency or voltage are low.
Fine, but what happens when the grid is disconnected? To charge, the Powerwall must generate AC voltage, both convincing the inverter to start up, and to power the house. Then it indicates its need for charge by keeping the frequency low, and when full, convinces the inverter to stop charging by raising the frequency.
When in this mode, turn on a decent load like a kettle. Watch the frequency fall, and the inverter re-start.
So the house is now an island with its own AC supply. But what about the poor lineman?

The gateway.

If external AC mains is disconnected, the gateway provides further isolation, disconnecting your island from the external wiring, and won't re-connect until external power is re-established. (It must provide some way of synchronising the two grids in the process. I don't know how).

Now I think we can answer your questions...

The Tesla signals it's full by increasing the frequency until the inverter stops generating excess power. Adding a load should reduce the frequency again.

If there's excess energy (sunshine), no grid, and full batteries, the inverter simply doesn't draw current from the solar cells, which dissipate the excess power as heat, getting slightly warmer.


You can add a load dump (like a water heater) connected when the frequency rises, to capture this energy.

the inverter SHOULD legally be set to UK (G83/2) standard if you're using it in the UK. Parameters like freq range, response time to faults, etc will then comply with UK rules.

If the PV meter is in the inverter, it must measure power exported by the inverter (= power harvested from the solar cells) whether you use that internally, charge the battery, or export it.


To measure power exported to the grid, you need a smart meter that reads import and export power separately.