Yea, as I thought about it more it makes less sense. It will be interesting to have a Powerwall system on my own house to be able to test on, and perhaps log what the power is looking like under various conditions. There's something going on that I don't quite understand here, and is demonstrated by
@chrisbailey13 Not quite sure what else would cause that behavior?
The powerwalls are designed to support ramping back the PV output, as long as the inverter supports it. However, it doesn’t appear that tesla normally configures this. (They certainly didn’t in my case at least). However, my solaredge inverters support ramping back production with a feature called P(f). I believe other inverter manufactures support this as well, but it’s probably called something different.
With the solaredge inverters you can define a low frequency level and a high frequency level and it will limit output when the frequency is in that range. For example you could define a low frequency of 60 and a high frequency of 61. As long as the frequency was at 60Hz or less the inverters would output 100% of the solar production. If the frequency started going up then it would scale back. At 60.1Hz it would only output 90% of the solar production. At 60.5Hz it would output 50% of the production, etc, until it got up to 61Hz and then it would turn off.
Because the powerwalls will slowly raise the frequency as they approach 100% you can configure your inverter to scale back production as the frequency increases and (theoretically) at some point it will reach an equilibrium where the amount of power the inverters are producing is equal to the amount of power your house is drawing.
However, in practice this may be a bit difficult as the inverter scales based on percent of current production. So as clouds come and go or as the sun moves across the sky the output will still be changing. Additionally the power demand from the house will go up and down. But the system can use this to scale back solar production as the powerwalls get close to being fully charged.
I spent a little time trying to configure this on my system, but eventually I gave up. The first problem I had was that even if I set the high frequency level to, say 61 or 62Hz the inverters would still shut off when the frequency got to 60.5Hz. Apparently that cutoff was defined elsewhere in the settings. So at that point I could only work with the range of 60Hz to 60.5Hz and that just wasn’t enough range to really make this effective. The frequency would get up to 60.5 before the inverters had a chance to react and scale back and they would just shut off instead. Additionally I really wanted to set the low frequency level to like 60.2 or 60.3 just because I didn’t want my solar production scaling back if the utility happened to be sending power at a slightly higher frequency than normal, but that would just make the scale back range even smaller.
In the end I decided that it just didn’t matter. It seems a little more elegant to allow the system to scale back production, but really it will be no different one way or the other. In one case the system might run at 50% power for a while and in another case it might cycle on and off for a while, but both cases it will produce the same amount of power and have the same amount of power ‘lost’.