@miimura there is a way with the API to command grid charging. It requires scripts to make API requests at the right time that set the Powerwall into
self-powered mode and adjust the
power reserve percentage. However, in addition to needing the skills to do that and a device to run on, there are flaws:
a) It will only charge at the slow 1.7kW rate, not the 3.4kW it is capable of doing.
b) It only works when there is an internet connection AND Tesla servers are running
I currently do that every night when I have 4 hours of at off-peak and can charge my Powerwall by ~50% at most. That will not be adequate for winter when solar generation is at a minimum and power use at a max. The script has to make multiple attempts because the Tesla servers often do not respond. The changed settings values are sometimes not applied until hours later leaving the Powerwall doing the wrong thing in that period.
It would be so simple for Tesla to provide a facility to input the percentage you want the Powerwall to charge to during the off-peak period, why won't they do that?