It's pretty well established that Tesla won't configure the PWs to charge from the grid in the US and gives customers no way to force this (API or otherwise), and most utilities won't approve it to be used this way. A couple people on these forums seem pretty confident they can convice Tesla to configure their PWs/Gateway to charge from the grid, but I haven't seen anybody post in regard to "actually" having accomplished this in the US.
Also, in terms of stormwatch, Tesla totally controls when your system will go into stormwatch mode and in turn then when it could charge from the grid on a very limited basis. You can only choose to allow that to happen (On) or not to happen (off) should Tesla trigger a stormwatch condition for your area, which are pretty rare - i.e. only during severe conditions where power his likely to be lost due to major weather situations or other disaster scenarios.
I believe Tesla is forced to limit and totally control grid charging like this in the US due to agreements with regulators and utilities that were required for Tesla to actually be allowed to move forward with installing Powerwalls at scale, and to maintain various required product licenses and certifications that are required to sell PWs in the US for grid connected systems. Even if a given utility says they support grid charging, Tesla may not be able to allow that due to product certification requirements (just a guess here) that limit what has those products operate on the grid in the US. Clearly Tesla otherwise supports charging from the grid because it's possible in other countries.
some people in Arizona do not have solar but have Powerwalls. So the general idea of charge from grid is possible . It’s when you have solar and I think that is more or less tied to tax credit . The tax credit will go away eventually . Then Tesla might revise their firmware for solar units .