No joke, when I was on the phone with Tesla to request the feature that you describe as a no-brainer, they said it couldn't be done.
The guy on the phone said, if the load isn't backed up, it cannot get power from the batteries regardless if the utility was online or not. If you want the battery power to get to the load, it has to be on the backup side no matter what even if the power company was operating as normal.
For proof, here is the first ever post I made here. Back then prior to signing up for TMC, I had been "educated" by like over 5 ESS installers that what Tesla did without you asking was actually impossible or would damage the Powerwalls.
First time poster here - I'm trying to get new PV and Powerwall partial backup installed in my home in Northern California. If you have a partial backup system, can you help me understand if my goal is possible? My goals for the PV + partial backup: 1) When the Utility is working fine, I...
teslamotorsclub.com
PS, Vines and wwhitney is amazeballs. If they weren't on this forum, I guarantee you I'd have a partial home backup solution right now with only like 5 loads getting energy from my Powerwalls during peak time (even when the utility was operational).