Yes - and it can be manipulated to force the battery to grid charge. For example, I get cheap power from 1am - 6am and 10am - 3pm. I force the reserve to 25% at 1am to ensure I have at least 25% charge at 6am - which will get me through until the solar cranks up. When winter arrives - I’ll probably increase this reserve % further due to heating being used in the morning.
I’m certainly not aware how it can be done using the Tesla API.
I’ve done some experiments with curl to help me understand how to use the API, now I just need to sit down and code it. Now I’ve looked - I will probably also support reading the current situation like battery charge, solar power, grid power, etc - as it looks to be a single API call then just decode the information.