I heard back from Tesla today, so I guess I owe them an apology for implying that their customer service wouldn't provide an answer in a reasonable amount of time. The voice mail message stated that the designer ran the load calculation and I should be fine to add the hot water heater. I had provided them with the Rheem model number and all of the specifications, I'm not sure if that influenced the calculation or not, or if they just assumed a worst case 30A load. They said that if there are any questions from the electrician to have them call Tesla and there is a department that handles that type of issue, though they didn't specify what department that is. Since I'm pulling the permit and will be doing the work myself, the issue of who to call is likely moot, though it might come up if the building inspector wants to verify the load calculation.
They also noted that the sub-panel had a 30A dryer circuit in it, so they seem to be extracting fairly detailed information from the pictures that are collected either during the purchase process or the site visit. Their advice was not to run a dryer and the hot water heater during an outage or it may shut down the Powerwall. When I moved into the house it included both a dryer circuit and a gas line; we've been using the gas dryer that we already owned, but this clears the way for electrifying that appliance as well, though I'm not sure if we'll wait for its EOL or we'll convert it before then.
So at least in our case, the 30A circuit per Powerwall rule of thumb seems to hold (new 30A circuit in back up load panel, existing 30A dryer circuit in a sub-panel fed by the backup load panel). Anecdotally the 50A charging circuit doesn't seem to be of concern, perhaps because they know our Model 3 will limit its current draw during an outage.
Answering some of the other comments that have come in since I last posted...
@arnolddeleon @Ampster @Vines I considered removing the sticker, but if it was added at the request of the local inspector I would be creating problems for myself when the inspector came out for the new circuit. (I'll be sure to ask the inspector when they come out if they required it).
@charlesj I could have wired via the 90A sub-panel, but a) that wiring path would have been very painful, and b) violating the spirit, but not the letter of electrical requirements isn't really my style.
@arnolddeleon I'm not in SVCE territory, but
Electrify Marin is offering a $1K rebate, and with the $300 federal tax credit for a heat pump hot water heater, it makes the high upfront cost of these less painful.
Personally, I don't think it's really sustainable to have all of their customers consult back with Tesla to perform any electrical upgrade or remodel for the purpose of them performing the load calculation. SInce the formula for the load calculation (based on the Powerwall output specs?) doesn't seem to be published anywhere, I guess I still don't understand how a licensed electrician knows exactly what Tesla wants them to consider. I think it's the whole ambiguity of the sticker that raised questions in mind, and it sounds like others here. Is it to protect the Powerwall warranty or some other overcurrent condition? Is it a local AHJ requirement? It doesn't seem to be related to any NEC rules for sub-panels.
Anyway, I got the outcome I wanted in a reasonable time frame. I'll report back if I learn anything relevant from my inspector.
Also attaching some pics since there were some questions on the connection topology (and to let everyone else appreciate the glory of "the sticker")
