I have found this thread very interesting, and I would like to chime in on this, and am curious if you have come to any decision.
1. You have what is called a "meter main. Here is what they look like inside. This one is 200 amps. Yours is very likely only rated for a 100 amp breaker. A 200 amp breaker would be much larger and not fit in your box.
2. Just because the meter has a CL200 rating does not make your meter main capable of 200 amps
3. I have a hard time believing your POCO installed wire good for 319 amps to your meter. You need to have someone open the meter and verify.
4. I have seen 200 amp service here with #2 AL wire coming from the POCO. They size the wire for the expected load and voltage drop, not for 200 amps. A typical example would be a 2000 s.f. house with a 4 ton air conditoner with 10 KW of electric heat. I have also seen 200 amp services with 4/0 AL wire on a larger houses with, say (2) air conditioners and 20,000 KW of electric heat.
5. Then there is the transformer to which your house is connected to consider.
6. Replacing a meter main in my area would cost about $3,000 to $4,000 if you use a typical small business electrician, more if you use one of those large electrical contractors. Replacing underground service wires by the POCO would be extra, replacing the overhead wire in my area would not cost much or maybe cost nothing. In my area the POCO would upgrade the transformer, if needed, at no additional cost. The one serving my house and 2 of my neighbors was upgraded from 50 kVa to 75 kVa about a year ago.
7. I cannot imagine Tesla or any other solar company would replace your meter main at no cost. They quoted adding solar to your existing electrical system and upgrading the meter main is beyond the scope of their job.
8. But why Tesla would not want to make an upgrade part of their installation, at whatever it would cost, I cannot understand, other than they are just so busy they might not want to fool with it.
9. If your load calculation comes to 98 amps right now, you would probably be best served to upgrade to 200 amps if you want to add electric water heater and a heat pump, along with unfettered EV charging, which requires backup electric heat which runs when defrosting the heat pump and is necessary to have available in case something goes wrong with the heat pump.
10. Even though your load calculation comes to 98 amps, you rarely use that much power. 98 amps running 24 hours a day would consume almost 17,000 kWh in a month! So if you do not want to upgrade your service (but you would probably not be able to add electric water heating or the heat pump) if you want to get by with the existing 100 amp service, there are devices that can monitor your home's power consumption, and if the consumption is too high will interrupt your EV charging, such as this device:
Get faster electric vehicle charging without upgrading an existing electrical service or panel when installing an EV charger, by using Automatic Load Management Systems (EVEMS or ALMS) by Black Box Innovations.
shop.blackbox-in.com
and this device:
DCC Electric – DCC-10 EV charging hardware for a full electrical panel in a house. DCC-9 EV charging hardware for condos and apartments.
dccelectric.com
There are others. This would be "fettered" EV charging!
11. The Tesla Backup Gateway 2 is capable of being configured to be Service Equipment assuming your local POCO and AHJ will allow it. This would allow you to simply install a new 200 amp meter socket and feed the gateway direct from the meter, in which a 200 amp "main" circuit breaker would be installed. The Gateway would become the main disconnect. See page 25 of this manual:
This is how it would be wired:
12. I cannot imagine why Tesla would not want to install such a configuration. Initially it could be installed and fed from your 100 amp meter main, and later you could replace your meter main with just a new 200 amp rated meter socket (if allowed) or a 200 amp meter main (if required). Since your existing meter-main (and its replacement) is before all the Tesla stuff, you should be able to find an electrician who would do such a job. This should have been seen as an option when you had your site survey.
Let us all know what you end up getting, and good luck.