Spaces vs Circuits
First let's explain the 8/16 thing, or "8 spaces but 16 circuits". At some point, panel manufacturers developed what I (alone) call a "double-stuff" breaker, which crams 2 breakers into a single space. This is intended to solve crowding problems in too-small panels.
photo source: Home Depot
Relying on these, marketing called their boxes 2X-circuit (X-space). Around 2014 they mysteriously stopped doing that. Recent NEC changes required AFCI or GFCI breakers on damn near every circuit... AFCI and GFCI are not made in double-stuff breakers.
Your 16/8 service panel is a legacy of that bad labeling. You have an 8-space panel, intended to replace an 8-space fuse box.
Panel space is dirt cheap. Go nuts.
You are dreaming of a 16-space panel. And even that is dreadfully small and you will find it rather limiting even with 100A service.
The practice of many electricians is to give you the smallest panel possible. That's first, to force you into more subpanel work if you expand further (more money for them) and second to save themselves $20 in part cost.
The cost of a larger panel is trivial tiny compared to total project cost. It is very correct to be extreme. Please spend an extra $100 for a 42-space panel, unless you just can't make it fit. Honestly I'd go 60-space. Not least, those larger panels are 225A-ready.
You never know. You might do a kitchen remodel and go hey, I'd like a separate circuit for refrigerator done microwave done dishwasher done disposal done 3 outlet circuits done. You have the space.
When you buy a PC, do you really calculate your needed hard drive space and buy just what you need... Or do you just buy the big 3TB drive? Of course you do. Same thing here.
More work to do at the meter
OK so the power company says their side of the cable is 320A. If they say so.
They say their meter is 320A. If they say so.
But your meter housing is not 320A.
To support more than the listed 125A, you will need to upgrade your meter housing. There's no question of that. That's your equipment and you have to pay for it, probably, unless your power company does something different with cost sharing.
You won't be able to replace parts of it, you will probably even have to replace both sides of it as a single unit. However it is very nice to have your main breaker in the meter. It means you can fully de-energize your main panel, which makes it safer for you to do yourself.
The 320A may be shared
Since it's multi-unit, the power company provisioning 320A makes more sense. Trouble is, this 320A service is probably shared between both units. So it's not as much as you think.
You really need to talk to your power company about what they consider this "320A" to be. It may be a relabeling of what other power companies call 400A service. So they may be willing to feed two 200A panels, or 100/100/200. This is a conversation you can have only with them.
More food for thought is sub-metering units, and separately metering common spaces. The new thing in rental properties is to have one main meter and the landlord has sub-meters per tenant. Many landlords prefer (and some law requires) commons space usage to be on a separate (sub) meter, i.e. heating, yard and commons lighting, anti-freeze pipe wrap or roof/gutter heat, coin laundry, Christmas decorations etc.
Main breaker
There's no problem with having a main breaker in a subpanel. It's required if your panel is in a separate building and has more than six breakers, but there's nothing prohibiting it in any other case.
Wire size
You need to ensure the wire between the panels is sufficient size. See What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run?
Neutral-ground bond
You need to ensure the only bond between ground and neutral is in the main panel. The subpanel must have separate ground and neutral bus bars. Since your panel is used a main panel now, it will currently have a bond between ground and neutral, so that'll have to be removed.
- It's usually a metal bar or connector that screws between the two bus bars, or between the cabinet and neutral bar.
- If you currently have neutrals and grounds mixed on both bus bars, you'll have to separate them out
- If you have only a single bus bar (with grounds and neutrals mixed) you'll have to add a neutral bar (which is not bonded to the cabinet). Check with manufacturer first to see if they have a documented way of doing this. If not, I'd suggest you check with your AHJ (inspector) first to be sure whatever you do will be acceptable.
Best Answer
Yeah, that's a thing. The problem is you have potentially two energy sources feeding the panel: The 200A main breaker and the 60A Tesla PowerWall. That's 260A on a breaker bus that is rated either 200A or 225A - I suspect the former, though the label will say for sure.
Now, they give you a mulligan for 20% of the panel ampacity. So they'll let you slide up to 240A on that 200A busing. But you need 260A so that is not enough.
So, one option is to derate whatever breaker happens to supply this panel. Now you have 175A + 60A = 235A = you're inside 240A.
Another way is to go talk to the dealer who sells those lines of panel. They put a large variety of panels on a limited number of panel box sizes. The goal is to find a panel with 225A-rated busing, where the bus assembly will bolt up to the same mounting holes that are in your panel.
Now you'll have 225A x 120% = 270A, which covers your 260A.