A licensed electrician will know the answers to your questions. If they don't, don't hire them. There are established formulas for determining load. Most modern homes are just fine on 200A. Remember, you don't use everything at once. It's certainly possible to run out of room for more breakers, though. Modern codes call for a lot of separate circuits, as opposed to the old days, when they combined all sorts of things, and fuses blew. Our small house has 28 circuits, with 8 in the kitchen alone (not including lighting). If it turns out that you need more total capacity, it may involve new feeder cables into the house, a new meter, and a bigger panel. This adds up to serious money. If 200A is enough, but you're out of space for more breakers, they can just add a subpanel- considerably cheaper.

In principle, an electrician should be able to take a list you provide of all of the electrical devices in the house (including in this case future devices) and determine the peak load and a breaker box architecture that best fits it. 200A at 240Vac is 48 kW; that is a lot of power to be using at once in a house of this size.

Given such a list, you could add up power loads likely to be operational together to get an idea. There are rules in the National Electrical Code (NFPA 70) for estimating likely power drain of multiple circuits that might be on or off, but you will know best what is your worst case light and appliance usage.

Solar panels provide power to the house, and perhaps in reverse to the power company, but should not in themselves increase the house's power draw from the power company.

So there are two factors at play here. One is the amperage size required for the electrical service at the power entry; the other is the number of breakers needed/desired to provide the degree of granular control over circuits that you want to have.

I have four main breaker boxes and an auxiliary box for load shedding when my generator is running. I likely have (never counted) well over 100 breakers depending on how you count poles. I'm sure that they add up to hundreds of amperes. But my service is only 200A. My town's former electrical inspector once bet me that I couldn't actually draw more than 120A, as I recall, and while I probably could have done so, it would have required an unusual combination of device operations that would not normally occur in the same season plus oven and cooktop power usage that would be extremely improbable under normal circumstances.

In one architecture you could have, the main entry box can have two existing breakers replaced with a 100A breaker to power another breaker box. This second box gets the two breakers you removed from the entry box plus other new breakers. This can be expanded to multiples and auxiliary boxes can proliferate to your heart's content.

Another valid architecture is to split the service entry after a single point of service disconnect (a generator transfer switch might be one example) into a set of secondary circuits, one to each breaker protected breaker box.

Difficulties arise finding room for breaker boxes, rerouting cables or conduits to breaker boxes without splicing, and, in general, assuring compliance with NFPA 70. It is more difficult and messy to do this repeatedly over time as the house is revised than doing it initially. Also, the homeowner usually would like to not lose electricity for an inordinate amount of time while the electrical system is reconfigured.

Perhaps your question to the electrician was too vague for him to specifically address, or perhaps he wasn't the right person to deal with the complexity.

P.S. When I wrote the above, I hadn't noticed the car charger detail. I have no data on these nor do I know if the charger is to run only when solar power is being generated, or at night when other circuits may be in use. You truly need to use a spread sheet to determine power usage, both draw and generation, to see what the peak current is that flows into your house under various assumptions about device usage.

The utility company supplying your power may or may not have to revise the feed to the household. Usually, electrical lines that go from pole to house in the air can be rated higher than one might expect from the NEC tables for interior wiring. (A utility tech once told me that they had "special dispensation.") But from the meter onward your wiring may need to be upgraded, and this is why I mentioned the second architecture. It may reduce the amount of high current wiring that has to be changed. The next time you question an electrician, have some photographs handy of your present service entry situation and breaker box installation, along with your spread sheet.

Adding a sub-panel does not add capacity, only the opportunity to add more circuits. Important : refer to my post and ask your electrician about your solar feed to your current bus.

Panels mounted to naked garage or basement walls are easier to deal with than those embedded in sheetrock and studs. In the latter case there can be a lot of "demolition" needed to move cables about and put in additional panels, depending on how it is done. For finished sheetrock, you or someone skilled in the art will have to deal with the sheetrock refinishing.

"My contractor told me that to add a subpanel would require them opening walls and rewiring and would be the most expensive option." - What were the other options?

"You said you have a lot of solar panels. Your solar power produced my require you to de-rate your main bus." - You do not de-rate the main bus. The bus has the same rating regardless of the setup. The total of the Main breaker and the PV breaker can not be more than 240 amps (120%). That would more likely suggest (economically) changing the Main breaker to a 175 amp breaker if the PV breaker is more than 40 amps (and less than 65).

"That with maximum existing capacity would suggest an upgrade to 400amps service." - Often, a 400 amp service is actually 2- 200 amp panels. The rating of the bus bars would not change. If the PV installation was done in accordance to NEC 705.12 (D)(2) to begin with, adding more load breakers such as hot tubs or car chargers will not change that, nor adding a subpanels , or even another 200 amp main and CT can... But, apparently Sindy found a service load calculation explanation, and it is only 123 amps...

Fred: The other option is to upgrade from a 200 amp to a 400 amp electrical panel which would have more breakers.

For adding subpanel option: My contractor said that the subpanel has to go in the laundry room and he has to rewire a few rooms of electrical wires to that subpanel and open up the walls. He also said people who are lazy would do the subpanel because it's faster. And he cannot put the subpanel in the garage or outside where the main electrical panel is. I am pretty confused by what he's telling me.

Thanks for your insight on the PV. Looks like the PV breakers are in the main electrical panel and takes up a bunch of slots in there, which is why we don't have any left over for the last few items, ugh.

"Now all the breakers are used up but we need to add a few more lights in
the kitchen and bathrooms, power for the jet tub, yard lights, and an
electrical car charger." I don't see how you would escape opening up walls anyway, unless you have a basement with easy access to the bottom plates of walls and free passage of cables to the breaker box.

Having a breaker box on the outside of the house is unusual in my experience. Does the area have no rain ever and no vandals or practical jokers? Is it ever unpleasantly cold out there for a breaker resetting exercise? Is this configuration related to having someone else's solar panels (e.g., Elon Musk's) on your roof making power for the neighborhood and easy exterior disconnection means is needed for the panels?

In my area, outdoor panels are common (perhaps every house?). Solar fed into the outdoor panel. Indoors - subpanels where majority of breakers are. It rains here and it is 20 degrees right now. Strangely - no joker has ever shut off my house. I have had to go out in the cold and flip a breaker - it was my own fault and not a common occurrence. But the electricians did add a forgotten (during construction) circuit to the outside panel but it never tripped in the 8 years I was there. The HVAC breakers were on outdoor panels - and they were right next to the outdoor units.

This thread may be a helpful reminder for people to go with 400 A service in a new house. Electric cars are coming - don't limit yourself or come up with 10 times the cost later. You certainly don't need that much juice for an electric car but if you have 2 and solar, it makes for a lot of flexibility - like very fast charging.

The only issue I can remember with solar is that you are limited in number of panels (and thus generating capacity) by your total service connection. Even though solar is actually reducing the electricity you use, it is increasing the electricity going through the panel.