undersize inverters?

[Ampster]

It is your money, but if it was my money I would want to know the details. What assumptions are being made in terms of the load calculations? I agree with @nwdriver about obvious ways to reduce power in terms of the load calculations that the builder is assuming.

He may be assuming that you are going to charge two cars at the highest speed on the hottest day of the year when your pool pump is running and your wife is cooking in both ovens while using the electric clothes dryer and every bathroom has someone blow drying their hair. Oh, and you have 100 Watt incandescent bulbs in every light fixture and all the lights are on and you are charging your golf cart.
Or it could be a combination of the busbar rating because you mentioned you wanted the inverters upgraded.

 

[Laketime]

Our builder said we are very close to maxing out 400amp service and there is no option for 600amp service so the next step up is 800amp service.

I've been debating backing up two 200amp panels but then I'd need at least 6 power walls because I don't think 2 power walls each would have enough output power (I think 5kw per pw). Thoughts?

Typically there will be 200amps per 5000sq ft. for a loaded luxury home. I've even seen 6000ft homes with 5 CACs get by with a single 200amp service. I would ask what is going on with construction for you to need such a massive power supply. You should be concerned because if you are drawing that much power your bill will be in the $1000's and power walls will have to be numerous to just get a few hours of backup.

 

[jrweiss98020]

It all depends on what is on each of the panels, and how much of that you are actually using all/most of the time. I have 2 PW on a 200A service, but the base load for the entire house is around 0.5 KW. Unless I'm using the dryer or oven or charging the car (which I would NOT when the grid is out), typical max load is ~2.5 KW.

Yes, each PW can put out 5KW continuous. Tesla also recommends you have 1 PW for each 7 KW of PV output, so the PW can absorb all of it when necessary. So, 6 PW is not out of line for 35 KW of PV.

 

[Ampster]

So, 6 PW is not out of line for 35 KW of PV.

I would much rather put my money into more panels, and another Powerwall which would give me more resiliency than into unneeded grid capacity. Additional subpanels to accommodate that would be a lot more cost effective than 600 Amp service.
I have spent so much time around construction contracts and contractors that I think it is always useful to have a dialogue about cost versus benefits anytime changes are proposed. Cost engineering is also a valuable process to get projects to the finish line with the best value. In home convenience. aesthetics and Wife Acceptance Factor are more important.