[RE-wrenches] Backing up a 400A main with two 200-amp main panels: separate or combine with more complexity and use external ATS?

[Jason Szumlanski]

If you can actually run the whole house on two Sol-Ark 15K (100A combined
output), realistically, they probably don't need 400A service. Have you
done a load calc on the home? We see lots of 400A services where they could
have "gotten away" with a 200A service.

In these cases, I always encourage the homeowner to just back up one of the
two 200A panels and move all critical loads to that panel. If you need more
spaces, add a subpanel. If they have 400A service but only need 200A
service and demand truly whole-house backup, simply disconnect one of the
panels from the utility and instead feed it from the backed up 200A panel
as a subpanel. There's nothing that says you can't have a 400A meter
feeding a 200A main panel protected by a 200A main breaker, and use the
other one as a subpanel, as long as the load calcs work out. You will just
have to separate neutrals and grounds and remove the bonding jumper.
Depending on the panel brand, you can usually get a 200A breaker or
feed-through kit that takes up four spaces.

Even if the load calc for the home comes back over 200A, you can probably
easily get the backup panel under 200A of calculated loads and just back
that up. It makes no sense to try to back up a whole home that needs over
200A service unless you are installing a massive system, which would be
more than two Sol-Arks can handle. We encourage people to choose
non-critical loads to leave out, like electric dryers, pool heaters, pool
pumps (although this is nice to have on a smart load), EV chargers,
electric ranges and cooktops, outdoor kitchens, hot tubs, electric saunas,
etc. This should easily get the backup panel under 200A. If you present it
as a value engineering option, it's relatively easy to get people to move
away from their "whole home or nothing" mentality.

With that said, if I am truly backing up a 400A service here in Florida,
I'm installing at least four 15K inverters, 35kW of PV, and 120 kWh of
battery capacity. And I'm still dumping EV chargers and pool heaters during
a utility outage. I would put a 400A main service disconnect ahead of the
inverters and install a 400A DPDT transfer switch for bypass purposes.
Auxiliary generator input goes directly to the Sol-Arks, unless the owner
wants to spring for another DPDT transfer switch to send generator power
directly to the house in the event of grid and inverter failure.


Jason Szumlanski
Principal Solar Designer | Florida Solar Design Group
NABCEP Certified Solar Professional (PVIP)
Florida State Certified Solar Contractor CVC56956
Florida Certified Electrical Contractor EC13013208


On Fri, Jun 27, 2025 at 3:06 PM scot.arey--- via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Seeking your feedback and best practices.
>
>
> We have a existing customer who wants to add ESS for whole house backup.
> He has a 400-amp meter box with two 200-amp panels on each side of the
> meter can. We want to use Sol-Ark and HomeGrid which is our go-to solution.
> Today, we have two 10KW ground mounts feeding a grid-tied system with
> Fronius inverters aggregated at the array with single set of conductors
> back to the house and meter
>
> Option 1: Add a SolArk 15KW and HomeGrid '4-stack' per each 200-amp
> panel and in essence have two separate systems each backing up their own
> panelboard. Seems easy but I know the panels are not totally balanced and
> this seems like I'm having excess capacity at the panel that powers
> circuits to run less than all the time.
>
> Brings me to option 2:
>
> Install two paralleled SolArks with two batteries again (and now a
> battery combiner) and add a 400-amp transfer switch and take grid output
> from meter and bring to ATS first. The GEN inputs on ATS will be fed by
> aggregated LOAD outputs of the SolArks so if grid fails, the ATS "sees"
> 240V and transfers to the inverters only  and opens circuit to grid. When
> grid power is there, all is grid-tied and bi-directional. I've done similar
> setup with Outback Radian and ASCO 200A ATS for whole house backup.
>
> But this seems really complicated with lots of extra boxes.
>
> Take the simple-easy route even if one inverter is a bit too much for one
> of the 200-amp panels? Ot aggregate and use ATS so we can put two SolArks
> to work together to address full load as one operating output? Wall space
> for this is pretty good for both options.
>
> *Howard "Scot" Arey*
> *Owner, Solar CenTex*
>
> NABCEP PV Installation Professional
> TECL 29755
> 254-300-1228
>
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