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

Sun Jun 29 16:55:53 PDT 2025

I second the idea of just backing up one of the 200 A panels, and moving 
critical loads there.  The trouble I've seen with total backup, is that 
the customers are not even aware that the grid went down, and make no 
effort to curtail their usage.  Then, once the batteries are down, they 
are truly off line.  I've had customers even compliment our choice of 
circuits; they knew the power was out because certain things went off, 
but still had a very usable home. Also, on larger homes there are all 
sorts of loads like steam showers, etc. that really don't need to be on, 
but will drain the batteries quickly in an outage.

Finally, unless power outages are common, a GTB system can make the 
power less reliable, not better.  I quit doing them, unless folks had 
numerous outages, like in Puerto Rico.

Ray Walters
Remote Solar

On 6/29/2025 12:35 PM, Jason Szumlanski via RE-wrenches wrote:
> 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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