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

[Chris Sparadeo]

Hi Scot,

In my experience backing up a 400A service with two Sol-Ark 15K inverters,
it's critical to use a single battery bank and to parallel both the
inverter inputs and outputs. I initially attempted to configure two
interdependent Sol-Arks to work together, but encountered monitoring
communication issues and was informed by Sol-Ark tech support that data
would be skewed if the inverteres were on the same utility transformer and
were not run in parallel. Hope this helps.

Best,

Chris

On Fri, Jun 27, 2025 at 3:05 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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