[RE-wrenches] Adding Battery to Grid Tie

[Jason Szumlanski]

One thing nobody has mentioned is rapid shutdown. If you have an existing
DC system with no RSD system (older installation), most AHJs will likely
require you to upgrade to RSD if you are changing the way the system works
by installing a new inverter, especially if you are rewiring the strings.
That adds a cost and complexity that suggests AC coupling to the existing
system would be better. If you aren't touching the existing system on the
DC side, there is a strong argument that you don't need to bring it up to
RSD compliance.

Certainly, if there are microinverters or power optimizers on the roof, the
path of least resistance is AC Coupling.

All of the manufacturers are moving in that direction, probably because
there is a considerable market for Enphase and SolarEdge systems installed
in the last 10 years that could have batteries added to them.

I am finding fewer and fewer systems every year where it makes sense to add
batteries and DC couple the existing panels. Frankly, if that was an option
in recent years, there has usually been an argument to "start over,"
especially with the ability to get a 30% tax credit and new warranty. With
the tax credit going away, that will change the calculus for do-overs.


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 Sat, Aug 9, 2025 at 3:10 PM Zeke Yewdall via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> There's two main ways to do it.  Either keep it as is and AC couple it
> into the new battery system, or commandeer the DC from the PV array and
> feed into a new battery system.
>
> While AC coupling is common for a lot of the options -- Franklin, Tesla,
> many others, and just about all of the AIOs, I don't like it much if I can
> avoid it. Usually less control options if you get into limited or zero sell
> back situations instead of net metering, compared to a DC coupled solution,
> and difficulty with black start.  If you already have embedded
> microinverters on the roof, it's probably the best option though.   I think
> the SolArk can choose either AC coupled PV array or generator input, but
> not both.  Midnite can do both, and not sure about EG4.
>
> Most of the AIO's can take a 600 volt string inverter array into their DC
> inputs, and just get rid of the old string inverter.  Just make sure the
> old array doesn't have existing ground faults -- many old inverters were
> pretty blind to ground faults on the negative leg especially, and the AIO's
> running a floating array will not be happy with multiple faults in old
> arrays.
>
> There are also a bunch of battery capable string inverters from Fronius,
> SMA out now... typically pretty poor surge and imbalance performance from
> them, so if you are doing backup for well pumps or septic pumps, it's
> something to consider.
>
> Zeke
>
> --
> Zeke Yewdall
> PV Engineer
> NABCEP #031508-89
> zeke at darkforestsolar.com
> 303-523-3592
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