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diy solar

Adding removable battery storage to existing grid-tied residential system?


New Member
Mar 8, 2020
Oakland, CA
I have an existing 4kW residential rooftop PV system - with a SMA Sunny Boy 4000TL inverter, grid-tied. California, fully permitted.

I've got experience/understanding in off-grid solar, and am building a semi-portable server-rack rig (in a cargo trailer) ~20kWh. Currently debating EG4/SOK batteries, and likely ~6-8 kW inverter tbd.

For the 90% of days where the trailer is parked in my driveway - I'm wondering what my options are to roll the battery rack into my garage and tie into my home PV system (via an Anderson plug or similar?) - more for peak load shaving than outages. Do I need a AT switch and/or Sunny Island alongside the Sunny Boy? Or would replacing the inverter entirely with a EG4 or hybrid inverter (that communicates easily with the batteries) make more sense? (the SMA is ~9 years old). I don't know anything on the on-grid/permitting side of things - but would like to do this by the book.

Appreciate any guidance - especially as I look to purchase batteries soon, and want to make sure I am future-proofed so I could roll them into a grid-tie usage.
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See the attached manual for details on how existing solar is AC coupled to this Hoymiles Hybrid inverter. I'm working on installing one on my existing solar now, AC coupled. The AC power meter is included with the inverter to do export control.


  • HYS-LV-USG1-Series_Datasheet_EN_V1.0.pdf
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  • HYS-LV-USG1-Series_User-Manual_EN_V1.0.pdf
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  • 1-Line_Sample_Hoymiles_AC_Coupled.pdf
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Nothing temporary / with rolling batteries is going to be considered "by the book". So it's by degrees of how off the book you are willing to accept?

Since you have a permit that allows exporting, your easiest option would be to just connect a GTIL up to your batteries and run an AC extension cord to an outlet of your choosing to feedback into the system. You could use a dedicated circuit if you want to keep it safer and more respectable than an extension cord. Depending on what else is on the circuit, and where the plug you use is on that circuit, you could overload the wiring. If you are sure nothing else is on the circuit, then it's not particularly unsafe and requires no modifications.

The most popular unapproved 48vdc GTIL would be this one. You probably don't need the limiter function of it, as you are permitted on your other system. The limiter would keep you from exporting (for the most part, not 100%) from the GTIL.. supporting only local loads. If you don't use the limiter, it will just export full blast, just like your existing system probably does.

I suggest two of them at 120vac, one per leg. Some others like to use the 240vac models instead of the 120vac models.. but it requires modifying the CT's that come with them / buying another CT. If you only do 120vac, you can start with one and just offset some of your 120vac usage on that leg and see if you like it.

When connecting to a battery system, it may be necessary to consider the use of an AT switch (automatic switching switch) to ensure safe switching to battery power in the event of a power outage. In addition, if using equipment that can work with the battery system, such as Sunny Island, it may be necessary to configure it appropriately as needed.