diy solar

diy solar

ATS for grid-tied solar with battery and portable generator


New Member
Jun 26, 2020
I'm trying to add battery storage (highlighted in yellow on the screenshot below) to my existing grid-tied solar installation with the primary goal to mitigate demand fees during peak time.

You can see that discussion in a separate topic:

SRP (the utility company) has this diagram for cases similar to mine:

They require the ATS (Automatic Transfer Switch) to be installed, preventing backfeed to the grid from Solar or battery storage.

The requirements for the ATS are the following:

Based on them it looks like any ATS on the market certified with UL-1008 would do that job by disconnecting the DER system from the grid if the latter goes down. E.g. "Generac RXW200AZ."

But I want also to be able to hook up a portable generator to that ATS in case of emergencies. E.g. long dark rainy nights with power outages, no sun, and depleted battery bank.

I read the documentation for "Generac RXW200AZ." It requires a "Generac" brand name generator ($$$) and will NOT support a "no-name" portable generator that I have. While it might disconnect my DER system from the grid during a power outage (meeting the SRP requirements), it will NOT activate the generator input even if the voltage from the generator has been detected unless it's a "Generac" brand.

I look at the other brands and models of ATS. All of them (as I found on the market so far) would require a generator matching their brand.

My questions to the community in this forum:

Which brand/model of the ATS could meet my needs?

The criteria are the following:
  • 240V/200A single phase
  • UL-1008 certified
  • Automatically disconnects from the grid if the latter goes down before reconnecting to the generator (open transition "break-before-make")
  • The generator connection in the ATS should activate by detection of the voltage in the generator inlet without communication to the generator. Or it can be done automatically after disconnection from the failed grid waiting for the generator to be started manually
I understand that the latter two criteria would be better met with a manual transfer switch. But, unfortunately, it has to be ATS to meet the utility requirements for safety.
That link wouldn't give an answer to my question. It points to the Website of another solar installer company, which I'm absolutely NOT interested in.

I'm expanding my existing interconnected DIY system and would like to find a make/model of the ATS, compatible with no-name generators providing no interface with the ATS.
After doing some research, I believe that the ATS Cummins RA212N3 (aka RA200NSE) can do my job by adding a simple 12V AC/DC adapter to its entrance lines.

Cummins has great documentation describing how their ATS works on pages 35-36:

When the portable generator starts, the AC/DC adapter is energized and sends a 12V DC "Generator ready" signal to the relay K1. It will flip the actuator in the ATS to the "Generator" position, thus connecting loads to the generator.

When the utility comes back, the ATS will disconnect the generator. The latter will keep running idle until I manually quench it.
This is the suggested wiring. Any concerns about it?


Can somebody suggest a better make/model of the ATS for my needs without that hacking?
I just realized that if the Grid power fails, the DER system with battery and solar (as shown on my first posting here) keeps back-feeding the ATS via the lines L1,L2,N, ("Loads") it might never disconnect the DER from the utility. That's a dangerous situation the ATS is intended to avoid.

Are all ATS the same? How do they work with backfeeding them from the "Load" side when the grid is down?