Adding Batteries and Generator to Sunny Boy 4000 Grid-Tie System

rimcanyonrimcanyon Registered Users Posts: 5 ✭✭
I’m looking for suggestions how to add battery backup and a natural gas fueled generator to an existing grid-tied system that uses a Sunny Boy 4000TL inverter and (12) Sunpower X-21 345W panels.

The grid-tie for the existing system is in a 100A sub-panel that supplies the house.  OTOH, I would prefer that the generator’s transfer switch be attached at the 200A service entrance panel, which has three circuits (feeding house, shop and guest house subpanels).  That way I can get power to the shop during an outage, which is currently not possible.  So the first question I have is whether or not this topology is even possible, since the grid-tie is through one of those circuits managed by the transfer switch.  The issue being that the inverter will back-feed the generator when both are running.

I can run communication cables between the transfer switch and the inverter if there are compatible products that would allow this setup.

My ultimate goal is to have an automated system, where all three components can coexist with reasonable behaviors.  However, I would accept some compromises, such as having to turn the generator on manually when solar and battery storage are at zero.


  • BB.BB. Super Moderators, Administrators Posts: 31,685 admin
    Welcome to the forum Rimcanyon,

    You are asking some pretty tough questions... In general, generators (really AC Alternators) do not like to be back find power from a GT AC Inverter (or other sources). So, it is best never get into that situation (could damage your alternator/AVR/etc.).

    Yes, some GT inverters (more now?) can be controlled with a remote on/off input. But this may not be straight forward. The SMA 4000TL manual:

    Page 15:
    4.2 Interfaces and Functions

    The inverter can be equipped or retrofitted with the following interfaces and functions: BLUETOOTH Via BLUETOOTH, the inverter can communicate with various BLUETOOTH devices (for information on supported SMA products, see

    SMA Speedwire/Webconnect

    The inverter is equipped with SMA Speedwire/Webconnect as standard. SMA Speedwire/ Webconnect is a type of communication based on the Ethernet standard. This enables inverteroptimized 10/100 Mbit data transmission between Speedwire devices in PV systems and the software Sunny Explorer. The Webconnect function enables direct data transmission between the inverters of a small-scale system and the Internet portal Sunny Portal without any additional communication device and for a maximum of four inverters per Sunny Portal system. In large-scale PV power plants, data transmission to the Internet portal Sunny Portal is carried out via the SMA Cluster Controller. You can access your Sunny Portal system from any computer with an Internet connection.

    SMA Speedwire/Webconnect enables, for PV systems operated in Italy, the connection to or disconnection of the inverter from the utility grid and definition of the frequency limits to be used with IEC61850-GOOSE messages.

    RS485 interface

    The inverter can communicate via cables with special SMA communication products via the RS485 interface (information on supported SMA products at The RS485 interface can be retrofitted. Grid Management Services The inverter is equipped with service functions for grid management. Depending on the requirements of the grid operator, you can activate and configure the functions (e.g. active power limitation) via operating parameters.

    SMA Power Control Module

    The SMA Power Control Module enables the inverter to implement grid management services and is equipped with an additional multifunction relay (for information on installation and configuration, see the installation manual of the SMA Power Control Module). The SMA Power Control Module can be retrofitted.

    Multifunction Relay

    You can configure the multifunction relay for various operating modes. The multifunction relay is used, for example, to switch fault indicators on or off (for information on installation and configuration, see the installation manual of the multifunction relay). The multifunction relay can be retrofitted.
    Which (if any) of the above can be used for remote shutdown... I don't know. And not sure if you want to use a small computer (Raspberry Pi or similar) to setup RS485 Communications to do the coordination (software/hardware failure and failed shutdown).

    The typical method would be to have the first panel after the meter connect to your GT inverter and the a second panel (if whole house generator)... Or to bring out a branch circuit (i.e., maybe 30 amps at 120/240) to a transfer switch then to a "protected circuits" sub-panel. That would let you decide what stays in the main panel (not protected power) vs in the sub-panel (protected power from genset and/or AC inverter).

    Given that solar power systems are expensive--Generally you want a second sub-panel just for solar (i.e., first sub-panel with 7.5 to 10 kWatt genset, and second sub-panel for a 2 kWatt AC inverter just to power the refrigerator, some lights, cell phone chargers, laptop, etc.

    And--There is the old argument of keeping it simple. Avoiding automatic start generator systems. There is a lot that can go wrong if nobody is home (crank till dead, shutdown from overheating, no oil, or decides to start when not needed and uses up all your fuel, etc.). Many wild and wonderful way things can fail. Manual generator start can make things less crazy when failures eventually happen.

    If you do not have many outages (may a couple days a few days a year)--A genset may be the better backup power solution. You can have a small solar system for lights, laptop, radio, tv of 300-1,000 Watts--But when you add a refrigerator, you are looking at a much larger/more expensive solar power system.

    Regarding generators and GT Inverters--There is a school of thought about not worrying about connecting the two together. Generally GT inverters have very tight frequency requirements (something like +/- 0.5 to 1.0 Hz).

    If you have a standard mechanically govern generator, most of the time they cannot hold that tight of frequency for the 5 minutes that are needed to for the GT Inverter to "qualify" the frequency (and line voltage, etc.).

    Another thing to watch for--How loud is the generator. You don't want to keep your neighbors up at night if you houses are on 50x100 foot lots.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rimcanyonrimcanyon Registered Users Posts: 5 ✭✭
    Bill, I much appreciate the welcome and overview.  I’m not interested in doing a homebrew software project.  I’m a retired SW engr and at this point in my life I want to assemble a system that works and has a lot of resilience and dependability.  As you implied, the integration of a generator into the system is the hard part.  I may have to do it manually, i.e. use a transfer switch to change my home wiring topology around and provide a way for the generator to take control of specific circuits, isolated from the solar and battery circuits.  I think I will leave that problem until later, and just work on battery backup for now, but with an eye towards an eventual solution.  I also contacted a couple of the big companies that supply whole house generators, and they said they do not have a solution.  Its a strange situation, because grid-tied generators are well-understood and have been for 100 years or more.

    I have been looking at other SMA products, and the combination of Sunny Boy Storage and the SMA Automatic Backup Unit appear to fit my needs for battery backup, except they don’t support the topology of my existing installation, because the Sunny Boy 4000TL inverter would have its output plugged directly into the ABU, and it would be controlled by AC coupling.  So I do not see a way to add the ABU at the service entrance, it would need to be co-located with the SBS and the PV Inverter at the house subpanel.

  • BB.BB. Super Moderators, Administrators Posts: 31,685 admin
    You are very welcome.

    If you have a limited number of circuits that you want to have a manual (or possibly even make automatic), a simple 6/10/more(?) manual transfer switch. Basically you put the wire from xfer switch 1 to the Circuit Breaker output. And take the output wire from xfer sw1 to the disconnected breaker wire. Do this for each circuit you want to have alternate power (genset, inverter) you want to power. And example of some (relatively inexpensive and simple) manual transfer switches.

    The other connection is a socket/cable that you connect to your genset or AC inverter... If you want to further "automate", you can use a transfer switch to "power" the manual 6/10 circuit xfer panel. Leave the xfer panel in "bypass". And then the auto transfer switch will use Mains or Alternate Power (genset/inverter/etc.) to decide which power your "protected" circuits need.

    You can use a simple relay transfer switch too (start the genset, the transfer switch goes to generator AC source): (this is 30 amps of AC, they also have a 60 amp version too):

    It is a pretty easy setup and better than other "backyard DIY" options. And you don't need to install a new sub-panel and rewire all those circuits from the main panel to the sub panel.

    If course there are other options out there too... And more details to discuss (size of the backup system, PSW or MSW inverters, etc.). There are "interlocks" that can be installed in main/subpanels (interlock lets you choose between source A or Source B, but prevent you from turning on both A&B at the same time, etc.).

    In this world, details matter. And the solution should meet your needs (and wallet).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,912 ✭✭✭✭✭
    edited August 2020 #5
    The sunny storage inverter, the sunny island, has a gen input. I don't use this equipment (I am Schneider solar installer/dealer) but I would think that it is smart enough to disconnect the genset when the grid comes back. Ask SMA !  If you want, after you try and get an answer from them, I can post this to my grid bud installers. And yes details do matter  ;)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • rimcanyonrimcanyon Registered Users Posts: 5 ✭✭
    edited August 2020 #6
    Dave and Bill, thanks for your replies.

    There is one detail that needs more explanation, because this is the problem I am trying to solve.

    My main panel, where I want to connect the generator, has three circuits only.  Each is 100A and the panel is protected by a 200A circuit breaker, because I will never use over 200A, even with everything turned on.  I have not seen a transfer switch rated for multiple 100A connections.  I.e. when the transfer switch is switched to utility power, the ampacity of each circuit needs to be 100A, but when it is switched to generator power, the input will only be 30A for both circuits combined.  I did find these single circuit transfer switches that come close to my needs (I could use two or three of these):

    They would be positioned between the service entrance panel and the feed to each subpanel, with a connection to the generator and the utility power.

    Most likely I would leave the house subpanel connection unmolested, because that is where the solar and battery connection would be made and I don’t have a solution for combining solar, battery and generator in the same circuit.  That way the house connection would always be connected to the grid.  So for now, the generator connection would be to power the shop and guest house, and the house would be solar + battery.

    I have no experience with this product, so I don’t know what the quality is.

    Dave, thank you for the information about Sunny Island.  I currently like Sunny Boy Storage because it accepts higher voltage batteries (three connections of 10A at 550V) while Sunny Island requires 48V batteries, but I did not know Sunny Island had a generator input.  Will investigate further.

  • Dave AngeliniDave Angelini Solar Expert Posts: 5,912 ✭✭✭✭✭
    Hi Dave,

    Keep a wary eye on 550v batteries. If this is residential and you have home insurance, you may not be covered.

    It is not just the lithium cell that must meet UL standards BTW. The whole battery must meet UL 1973 and UL 9540. Just went thru this with a local AHJ.

    Many that I have helped to do what you want, avoid all of the sub panel and hassle by adapting an offgrid mentality to the loads.
    They get rid of electric ranges, dryers and hot water heaters. Pumping, Cooling and heating is by soft start variable speed. 

    In this strategy they can power the existing panel with the typical 60 A AC coupled XW or Radian 7kw inverters. They stack more inverters if needed. It very rarely is needed if done right for residental. These inverters also have amazing surge capability that high voltage batteries can't supply. These inverters are split phase and do not need the ridiculous Sunny transformer. The last I looked, some of SMA electronics for battery were not listed in North America. Again I do not stay up to speed on SMA but I know the TL series work fine with XW for AC coupled.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • rimcanyonrimcanyon Registered Users Posts: 5 ✭✭
    I found this document to be quite useful in understanding how a Sunny Island battery inverter integrates with and controls a generator.  It discusses the generator requirements to work with Sunny Island:
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