AC Coupling – Adding A Battery Back-up Package To Your Existing Grid-tie System

Buyer’s Remorse.  “Why, oh, why didn’t I buy a battery backup PV system?” Or, “I hate looking at my new PV system shut off when the grid’s down.”

You may already have a fabulous grid-tied solar system installed on your house, making power when the sun is shining, and turning back your meter. That is absolutely fabulous! Grid-tied systems are the most cost effective and easiest systems to install. Until the day the grid goes down, and your house is without power while your neighbor with the loud, smelly generator is sitting in their house drinking frozen margaritas and watching TV. What to do now? No, you can’t run an extension cord to your neighbor’s house and hope they don’t notice. Electrical code requires that our grid-tied system shut down if the grid goes out to prevent accidentally electrocuting the linemen while they are working on a power lines to restore power.

You may think you are locked into the option you bought, or have to get rid of a lot of your equipment and start over.  Not so. There is an increasingly popular configuration called AC Coupling.  AC Coupling uses your existing system to feed into a grid-tied, battery backup (GTBB) inverter/charger to charge the battery bank. Here’s how it works.  You keep your existing system exactly as is, except you add a GTBB backup inverter/charger and battery bank.  The battery bank doesn’t need to be huge, just large enough to run your critical loads needed during a power outage; the fridge, well pump, furnace or boiler system’s electrical components, a few lights, and maybe recharge your cell phone or iPad to let friends and family know you are OK.

A GTBB inverter/charger connects to a new subpanel called the “critical loads panel”.  It’s the breaker box that the most important circuits are connected to. The GTBB inverter is also connected to the grid. When inverter detects that the grid is out, it turns off any connection to the grid, and only powers the items wired into the critical loads panel.  Therefore it complies with the requirement to not send power down the lines, but allows you to have some power in your house. In an AC Coupled system, your existing grid-tied inverter is also connected to the critical loads panel. When the grid is up, it sends power through the GTBB inverter, out to the rest of the house, and any excess power gets sent to the grid to spin the meter backwards. But when the grid goes out, the GTBB transfer switch flips, and it turns off any connection to the grid, and only to the critical loads panel.  Since the grid-tied inverter is also connected there, it instantly sees the output from the GTBB inverter, and thinks it’s the grid. The inverter doesn’t turn off.  It keeps sending solar power to the GTBB AC charger component, to charge the battery bank that the GTBB inverter is getting its power from. So you are powering your critical loads off the battery bank, while the solar panels are recharging the batteries during the day.  Once the grid comes back up, the transfer switch flips back, and the grid-tied inverter goes back to powering the whole house and spinning your meter backwards.

Certain restrictions apply depending on which inverters you have, give us a call to see if AC Coupling is a good option for you.

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Amy Beaudet

Solar Queen at altE
Amy Beaudet has been in the solar industry at the altE Store since 2007. She’s been a sales rep, an instructor, and an all around solar evangelist, sharing her passion for solar around the world. When not at work, she’s either sailing or skiing, depending on the season, but odds are good she’s still talking about solar on the boat or on the slopes.

About Author

Amy Beaudet
Amy Beaudet has been in the solar industry at the altE Store since 2007. She’s been a sales rep, an instructor, and an all around solar evangelist, sharing her passion for solar around the world. When not at work, she’s either sailing or skiing, depending on the season, but odds are good she’s still talking about solar on the boat or on the slopes.

10 Comments

  1. ben

    So in this configuration, you have a battery powered inverter generating split phase power to energize your solar subpanel, which has a branch circuit that supports the grid tie inverter.

    What I am interested in is how the inverter charges it’s batteries while it is creating AC to keep the grid tie inverter synced up. Or perhaps the better question to ask is how does a grid tie inverter actually detect a grid failure while it is pumping out AC.

    1. Amy Beaudet

      In general. the grid-tied inverter AC Out and the GTBB inverter AC Out are tied together in the critical loads panel. The GTBB inverter is always monitoring the AC In terminals for any voltage drop or frequency shift. While the grid is up, it is just passing the power through from the grid-tied inverter. Once it detects a problem with the grid, it throws the internal transfer switch to disconnect from the grid. The grid-tied inverter backfeeds through the AC OUT of the GTBB inverter to charge the battery bank. Different manufacturers handle this process differently, so I can’t give a general statement of how it’s done.

  2. Thomas Kasprzak

    HI,
    I presently have a 10KW system on my roof. It has 43 panels plus a separate larger one for the solar hot water. Am Interested in learning more about the battery back up system in case the grid goes down. Doesn’t make much sense to have the power on the roof and not be able to supply the house. I do not want to back-feed the system or harm anyone in any way. Are there code restrictions or other concerns or is this done by either you or a contractor hired by you to set up, install etc. What kind of warranty does this have? How much space for the batteries is necessary? Thanks. My phone is 941-661-0580 and you have my email. Thanks and look forward to hearing from you.

    1. Amy Beaudet

      A system like the prewired Midnite with 3 Magnum inveters would be able to work with your system, http://www.altestore.com/store/Inverter-Power-Panels/Midnite-Solar-Power-Panels/MidNite-Solar-Pre-Wired-AC-Coupled-System-3-MS4024PAE/p10546/ . The number of batteries needed depends on the amount of equipment you want to backup. We’d need to do a loads list to determine what you consider Critical Loads.

  3. David Brown in Sunapee NH

    Thank you Amy, I’m sending this to my friend in Florida..Well written to a subject of great importance.

  4. Anne Miller

    I live in the Palm Springs area of California and am hoping to get some solar power for my home, but when I found out that the solar was cut off when the grid went down I backed off and have been searching for a way to keep it up if the grid goes down. SCE has been extremely unhelpful, and I have not been able to get a straight answer from anyone as to whether there is a way to keep my solar operating. That is until I found your answer above. My question is, is it legal in California? They seem to really not want us to have our own power no matter what they say.

    Also, seeing as I have yet to choose a system, what would you suggest as to the type to get for the desert here, as we sometimes get up to 120 in the summertime and I understand that they have panels that are made for that kind of heat.

    What would be the cost of adding your GTBB inverter, sub panel, etc. to a new installation?

    Thank you for any help you can give me.

    1. Amy Beaudet

      Since you are starting from scratch, you can just start with a standard grid-tied battery backup solution. Both Schneider formerly Xantrex) and Outback make excellent solutions that are on the approved Solar Inverter list in CA. http://www.gosolarcalifornia.ca.gov/equipment/inverters.php

      We have a few packaged systems that give you an example of what’s involved at http://www.altestore.com/store/Packaged-Systems/Grid-Tied-with-Battery-Backup/c568/. These are small systems, but we can customize them to whatever size you’d need.

      Another solution can b to use one of the new grid-tied inverters that have the ability to power an outlet with up to about 17A of power when the grid is down and the sun is up. For example, the SMA SunnyBoy TL series has this capability. http://www.altestore.com/store/Inverters/Grid-Tie-Inverters/4001-TO-7500-Watt-Grid-Tie-Inverters/Sunny-Boy-5000TL-US-transformerless-5kw-Inverter/p10863/

      For a system that performs best in he desert, you’d probably want to go with monocrystalline panels instead of polycrystalline.

  5. Alexandra

    Do you offer the equipment for an A/C coupled battery system? I didn’t see it in your grid-tied with battery backup options, but I might have missed it. If not, any advice on where to buy these?
    Thanks!

    1. Amy Beaudet

      Most grid-tied battery backup inverters can be used for AC Coupling. DIfferent manufacturers handle it differently. Additionally, Midnite now makes some prewired AC Coupling systems that make it even easier to add AC Coupling to an existing system. Some examples are here, http://www.altestore.com/store/Inverter-Power-Panels/Midnite-Solar-Power-Panels/MidNite-Solar-Pre-Wired-AC-Coupled-System-with-Enclosure-MS4024PAE/p10542/and http://www.altestore.com/store/Inverter-Power-Panels/Midnite-Solar-Power-Panels/MidNite-Solar-Pre-Wired-AC-Coupled-System-3-MS4024PAE/p10546/ .

  6. Jerry McGhee

    Have 13 panel system @ 235 kw per panel w/ Growatt inverter3600 how many batteries & what size do I need and should I increase number of panels as they only supply about 75% of use. Cost ? Thank you for your response.

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