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Thread: AC Coupled Battery Backup Install Questions

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    AC Coupled Battery Backup Install Questions

    What's up everybody. Been a while since I posted but haven't had any crazy designs until now.

    I have a client in Western MA who is getting a (55) panel, 19.525 kW DC, 16.2 kW AC install (LG 355 Mono-X! panels and Enphase IQ7+ microinverters).

    He wants battery backup, and I was able to convince him to wait a little longer for the battery technology to develop further. I have my eyes set on Enphase's IQ8 technology, but it keeps getting delayed.

    Never the less, we will be using microinverters, so the battery backup needs to be AC coupled. There are no DC strings to intercept since everything is 240Vac off the roof.

    I am planning on adding a critical loads / backup panel, which will also act as the solar subpanel for the (4) 20 amp branch circuits. I want to have everything set up for the battery backup, and only have to add in the inverter and battery, or Enphase's batteries, whichever technology we go with.

    Here are my questions / concerns:

    It is looking like I will need a very large panel, possibly a 200 amp subpanel for this application. 80 amps for the solar branch circuits, maybe 60 amps for a 10 kW inverter for the battery circuit, and all the critical loads (maybe another 60-80 amps). This is leaning me towards a 200 amp subpanel.

    I have never wired a 200 amp subpanel off a 200 amp MSP. I do not want to do a lineside tap, since we would in theory be at risk of overloading the conductors coming in for the 200 amp service. I have not made note of their size yet, but I will next time on site.

    My electrician seems to think we will be able to use a 200 amp breaker like any other subpanel install, but I think he may be mistaken here. I can't imagine the size of the terminals to accept the proper sized wire for 200 amps. I have only seen 100 amp breakers, never anything higher.

    I am hoping some of y'all can help guide me through this. I will be having a professional electrical engineer stamp and approve my design, since our install is above 15 kW AC this is a requirement here in National Grid service territory. He seems pretty unfamiliar with all this residential backup technology so I wanted to look elsewhere for assistance.

    Thanks in advance and solar on my friends.

    -Alex DeFreitas

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    If your maximum system output (solar+battery) would be 120A after 125% factors are added, and your maximum backed up loads are to be well under that, then you don't need a 200A feed from an MSP. You may need a 200A backup panelboard depending how you qualify it under 705.12(B)(2)(3), but the utility side feed could be on a 125A breaker.

    You are also wrong to think you could overload 200A service conductors with only 120A of system output if you did a supply side tap. But I'm not saying this is the best option, since there are many other potentially relevant details you haven't filled in.

    Finally, 200A branch breakers do exist, at least for most newer equipment. But actually installing one in an existing panel might not be feasible depending on details, and certainly would be a pain in the a** on most residences. There's probably a better option.
    Last edited by jaggedben; 02-09-19 at 02:51 PM.

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    Also, like you I'm waiting for Enphase's new stuff and hoping it will compete well. But as far as planning this install, if you know anything about how Powerwall works, whatever Enphase comes out with is going to have to follow the same basic design principles. There will be some kind of getaway/automatic disconnect, and the solar, battery, and backed up loads will have to be all on one side of it. Plan your install so that can be easily inserted later, or at least so you're not making it harder.

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    Quote Originally Posted by jaggedben View Post
    If your maximum system output (solar+battery) would be 120A after 125% factors are added, and your maximum backed up loads are to be well under that, then you don't need a 200A feed from an MSP. You may need a 200A backup panelboard depending how you qualify it under 705.12(B)(2)(3), but the utility side feed could be on a 125A breaker.
    Ahh I see what you're getting at. The solar is like a generator, and backfeeds instead of drawing power. The batteries work both ways, so adding the solar output + battery output would be maximum backfeed, then the critical loads actually subtract from the amperage. So I just need to size for the solar + battery if I'm thinking correctly.

    Quote Originally Posted by jaggedben View Post
    You are also wrong to think you could overload 200A service conductors with only 120A of system output if you did a supply side tap. But I'm not saying this is the best option, since there are many other potentially relevant details you haven't filled in.
    So my thinking here is let's say we tap above main breaker. The MSP then can draw the 200 amps, and I didn't want critical loads + battery backup charging (if it's charging from utility) to exceed the amperage rating of the service conductors. I haven't checked the conductor sizes when on site (rookie mistake), but I will obtain next time I am out.

    By keeping all the new equipment below the main breaker I need not worry about overdrawing from the service conductors.

    Quote Originally Posted by jaggedben View Post
    Finally, 200A branch breakers do exist, at least for most newer equipment. But actually installing one in an existing panel might not be feasible depending on details, and certainly would be a pain in the a** on most residences. There's probably a better option.
    Good to know. I just can't physically imagine bending the conductors which land on the main lugs for 200 amps, and fitting them on a double pole breaker. My electrician is looking into it.

    Quote Originally Posted by jaggedben View Post
    Also, like you I'm waiting for Enphase's new stuff and hoping it will compete well. But as far as planning this install, if you know anything about how Powerwall works, whatever Enphase comes out with is going to have to follow the same basic design principles. There will be some kind of getaway/automatic disconnect, and the solar, battery, and backed up loads will have to be all on one side of it. Plan your install so that can be easily inserted later, or at least so you're not making it harder.
    Look a little more into Enphase IQ8. The technology is absolutely insane. It is what is called "grid agnostic" in that the solar and batteries can operate independently of the gridwithout a transfer switch. Even just the microinverters will be able to run offgrid without battery storage. What they are going to be doing is sampling the consumption of the home hundreds or maybe even thousands of times per second, and matching the potential solar power to supply just this amount, effectively not backfeeding the grid. Previously solar is either on or off, and any excess would be sent back through the meter and be a hazard for anyone working on the lines.

    All that being said, Enphase has delayed this IQ8 over and over, so clearly there are some details that are difficult here.

    With all the delays I am leaning towards doing a standard battery backup later this year if the client wants it sooner rather than later, but I do believe my client's best option is replacing their old micros with IQ8 and installing IQ8 batteries.

    You make a good point with the price, Enphase's battery is stupidly expensive compared to other battery technology on a $ / kWh basis, but they have a better DOD so in terms of usable kWh they are only slightly more expensive.

    -Alex

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    Quote Originally Posted by Alex-Sun Energy Solutions View Post


    Look a little more into Enphase IQ8. The technology is absolutely insane. It is what is called "grid agnostic" in that the solar and batteries can operate independently of the gridwithout a transfer switch. Even just the microinverters will be able to run offgrid without battery storage. What they are going to be doing is sampling the consumption of the home hundreds or maybe even thousands of times per second, and matching the potential solar power to supply just this amount, effectively not backfeeding the grid. Previously solar is either on or off, and any excess would be sent back through the meter and be a hazard for anyone working on the lines.
    I haven't looked into this tech yet, but it seems to me that some sort of isolation from the grid is necessary, else the grid would appear to be pretty much a dead short to the inverters. In a parallel circuit without isolation I don't see how you could power the local loads without trying to energize the grid.

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    Ggunn is absolutely right. The IQ8 will be multi-mode but you still have to disconnect from the grid to operate ... off grid. Not fundamentally different from Powerwall.

    I'm curious what you're using for 'standard battery backup'.

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    Quote Originally Posted by jaggedben View Post
    Ggunn is absolutely right. The IQ8 will be multi-mode but you still have to disconnect from the grid to operate ... off grid. Not fundamentally different from Powerwall.

    I'm curious what you're using for 'standard battery backup'.
    I'll just add this: In my experience (I have designed more than 500 residential PV systems) most solar customers are looking for a "set it and forget it" solution. Off grid solar isn't like that. It is incumbent on the customer to learn enough about off grid operation and hardware to be able to maintain and administer his own system and loads, otherwise you may be looking at what could be years of service calls. Be careful out there.

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    Quote Originally Posted by ggunn View Post
    I haven't looked into this tech yet, but it seems to me that some sort of isolation from the grid is necessary, else the grid would appear to be pretty much a dead short to the inverters. In a parallel circuit without isolation I don't see how you could power the local loads without trying to energize the grid.
    If you're sampling the consumption of the home in real time then all the power produced by the solar array will be used by the loads in the house, Enphase will lower the microinverter production to match the house demand if there is more solar energy available than what the house is drawing. I am not as sure what will happen in the opposite scenario, where you're trying to draw more energy than is available. I think they may end up using smart breakers which can toggle on and off. I haven't looked that much into the smart breaker technology but have heard about it a bit.

    I am interested to see some installation documentation for IQ8 but my reps at Enphase still don't have it available. IQ8 was supposed to be released Q1 or Q2 2019 but now got delayed until Q4 2019 earliest and I believe it may be at least a year away.

    At least Enphase isn't as slimy on unveiling then delaying products like TESLA (solar roof, etc).

    Quote Originally Posted by jaggedben View Post
    Ggunn is absolutely right. The IQ8 will be multi-mode but you still have to disconnect from the grid to operate ... off grid. Not fundamentally different from Powerwall.

    I'm curious what you're using for 'standard battery backup'.

    I consider standard battery backup to be what is readily available today. Usually it involves a transfer switch, critical loads panel, and a battery inverter with a battery off the inverter. There are a few different widely used ones. SolarEdge's Storedge inverter can function as both the PV inverter and battery inverter, but I personally don't like the SolarEdge PV technology. The DC to DC optimizers are points of failure and the central inverter is a single point of failure.

    None the less, a SolarEdge Storedge can be AC coupled with a Powerwall or LG Chem. Other inverter options are out there, as well as other battery options. I am a big fan of the Sonnen product line for batteries, but they are pretty expensive ($20,000 for the ECO 16 kWh unit).

    Quote Originally Posted by ggunn View Post
    I'll just add this: In my experience (I have designed more than 500 residential PV systems) most solar customers are looking for a "set it and forget it" solution. Off grid solar isn't like that. It is incumbent on the customer to learn enough about off grid operation and hardware to be able to maintain and administer his own system and loads, otherwise you may be looking at what could be years of service calls. Be careful out there.
    Very true. I am sort of on the customer's side here, as the end goal of offgrid backup would be to continue living your normal life. This, as you mention, is not that practical in reality. The Sonnen units do allow a lot of set it and forget it features (set points for DOD, standby generator add ons), but they are limited to 30 amps continual draw and thus can't really back up much of a home. I am waiting for something that can take a massive residential solar system and actually power a large percentage of the home. I am hoping Enphase's IQ8 makes a lot of this possible.

    The service calls don't really bother me too much. I offer a short but sweet warranty, and sell my clients on becoming their own DIY service techs down the road like you mention. They won't want me or anyone on my team driving 4 hours to setup a new SSID and PW for the Enphase Envoy once my warranty expires. Like you mention it will behoove the client to learn about the offgrid solar as to not incur service costs down the road.

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    Quote Originally Posted by Alex-Sun Energy Solutions View Post
    If you're sampling the consumption of the home in real time then all the power produced by the solar array will be used by the loads in the house, Enphase will lower the microinverter production to match the house demand if there is more solar energy available than what the house is drawing.
    All that may be true but it doesn't address the fact that if the house system is still connected to the grid when it is down, the grid will appear as a virtual dead short in parallel with the house loads. I don't see how it could operate without being isolated.

    UL1741 is often touted as protection for line workers, which is a valid concern if your feed from the transformer is isolated from the rest of the grid, but if it's not it's more a matter of physics. Your puny little 10kW inverter is not going to light up your town.

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    Quote Originally Posted by ggunn View Post
    All that may be true but it doesn't address the fact that if the house system is still connected to the grid when it is down, the grid will appear as a virtual dead short in parallel with the house loads. I don't see how it could operate without being isolated.
    Ahhh I see what you're saying. This is one of the reasons I wanted to be behind the main breaker, if it's just as simple as switching the breaker manually. Enphase must have something up their sleeve but like you and others have said it is likely a transfer switch which the other battery manufacturers rely on as well.

    I've seen the smart breakers, maybe there will be a SMART main that toggles off when there's no voltage. This wouldn't really help all my other clients who have taps above the main because of the 20% rule (can only backfeed 40 amps of solar on 200 amp service). Now that I think of it this may be an inhibitor for this system as well. We will have a total of 80 amps for the solar circuits. I may have to tap it in above the main breaker.

    Back to the drawing board...

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