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Thread: Grid tie to off grid conversion

  1. #1

    Default Grid tie to off grid conversion

    Hello everyone, happy new year. I'm looking for some advice. Here's my situation.

    I currently have thirty-eight 185w Suntech panels in a grid-tie setup using Enphase microinverters. The system has worked well for me so far. I generate 23Kwh/day on average, although here in PA the output varies considerably depending on the weather and season. Lately I've been thinking about how I could make use of my panels during a long term grid outage (aka SHTF). I have a Honda EU2000i to handle any short term grid outages.

    My goal is to have the proper equipment on hand to manually convert my grid-tie system to a light use off-grid system in response to a serious extended outage. I'd like to be able to run my household size fridge, microwave, toaster, and some electronics (not concurrently) during daylight hours, and just a CFL bulb or two at night. I understand that I'd need the following things:

    1.) MPPT style charge controller
    2.) Batteries
    3.) Inverter
    4.) Wire, connectors, breaker/fuses

    For the charge controller, I'm looking at the OutBack FM80. It seems like a quality piece that provides a lot of flexibility. Items #2 through #4 are where I'm in need of advice.

    Because I'm only planning to convert my system to off-grid in the (unlikely?) event of a long term grid outage, I'm not too keen on spending huge sums of money on lots of batteries that will just sit around degrading or on $2,000 inverter that I may never use. This has me leaning more toward a simpler 12v system. At the moment I'm considering just a single Concorde AGM 108Ah at $300, or maybe even just stealing the 12v batteries from our family cars. Anyhow, this also means a 12v inverter. I'm considering the Xantrex SW2000. It seems okay for $350 and I like that a decent 12v inverter is good to have around anyhow. So for not much money ($600 controller, $350 inverter, possibly $300 battery), I could have the main components on hand to go off-grid during a long grid outage.

    The downside of this cheap configuration (aside from the obvious fact that it's small) is that I don't think it will really allow me to leverage the thirty eight 185w panels that I have installed. If I understand correctly, with the Outback FM80 I'll be limited to 1250 watts with a 12v battery system. Does this mean that I'm limited to feeding a maximum of 1250 watts to the FM80 from the panels? Could I wire more (or even all?) of my panels to the FM80 so that I'm practically guaranteed 1250w of constant power delivery even on a cloudy day, since I have all these extra panels to spare? I think this is really the question that I'm most interested in.

    My original thought was to wire 3 panels in series (185w x3 @ 105v), and then run two of those in parallel to deliver 1110w @ 105v to the charge controller. Of course, that is on a perfect day - and not even then. Most of the time it would be producing substantially less than 1110w. I supposed that I could manually keep an eye on the weather conditions and connect more panels to the controller only on a cloudy day. But that seems dangerous. What if the sun suddenly pops through the clouds for a minute? Will the charge controller just ignore the excess power delivery? Will it simply trip the 80 amp DC breaker on the battery side of the controller? Do I need a breaker on the PV side of the controller to protect against this? Is this simply a stupid idea?

    I have considered the 'big budget' alternative - which would include a couple thousand dollars in a 48v battery bank and an Outback FX3048T 48v inverter, but I'm having trouble justifying the expense for an unlikely scenario. However that would allow me to legitimately feed in 5000 watts from the panels per the Outback FM80 specs.

    Lastly, regarding wiring, what is the acceptable gauge wire to run from the 3 panels in series to the charge controller over 75 feet? What about from the charge controller to the battery and inverter? I've seen recommendations for 3/0 or even 4/0 for 12v @ 2000w, but it seems like the Outback charge controller only accepts #2, so I'm confused about that.

  2. #2
    Join Date
    Sep 2006
    Location
    Willits, CA
    Posts
    5,564

    Default Re: Grid tie to off grid conversion

    Look in to the pricey (but effective) Xantrex XW 600VDC charge controller. Wait, you currently have micro-inverters.

    OK, you will need a MC connector Disconnect tool, to un-hook the cables off the micro-inverters from the panels, so you can connect to a charge controller.

    As you discovered - a 12V system won't get you far, but you could rig panels into 100V groups, feed your charge controller, and use a small 48V bank of batteries.
    At the first sign of SHTF, race to a box store and grab 4, 12V deep cycle batteries. Or the local golf club, and get 8, 6V cart batteries.
    You will have to have 48V inverter, Charge controller, MC cables & tools, battery interconnects, fuse, wire boxes, and a room to stash all the gear in, and save the battery purchase for the last moment. Rest of it sits idle.

    Or, trash the micro-inverters, and use a hybrid GT system, with battery backup, and you are all set for the next ice storm.

    You want 2 plans, you have to pay for 2 plans.

    Wire size, look at the caculator spreadsheet that Solar Guppy hosts.
    http://tinyurl.com/LMR-Solar
    http://tinyurl.com/LMR-BigLug
    http://tinyurl.com/LMR-NiFe

    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

  3. #3

    Default Re: Grid tie to off grid conversion

    Can you say "wiring nightmare"?
    Disconnecting the panels from the Enphase inverters and then re-connecting them to feed the FM80 is no small task.
    What you should ask yourself first is: "How often does the power go down and for how long?"
    Chances are the generator is the most cost-effective power-outage solution.
    If you want "quiet power" for outages you could buy a small battery & inverter and use the generator to recharge it during the day (with an appropriate charger which would also keep the batter up when grid power is available).

    Can your way be done? Yes, and if you really want to do it will tell you how. But it sure isn't easy.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  4. #4

    Default Re: Grid tie to off grid conversion

    Yes, I fully expect it to be a big and inconvenient job. I do have a Honda EU2000 generator for any short term outage. This would be only for a long term SHTF scenario - e.g. the grid has been down for several weeks and there is no indication that it's coming back up.

  5. #5
    Join Date
    May 2006
    Location
    Florida, USA
    Posts
    1,273

    Default Re: Grid tie to off grid conversion

    Based on the stuff you said you want to power you are going to need 3 kWH's to 5 kWH's per day. The refrig alone will be slightly over 1 kWH per day.

    The least cost and maintance for long term standby would be a sealed maintance free marine battery. I had really good luck with Delco Voyager Class 27 105 AH batteries. They lasted 7 to 9 years on mostly float charge. Walmart Everstart Maxx Class 30 marine batteries are pretty bad, lucky to get five years.

    You can 'Y' connector off microconverter DC panel connections and hook as many in series as charge controller input voltage can take. You leave the series connect down wiring feed breaker open during normal on grid tie feed. For backup you flip off the AC breaker going to micro inverter and flip on the DC breaker going to the series stack PV panels to allow the series stack to feed the charge controller. If you do more then one series string you have to bring down each series string as a separate branch to individual DC breakers, one for each series string. You can flip on all the DC breakers when offline and have multiple series strings in parallel feeding the battery charge controller. Leaving the series 'Y' connections at the panels is fine when operating the micro inverter on grid tie as long as you don't complete the series stack connection path by closing the DC breaker at the breaker panel.

  6. #6

    Default Re: Grid tie to off grid conversion

    So long as you understand that ...

    The first step is to determine how much power you will need in an outage. Get a Kill-A-Watt (or equivalent) meter and measure the actual power use of everything you want to keep running when the power goes down. You're looking for two things: what the peak total Wattage is and what the total daily Watt hours are. Remember that things like refrigerators (and other inductive loads, mainly motors) have big start-up surges which have to be taken into account. The K-A-W will not measure these.

    Once you have those numbers you'll be able to determine how big of an inverter you need and how much battery to supply it with. That number will tell you how much panel and what charge controller you need to recharge with. You may go back and forth with these numbers a few times, as it is easier to drop "essential" loads than to add power capacity. Cheaper too.

    There are other ways around this, including what Mike mentioned; dropping the micros in favour of a central hybrid inverter which has battery capacity built-in. Others will suggest "tricking" the micros into supplying power when the grid is down, but I've yet to hear of sure-fire and painless way of accomplishing this. When power is out you don't want a system that may work ... or may burn up.

    Or you could base your system on what power might reasonably be expected from utilizing all of your PV's, which at 7 kW would be considerable. But that means more charge controller capacity and bigger battery bank, which may not be desirable for long-term investment.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  7. #7

    Default Re: Grid tie to off grid conversion

    Thank you to everyone for these replies. Can you answer this question? Sorry but I had it buried in my lengthy initial post:

    >If I understand correctly, with the Outback FM80 I'll be limited to 1250 watts with a 12v battery system. Does this mean that I'm limited to feeding a maximum of 1250 watts to the FM80 from the panels? Could I wire more (or even all?) of my panels to the FM80 so that I'm practically guaranteed 1250w of constant power delivery even on a cloudy day, since I have all these extra panels to spare?

  8. #8
    Join Date
    Sep 2006
    Location
    Willits, CA
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    5,564

    Default Re: Grid tie to off grid conversion

    Quote Originally Posted by Ishmael View Post
    Thank you to everyone for these replies. Can you answer this question? Sorry but I had it buried in my lengthy initial post:

    >If I understand correctly, with the Outback FM80 I'll be limited to 1250 watts with a 12v battery system. Does this mean that I'm limited to feeding a maximum of 1250 watts to the FM80 from the panels? Could I wire more (or even all?) of my panels to the FM80 so that I'm practically guaranteed 1250w of constant power delivery even on a cloudy day, since I have all these extra panels to spare?
    1) you can over supply a MPPT controller, it's supposed to throttle back to save itself

    2) My 3,000 watt array, produces maybe 300w on a cloudy day. Clouds = Bad.

  9. #9

    Default Re: Grid tie to off grid conversion

    Is it hard on the controller when you substantially over supply watts? Or does it simply ignore the excess wattage such that 5,000 watts of excess isn't really any different than 100 watts of excess? My array produces about 6,300 watts on a typical sunny day, for example, so there is potential for a pretty dramatic over supply.

  10. #10
    Join Date
    Dec 2011
    Location
    Beltsville, Maryland
    Posts
    41

    Default Re: Grid tie to off grid conversion

    You asked about wire size. If you're still talking about six panels wired as 1110W for 10.5A at 105V I would think that in a SHTF situation you could get by with as little as 14 gauge for a 75 foot run. Sure that would be nearly a 5% loss of power, but in an emergency situation the ability to use a heavy duty extension cord or regular romex household wire could be important. Ideally you'd use properly rated outdoor wiring of probably 8 or 10 gauge.

    My question is whether a single 108AH 12V AGM battery could run a microwave or toaster. You'd probably be pulling in excess of 100A with the inverter, possibly 150A for a 1500W toaster. If you only did that while the sun was shining and the controller was contributing up to 80A you might get away with it.

    The next question would be can a single Concorde battery deal with up to 80A of charging current. That's way above the commonly recommended 5-13% charge rate. I know AGM batteries can handle higher charge rates than regular flooded batteries, but is .75C doable?

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