Grid Tie MicroInverter system and Battery Backup?

Hi,

I just had my 6kwh solar system installed with Emphase micro inverters. I am looking for a long term solution to power outages the just in case kind of scenario. As I understand it now, when power goes out so does production. I was wondering if there was a way to manually switch over to a off grid kind of system only if the power was out longer than a few days.

The power coming down is AC, so would I have to convert it back to DC to charge a battery bank? which means another inverter to convert the energy back?

Thanks,

Chris

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid Tie MicroInverter system and Battery Backup?

    this isn't normally done with straight gt systems. you could install a separate inverter/charger with batteries so that when the power goes off it'll act like an ups, but this will still need utility power to charge it back up. a generator can sometimes be used during the day to charge the batteries so using the inverter there's no noise during the evening or you will need to place more pvs with a controller to feed power to your batteries. i don't believe running the + and - leads from the present pvs will meet current inspectors' approvals being they are already feeding the grid, but you can ask them if there would be such an approved way to tap the pvs up there. do note that these would be individual taps on each pv selected as there is no simple answer to getting the dc from them otherwise. i would be concerned with the taps as the grid reappearing could cause problems.
  • doctai77doctai77 Registered Users Posts: 5
    Re: Grid Tie MicroInverter system and Battery Backup?

    I'm not sure I understand entirely all of what you are saying. But basically i think you are saying try to tap the solar panels before the microinverters?

    is there a way to charge batteries using a ac source?

    the problem with generators is fuel.

    thanks,

    C
  • doctai77doctai77 Registered Users Posts: 5
    Re: Grid Tie MicroInverter system and Battery Backup?

    Why can't I have a subpanel that is fed directly by the AC power from the solar panels that has a manual switch to prevent back feed when the grid is down?

    C
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Grid Tie MicroInverter system and Battery Backup?
    doctai77 wrote: »
    Why can't I have a subpanel that is fed directly by the AC power from the solar panels that has a manual switch to prevent back feed when the grid is down?

    C

    They are grid-tie inverters - they don't make power without a grid source to sync to. Take away the grid source and the inverters shut down.

    You could rig a battery bank with a charger and non-grid-tie inverter and use a transfer switch to run from that system when the grid is down, but it won't be getting recharged when the grid is down unless you add a generator.

    There are other inverters that can do grid-tie *and* battery backup, but you have no way to feed them from the solar panels without hacking into the wiring between the panels and the Enphase inverters (that's the "tap" neil was talking about).

    EDIT: Same with trying to charge the batteries off the solar. To add a charge controller, you'd have to hack the wiring between the panels and the inverters.
  • BB.BB. Super Moderators, Administrators Posts: 31,070 admin
    Re: Grid Tie MicroInverter system and Battery Backup?

    There is a way that can be done to run a pure GT inverter off-grid--But that requires a second True Sine Wave off-grid inverter, battery bank, and AC Transfer Switch (all of which have to be rated equal or greater Wattage than the GT inverter system).

    We have talked about it quite a few times here (aka "AC Coupled" or Back Driven Inverter)--But, unless you (or your friend) is very knowledgeable about electrical systems and electronics--It is not something that I would recommend for a first time project for a home owner. There may be one Off-Grid inverter designed/capable of doing this (Xantrex/Schneider XW Hybrid Inverter--Some other OG inverters are capable, but not tested/designed for function, and there are issues with properly charging the battery bank)--but I don't have any details.

    Other than that, there is the Sunny Island system which has its own GT/Off-Grid hardware (nice system, but not cheap).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Grid Tie MicroInverter system and Battery Backup?

    Yes, theoretically you can back drive a GT inverter with a non-GT inverter to simulate a grid source and cause the GT inverter to "light up" and produce power.

    There are technical issues, as Bill says. It has been discussed here quite a few times, I don't recall anyone saying they've actually tried it.

    Aside from the technical issues, there is also the issue of trying to get the Building and Safety inspector to buy it, as well as the utility company.

    Adding batteries to a GT system can also raise an issue about tiered rates, and charging the battery from cheap off-peak grid power, and then selling it back to the grid at higher on-peak rates. They frown on that.
  • BB.BB. Super Moderators, Administrators Posts: 31,070 admin
    Re: Grid Tie MicroInverter system and Battery Backup?

    Solar Guppy has done the test for a bit with at least one brand of GT inverter / OG Inverter (manually monitored/controlled battery state of charge and had A/C loads running at time--as I recall?).

    And, at this point, I don't think it makes economic sense for a homeowner to charge a battery bank at night and feed the grid during the day--Batteries are just too expensive and the power losses are non-trivial.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Grid Tie MicroInverter system and Battery Backup?

    The latest Xantrex XW-6048 has sanctioned support for it now, I ran it today for a bit as out of the blue we lost power in the neighborhood for about 7 hours last night, First time since I installed my whole-house ability 4 years ago that I actually used it. :roll:

    The Power company was still working on it past my sleep time, so I ran all night and let the GT's charge the battery bank this morning ( along with the regular pair of MorningStar Mppts that are used for Griditie )

    Its not automatic, I have to manually disconnect the main breaker and engage a bypass so the whole house is powered from the XW-6048, but it performed well

    Its been brutally hot this week, I'm watching the plasma, AC running, house lit up in a complete dark neighborhood ... Wouldn't be good in a "Mad MAX" situation, but felt good last night with the neighbors scurrying around wondering why my house has power;) Used close to 17khw or about 60% of the batteries to keep everything running,
  • mike95490mike95490 Solar Expert Posts: 9,033 ✭✭✭✭✭
    Re: Grid Tie MicroInverter system and Battery Backup?

    Be careful with all those lights, turn them off, and put a candle in the window, about day 3 of the outage, you will want stealth, when the village gets the pitchforks and torches....
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • doctai77doctai77 Registered Users Posts: 5
    Re: Grid Tie MicroInverter system and Battery Backup?

    Yes this is definitely something I would not have installed myself, but have a pro do. I know it's not practical, and I'm not trying to sell power back at different tiered rates. All I want to do is have the pV charge the batteries in periods of long power outages.

    Thanks for all the input. What I gather is that

    1) it's possible
    2) it's not practical
    3) hire a pro


    anyone know of an installer in PA?

    C

    ps i'm not as concerned about battery cost as i have a relative with deka batteries that provides significant price benefits.
  • BB.BB. Super Moderators, Administrators Posts: 31,070 admin
    Re: Grid Tie MicroInverter system and Battery Backup?

    Afternoon Brother (i.e., relative with access to discounted batteries;)),

    A really good system (if have not installed the GT inverters already) is the Xantrex XW system... The hybrid inverter can do both GT and off-grid inverting from a 24 or 48 volt battery bank. You set up the solar array with DC charge controllers to charge the battery bank (and run the GT/OG hybrid inverter).

    Not cheap--but a very nice system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,327 ✭✭✭
    Re: Grid Tie MicroInverter system and Battery Backup?

    It is very unlikely that a grid tie inverter will stay locked onto a general off-grid (non-grid tie capable) true sinewave inverter.

    Many use only an RC oscillator timebase which are likely not to have accurate enough frequency which would be first non-starter for a grid tie inverter to lock to.

    Even if frequency is within the -0.7 Hz - +0.5 Hz accuracy, required by a grid tie inverter to lock on, the off grid inverter will not react to anti-islanding tests like the normal grid.

    Anti-islanding test is done by a grid tied inverter periodically by trying to resist the grid, a little, either by attempting to raise/lower voltage or attempting to wobble the phase of the inverter. The grid tie inverter senses the increase/decrease in instantaneous current as the grid tie inverter attempts to move the 'immovable object', the low impedance grid.

    The general off grid sinewave inverter will not resist a raising of the sync'd AC voltage.

    If there is heavy enough load, a grid tied inverter might stay locked on, but this would still consume battery capacity by the the off grid inverter which needs to be supplying a signifcant portion of the load.

    You need a grid tied, battery hybrid, inverter like XW. The revised software for XW has the abillity to shed the grid tie inverter when AC load plus battery charging can not consume all the grid tie inverter generated power. I believe it does this by moving XW frequency outside the grid tie inverter acceptance range.
  • doctai77doctai77 Registered Users Posts: 5
    Re: Grid Tie MicroInverter system and Battery Backup?

    I already have emphase micro inverters installed on the roof, so i'm thinking none of the options people mentioned are going to work. I wish I knew more before hand.
  • BB.BB. Super Moderators, Administrators Posts: 31,070 admin
    Re: Grid Tie MicroInverter system and Battery Backup?

    I knew of some of the options regarding on-grid/off-off grid ~5 years ago when I installed my system (there are more and better options now). :D)--And I still chose to not install the batteries/off-grid option and just stick with a small Honda eu2000i genset and 20 gallons of stored fuel (plus car tank and siphon) for 10 days or so of emergency power--past that point, I probably got more problems.

    Our power is out for an hour or so once every few years (more from car accidents than storms here). It just was not worth installing $x,xxx worth of battery banks to cover a once in 50 year event.

    Plus the gensets are cheap and portable. A fixed off-grid/battery installation is neither... And my once in 100 year event is an earthquake. A fixed system may not be my best bet if the police are telling everyone to evacuate (which seems to be a common occurrence around here--Police Investigation--See that big hole in the ground with the gas pipe sticking up, investigate that!:roll:).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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