Mixed grid-tied and offgrid system?

SystemSystem adminPosts: 2,511 admin
Hi - I have a 3.5KW grid tied solar system (Magnavox inverter if it matters). The grid tied inverter is required to open circuit the panel if it sees no grid AC so as to not electrocute service personnel. This means on a nice sunny day with no AC, my house has no power!

The recent outages in the northeast had me thinking - can I put some essential circuits on a separate inverter fed by the same panel? The existing circuit essentially looks like:
Panel -> DC switch -> Inverter -> AC disconnect -> grid

If I put a new inverter (conceptually in parallel with the existing inverter), with battery backup and divert the essential circuits in the breaker panel to it, that new inverter could charge the batteries from both solar and grid as needed, and the essential circuits would be permanently off-grid. Ideally if the internal circuits don't need power the grid-tied part would still roll the meter backwards!

Any thoughts/feedback appreciated.

Comments

  • icarusicarus ✭✭✭✭ Solar Expert Posts: 5,301 ✭✭✭✭
    Re: Mixed grid-tied and offgrid system?

    Anything is possible, and there a numerous threads on the subject, but look at it this way. A Honda eu1000 genny, large enough to power most essential loads save a deep well pump, can be had new for under $1000. It will run about 6 hours on a litre of fuel +- and it will run all day and all night regardless of sunshine.

    In the real world, how often is your power off,, and for how long? Most grid power is very reliable, and it is usually much cheaper ( and more reliable) to ue a genny for those rare outages.

    Just my opinion.

    Welcome to the forum, and keep in touch. There are some very smart folks here, who have forgotten more about PV and RE than mot of us will ever know. They are generally very willing to share ideas as well.

    Tony
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Mixed grid-tied and offgrid system?

    Welcome to the forum.

    Okay, a brief explanation of how grid-tie systems work:
    When there's no grid there's no utility power for them to synch to and no place to send any power they produce. They shut down. This is called "anti-islanding" and is an essential safety feature. Even if they didn't do this you would have no power because there is no stable DC source to run from: solar panels put out varying Voltage and current depending on the amount of light falling on them and the amount of load applied. GT systems can not supply power in an outage without batteries entering into the formula somewhere.

    It is entirely possible to have a small, stand-alone off-grid inverter system that will take over automatically in the even of a power outage and maintain essential systems like refrigeration. It is not necessarily practical, as the cost for occasional back-up use can be whoppingly expensive compared to a small generator.

    Attempting to tie existing GT systems to off-grid systems to make use of panels & inverter already in place is a topic of much discussion around here. On the whole it is neither cheap nor practical, and in some cases it's just plain impossible. There are significant obstacles in synching a GT system to an off-grid inverter and getting it all to work.

    It's best to evaluate how much back-up power you need and how often in order to determine a good solution for your particular situation. In some cases it may be changing out the GT inverter for a battery-based hybrid one like the Xantrex XW or Outback Radian. Even that can require significant rewiring and expense.
  • System2System2 admin Posts: 6,290 admin
    Re: Mixed grid-tied and offgrid system?

    Hi all - Appreciate the thoughtful replies. Ignoring for a moment the $ vs !~ (for bang=noise + ~=fumes!) argument, my question really had to do with whether 2 inverters can be hooked up in parallel, the existing grid tied one untouched, a new one for off-grid essential circuits while preserving the net metering aspect of the original setup.

    I did find a commercial website with info about this (not affiliated with them etc..), if anyone else is interested:
    Thanks... Uday
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Mixed grid-tied and offgrid system?
    Uday wrote: »
    Hi all - Appreciate the thoughtful replies. Ignoring for a moment the $ vs !~ (for bang=noise + ~=fumes!) argument, my question really had to do with whether 2 inverters can be hooked up in parallel, the existing grid tied one untouched, a new one for off-grid essential circuits while preserving the net metering aspect of the original setup. Thanks... Uday

    That gets in to an area that's tricky for two reasons: for one thing, it would not be legal to have the off-grid inverter's output connected in any way that it might feed the grid. For another, it can be dangerous to do so.

    But as a sub-circuit it is entirely possible: power from an AC breaker in the main panel can feed the AC IN of the off-grid inverter, whose AC OUT goes to a sub-panel for essential circuits. There is no technical nor legal problem with that. The GT set-up really has no bearing on it (nor would it contribute if there were a power outage).
  • icarusicarus ✭✭✭✭ Solar Expert Posts: 5,301 ✭✭✭✭
    Re: Mixed grid-tied and offgrid system?

    I should note that thE Honda eu series gennies are so quiet, that in our remote, off grid site, one running right outside cannot be heard with the window closed. It Carlos cannot be heard through the bush after 150'.

    As for fumes, they are CARB compliant.

    T
  • ChrisOlsonChrisOlson ✭✭ Banned Posts: 1,807 ✭✭
    Re: Mixed grid-tied and offgrid system?
    Uday wrote: »
    If I put a new inverter (conceptually in parallel with the existing inverter), with battery backup and divert the essential circuits in the breaker panel to it, that new inverter could charge the batteries from both solar and grid as needed, and the essential circuits would be permanently off-grid. Ideally if the internal circuits don't need power the grid-tied part would still roll the meter backwards!

    I think, rather than having two inverters, I would sell the Magnavox inverter and install a Xantrex XW-series and battery bank. And put a good MPPT controller on your solar array to replace the built-in MPPT in the GT inverter.

    The advantages to this, as I see it, is that your inverter now always powers your loads and sells excess power to the grid, as opposed to selling power from the RE system to the grid and buying it back.

    The Xantrex XW can be set up to either maintain your battery bank for you while selling excess power, or it can be set up to only use the panels to charge the bank and do a Battery Transfer when there is sufficient solar power to run the loads in your house. Either way, you will never know the power went out unless you have something in your house that is not powered by the inverter, and it will accomplish what you would like to have with just one inverter instead of two.

    The Xantrex XW meets all the requirements of UL1741 for GT inverters and has a lot of nice accessories available for it like AC and DC conduit boxes, remote monitoring, auto-generator start, etc..

    The only reason I can see, if you want a off-grid backup system, that this would not be the way to accomplish this is if you get paid some sort of FIT or premium for your power you sell to the grid and can buy it back at a lower price. For some folks in Canada that's the case. But I know of no utilities, or programs, here in the US that do that.
    --
    Chris
  • techntrektechntrek ✭✭✭ Solar Expert Posts: 1,372 ✭✭✭
    Re: Mixed grid-tied and offgrid system?

    Uday, Xantrex recently released a high-voltage charge controller which most likely can be used with the output of your existing PV array depending on its voltage output. You would leave your grid-tied system as-is, add a separate battery-based off-grid system and charge it using the Xantrex charge controller (or charge it using a standard AC charger and only use the Xantrex during outages).
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • solarixsolarix ✭✭ Solar Expert Posts: 713 ✭✭
    Re: Mixed grid-tied and offgrid system?

    My more affordable solution to this is the Xantrex PowerHub system which is a compact battery backed unit with 2 batteries, an 1800w msw inverter and all the interconnects, fuses etc. You have to add the appropriate charge controller though. You can put the whole thing together for under $1000 and it will keep your critical loads alive for most of a day during an outage and potentially as long as you want if you tie it into your array.
    The array connection is the tricky part in running a GT inverter in parallel with a OG system. The GT inverter wants a H.V. array and the OG wants a low voltage one. You can either spend another $1000 on the Xantrex 600V charge controller and just tie right into the GT inverters inputs, or rig up some switching so that during an outage, at least part of the array can be reconfigured to a lower voltage. There are other brands coming out with higher voltage controllers. Midnite goes to 250V now and I'm not sure if the Apollo HV is available yet.
    There are a lot of GT customers spending many thousands on their solar PV systems who are surprised, disappointed and a little upset to find out that they are going to be s.o.l. just like everyone else in spite of "having solar". This low cost way of providing critical back-up capability is my answer to that.
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