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question about backup generator with grid tied solar

WillyP Member Posts: 45
I know with a grid tied PV system I am supposed to have an automatic cutoff, if the grid goes down. But I am curious. Does anybody know if there is a switching system that will allow me to use a backup generator when the grid goes down? I plan on having a large propane tank (the wife demands a gas stove) so there will be plenty of fuel, to last a few days without the grid.
I am not talking about cloudy days here. I am asking about when the power lines go down...


  • ratio
    ratio Member Posts: 2,925
    Google "Automatic Transfer Switch" for the primo, automatic version, or "generator breaker interlock" for the (significantly) cheaper, but manual, version.

    I'm not sure how the solar would feel about things when it's suddenly no longer grid-tied but generator-tied. I'd suggest a call to the solar installer or mfgr to see if they have any experience with this setup.

  • Jamie Hall
    Jamie Hall Member Posts: 17,080
    If you have a generator which can connect to the house wiring, you have to have an approved, positive, break before make, transfer switch. It can be automatic or manual, but it has to be there. It's not a "supposed to" -- and the electric company has the authority to disconnect you permanently from the grid if they find you without it. The automatic cutoff for the grid tied PV is similarly not a supposed to -- it's a must.

    Now, that said, there may be a problem with the solar PV, in that if it's output at some moment is greater than the house demand, and the house is disconnected from the grid, you could easily damage or destroy the converter and controller. As @ratio suggested, get in touch with your installer or manufacturer -- who may or may not know.

    I can think of at least two options -- one, obviously, is to tie the solar into the grid lines on the grid side of the transfer switch, with the automatic cutoff. You'd not have solar when the grid was down, but... you would have to get advice and approval from your power company and have it installed by a licensed electrician. The other is to have a battery bank which was arranged to be cut in along with the generator, and which was big enough to handle the solar output, with a suitable controller for the solar to prevent overcharging.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • WillyP
    WillyP Member Posts: 45
    Okay. I used a poor choice of words. I get it. Of course the system has to disconnect from the grid, so no linemen get electrocuted. The ability to off load extra power from the pv panels is something I hadn't thought of. Is that really a problem?
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,284
    Hi, I'd contact these folks: and see what they can tell you.

    Yours, Larry
  • jumper
    jumper Member Posts: 1,710
    Integrating solar with grid sounds funny to me unless solar is substantial. Three or four Kw only produces a dollar or so's worth per day for utility but owner saves double that? Eventually it'll pay to somehow store your surplus.
  • WillyP
    WillyP Member Posts: 45
    I plan on installing 20 340W panels, with built in micro inverters. SO fairly substantial. SOmeday, somebody is going to reinvent the battery and grid tied systems will become obsolete. Until that happens, it i sthe best system for storing solar electricity.
  • hot_rod
    hot_rod Member Posts: 15,033
    The micro inverter solar modules basically feed 240 V backwards from the panel mounted inverter into a electric panel via a breaker. Check with the inverter manufacturer, I think the breaker panel could be fed from either the utility or generator without an issue.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 17,080
    "SOmeday, somebody is going to reinvent the battery and grid tied systems will become obsolete."

    Aren't you the optimist. Have you actually done the math on how much panel area is required, even for a residence, never mind for a city? Clearly not.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • WillyP
    WillyP Member Posts: 45
    In Maine a lot of houses run completely off grid. Battery cost is a major factor. But having enough panels to run the household is not a big problem. The math is pretty simple. Of course with new construction there is some guess work involved, because there is no electric bill to go off of. As a general number I have been using my house in Virginia for my calculations. I figure I have an old house in an area that uses a lot of AC, so it should do pretty well as far as how much electricity my wife and I use on a regular bases.
    But I am not building off grid. I will be tied to the grid and using the grid as my storage.
    The numbers I have come up with are, twenty 375 watt panels as a minimum, or thirty 375 panels as a maximum. This is based on using 9000 KWH per year at an efficiency rate of 1.31 (for Maine). I will be installing LG neon R panels.