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I recently installed a 3.6kw solar panel setup at home. I added an inverter and a 3.6kw battery.

When I asked if that would work on case of a blackout the installer told me it won't - backfeed protection - as it is connected to the grid. The only solution that they offer was to install a completely separate circuit not connected to the grid and power that one from the battery.

Is there a not-too-complex way of having blackout protection with this setup so if there is sun and/or the battery has power it keeps providing it to the house during the blackout? I think more expensive set ups, like Tesla's, support this.

What would I need to add to my installation to cope with this?

EDIT

The installation is composed of

Juan
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  • Seems odd setup. Solar grid feed does not need batteries(as far as I know). A battery set up is to power a house. Most people with generators(big battery and inverter is the same) have an inexpensive(maybe 100 to 200 dollars, maybe less) lockout to prevent back feeding into the grid. – crip659 Sep 06 '22 at 14:21
  • Well the solar panels feed excess into the grid - the house drains power from the battery and the solar panels if available, from the grid otherwise, and sometimes a mixture of the 3. Battery only charges from the panels – Juan Sep 06 '22 at 14:22
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    The key you know, but are not processing is "more expensive setups" - anything can be done with enough cash applied, but if you got a cheap grid-tie-only setup then it's probably useless without being wholesale replaced with a grid-tie-and-backup solution that will cost more. Thems the breaks... – Ecnerwal Sep 06 '22 at 14:41
  • Thanks @Ecnerwal so I guess I would be looking at probably having to replace the Inverter? I guess panels and battery should be still ok. And the answer would be "yes you can get black out protection, you just need XXX and YYY"? – Juan Sep 06 '22 at 15:01
  • But you've given no details on what VVV, WWW, and ZZZ are other than the kW rating, so nobody can tell you what XXX and YYY would need to be to work with those. Some things are far less interchangeable than you might assume (Manufacturer Q's inverter may not play well with Manufacturer R's battery, since they like to make the whole thing vertically integrated to force you to buy their stuff.) So there may not be a simple swap solution, depending on specifics. – Ecnerwal Sep 06 '22 at 15:21
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    I have a 12KW solar, grid tied system, which is great when the power is on, but it frustrates me that it won't work when the power is off (blackout). I knew that when I bought it, but since then, I've wanted batteries for backup, but haven't found a good solution. Currently, I have a generator with a 200Amp automatic xfer switch to power an entire panel. Problem with batteries and an inverter is you have to decide which circuits to power up from the inverter. Generators & solar don't play well together. Hopefully someone will invent an integrated system that can control it all. – George Anderson Sep 06 '22 at 15:23
  • @George is your solar a DC system or microinverters? You could always throw the DC line over so it feeds a battery charge controller instead of the grid-tie inverter, and then it helps keep your battery up, then have an inverter running off the battery that feeds a critical-loads panel. Or the whole house if you are willing to re-engineer for efficient appliances (slapping batteries on That 70's Guzzler House will require ridiculous sized batteries, and there's a cost breakover vs efficient appliances or "just sell it and get a batter house"). – Harper - Reinstate Monica Sep 06 '22 at 22:30
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    OP you have not given us any of the essential details of what your system even is, that will be **required** to provide a useful answer to the question you asked. You describe the system only in vague categories. Please [edit] to describe your system chapter and verse in every possible detail (hint: if you don't think it matters, *it matters*), with photos if needed, and we can try to figure out what you even have so we can get to advice. – Harper - Reinstate Monica Sep 06 '22 at 22:44
  • @Harper-ReinstateMonica It's not a micro-inverter system, there are 2 large inverters. I don't think the micro inverters where avail when I installed. Fortunately I don't have a "70's Guzzler" house. Propane heat, propane range (both the burners & oven), propane water heater and backup propane generator. Question: Wouldn't disconnecting the DC line from the net metering setup and use it to power batteries and critical circuits defeat the credits I get for solar production and reduction in my power bill? – George Anderson Sep 06 '22 at 22:52
  • @George only if you did it permanently. I would put the panel infeeds on DC-rated SPDT switches so you can switch both banks of panels between their provided grid-tie inverter(s), versus charge controller(s) that feed the battery. Then the battery feeds an off-grid inverter driving into the critical loads panel. The critical loads panel could be the main panel if everything is a critical load lol. – Harper - Reinstate Monica Sep 06 '22 at 23:11
  • I think I get what you are saying, in summary: 1) provide a switch that can transfer PV output between the grid tied inverter and a battery charge controller, 2) feed the battery output into an inverter. 3) Feed the inverter output to a sub-panel, 4) Re-wire critical circuits to the sub-panel that is fed by the inverter. hmmm...in the sub, I'd probably put in a MTS to power it from mains rather than the inverter all the time. Keep the batteries up to snuff with a battery tender. The only downside is I'd have to manually transfer from mains to the battery powered inverter. ...continued: – George Anderson Sep 06 '22 at 23:38
  • But it would give me the option to keep the sub powered by the inverter all the time. Again, it would require a MTS with a listed interlock. Right now, I have a 25KW Onan genset on a 200 amp automatic xfer switch. I might wait a few years and see if there are better options. OK, we'd better shut up now or we'll get kicked over to chat! Thanks. – George Anderson Sep 06 '22 at 23:42
  • Thanks @Harper-ReinstateMonica - I thought it was simpler than it is! I will add as much details as I can when I have a moment at home – Juan Sep 07 '22 at 12:56
  • I have updated adding the details of the different parts of the installation. Let me know if anything else would be of help! – Juan Sep 08 '22 at 14:13

2 Answers2

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There are 3 basic types of alternative energy installations:

  1. Grid-tied
  2. Off-grid
  3. Hybrid

What you have is grid-tied which is absolutely required to shutdown within just a few milliseconds in the event of a grid power outage. As long as power is out the inverter(s) will not come back online. Once they detect suitable power for a suitable amount of time, they will come back online.

Off-grid systems supply power at all times when there is power available to supply. Since there is no grid there is no requirement to shutdown.

Hybrid systems, which are significantly more complex and therefore expensive, have the capability to switch from grid-tied to off-grid and feed your home via a transfer switch similar to one you might use with a backup power generator. There are a set of circuits that are handled by the transfer switch and these will be isolated from the grid during off-grid operation.

It seems from your question that what you want is a hybrid system that allows you to power some number of vital circuits from your PV system during grid outages. My recommendation is to talk to a local installer who is experienced with such systems and take it from there. There is a significant amount of electrical design needed as well as quite a bit more equipment. These systems also often employ battery backup, such as the Tesla Powerwall you mentioned, but combining all these systems and ensuring they work together properly and safely, takes quite a bit is know-how and experience. An answer from someone on an online forum hardly qualifies as "know-how"...

jwh20
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  • It's not really all that complicated - just expensive. Automatic break-over systems have been available for generator systems for decades. The (presumably 230 VAC) output of the solar panel system is indistinguishable from a generator output so long as you have a breakover switch. True, generator systems are never (so far as I know) designed to let you be grid-tied, for obvious reasons. – Carl Witthoft Sep 06 '22 at 17:02
  • Thanks! The question was more to get indications of what could be needed than to do it myself. I definitively get a professional to deal with it. The idea is to get the whole house from the backup not just a few systems, although probably a 3k battery is not reliable enough for that. – Juan Sep 06 '22 at 17:24
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    That's confusing "reliable" with "big enough" - and wanting to power the whole house rather than a few critical things just amplifies your cost problem even more, for what is usually not a realistic need. – Ecnerwal Sep 06 '22 at 17:43
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I'm doing the same thing, so here we go:

backfeed protection

Yes, when the grid goes down, the grid tied inverter must shut down. Otherwise, when the guys from the utility company cut the power to work on the wiring, your inverter would zap them. Hence there is a legal obligation for the inverter to be guaranteed to shut down.

Is there a not-too-complex way of having blackout protection with this setup

Yes but basically, you don't want it. If you use a hybrid or off-grid inverter, you can absolutely wire your whole house on the inverter backup power, if the power rating is high enough. However, in case of a blackout, all the stuff that's idling in the house and drawing power will suck your battery dry in no time.

If you have a 3.6kWh battery and 3.6kWp panels... if the blackout happens on a winter day with very overcast weather, your system will produce a peak power of maybe 300W and an energy of 1 or 2 kWh during the day. That's not much at all! You absolutely want to cut off anything unnecessary, which means you'll power only the freezer and one LED light and a phone charger, stuff like that.

So the big expensive installation you're thinking about, which powers the whole house... it would be a really bad idea anyway because of all the idle loads.

In this case it makes perfect sense to just wire a few sockets on the inverter's backup output and simply plug your freezer in.

I have hydronic central heating with a heat pump. Solar can't run that on batteries, but it can definitely run the circulator which uses little power, and I have a wood burner. So I can switch the central heating to wood if necessary.

bobflux
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  • 3.6 kWh battery ???? Have you **seen** the specs for a PowerWall? Or , for that matter, the optional system from FoMoCo to use their electric pickup truck as a source? – Carl Witthoft Sep 07 '22 at 09:57
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    @CarlWitthoft the questions says 3.6kW for both solar panels and battery, which I interpret as 3.6kWp panels and 3.6kWh battery. – bobflux Sep 07 '22 at 10:23
  • Thanks @bobflux really informative! And yes my battery is only 3.6k, I see your point above maybe being over kill setup. Our daily consumption is between 7-9kw a day, so during summer the battery would be ok to keep things running for a short blackout, in winter I doubt I will be able to charge it completely though (UK) – Juan Sep 07 '22 at 12:54
  • @bobflux Ahhh, I see. But it's not clear the OP understands the difference between kW and kWh – Carl Witthoft Sep 07 '22 at 13:14
  • interesting I thought I did. I have a 3.6kw panel set up - which I understand that at maximum efficiency it will generate 3.6kw in an hour? And my battery is a 3.6kw(x) which I understnad is that it can supply 3.6kw for an hour? @CarlWitthoft – Juan Sep 07 '22 at 13:56
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    One kW for one hour is one kilowatt-hour (kWh) – bobflux Sep 07 '22 at 18:34
  • Basically an off-grid home has to be carefully thought out and every watt accounted for (what is the power use of your wifi router?) To run a "normal" (ie, energy-guzzling) home off-grid **in all weather conditions** you'd need a ridiculously large and expensive solar installation (10x the panels for overcast days, huge batteries...). I sized my installation to run the freezer and have some comfort in case of blackout, which basically means run the wood furnace circulator to have heating, and a few lights, that's all. – bobflux Sep 07 '22 at 18:37
  • However a small (3.6kWp) installation will still greatly reduce the bill for heavy loads like a clothes dryer or electric water heater **IF** these run when there is enough sun to produce enough power. So you need a "solar router" device that switches the water heater in a smart way, and just do the laundry on sunny days. – bobflux Sep 07 '22 at 18:45
  • During this summer I have managed to only "buy" about 1kwh from the grid a day, and use the other 6-8 from the generation alone. Also charge the car during sunny spells. All pretty manual, but easy to deal with. I guess the simpler solution in my case would be to have an extra set of sockets connected to the inverter (if it can do that) and in teh even of black out - disconnect from the grid and feed only those extra sockets? – Juan Sep 08 '22 at 14:15
  • Yes that's what I do, last time the grid went down I plugged the freezer on the inverter socket. It makes sure there are no other loads wasting current. It's possible to automate car charging or routing solar surplus to the water heater. – bobflux Sep 08 '22 at 20:09