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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

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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
    Commented Sep 6, 2022 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
    Commented Sep 6, 2022 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
    Commented Sep 6, 2022 at 14:41
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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
    Commented Sep 6, 2022 at 15:23
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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
    Commented Sep 6, 2022 at 22:44

2 Answers 2

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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"...

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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
    Commented Sep 6, 2022 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
    Commented Sep 6, 2022 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
    Commented Sep 6, 2022 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.

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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
    Commented Sep 7, 2022 at 9:57
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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
    Commented Sep 7, 2022 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
    Commented Sep 7, 2022 at 12:54
  • @bobflux Ahhh, I see. But it's not clear the OP understands the difference between kW and kWh
    – Carl Witthoft
    Commented Sep 7, 2022 at 13:14
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    One kW for one hour is one kilowatt-hour (kWh)
    – bobflux
    Commented Sep 7, 2022 at 18:34

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