How can I use an on-grid solar inverter in an off-grid setup?

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I have a solar system with an on-grid (grid-tie) inverter, which only works when there is an active grid supply. Unfortunately, I live in an area where there’s no grid power for most of the day, so my solar setup goes unused during that time.

Buying a new hybrid inverter is quite expensive and would make my existing on-grid inverter a waste, which I want to avoid.

Here’s what I’m thinking:

  1. I have a 12V 150Ah battery bank connected to a pure sine wave inverter, which generates a stable 230V AC output.

  2. I’m considering disconnecting my solar inverter from the actual grid and connecting it instead to the output of the battery inverter.( In image connecting Point S-B)

  3. This would act as a “fake grid” to simulate grid presence, tricking the on-grid inverter into syncing and starting solar power production.

  4. Then I would run my AC loads using the combined output from both the solar inverter and the battery inverter.

My questions:

  1. Is this approach safe and practical?

  2. What are the risks of backfeeding from the on-grid inverter into the battery inverter?

  3. How can I protect the battery inverter from overcurrent or overvoltage due to solar surplus?

  4. Are there proven dump load controller circuits or open-source solutions to balance this kind of setup?

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Once the solar inverter syncs, it will start pushing power onto the AC line.
If your load is less than the solar output, the extra power will go backward—into the battery inverter’s output.

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Hi MrNams175178
Overall what you want to do I don’t think is a very good idea.

Your sine wave inverter is designed to SOURCE 240VAC. What would happen if you feed 240VAC back into it is anybody’s guess. And personally I am not going to try it to find out.

I have had no experience with this sort of thing but I would have thought the solar system would work alongside the grid supply and not just simply stop when the grid fails. Maybe that is correct and if some provision has not been made to isolate the grid your solar system might try to power everybody else on that grid circuit.

Then again your house might need some sort of steady supply and the solar system on its own might not be able to supply that and your house could experience all sorts of stops and starts which the electrics would not like.

I personally think an approach to the Solar specialists would be the way to go. See what they can suggest and try to find out if your Solar system can operate your house stand alone, I think not. And as I said you would have to isolate the grid so you don’t try to power everybody else as well. I think all these provisions would be designed into a battery backed system but as you say, damned expensive. But at the moment with all these rose coloured pie in the sky phantom money election promises floating around you never know just what is out there.
Cheers Bob

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Sounds correct Bob.
In Australia, any solar system MUST turn off when the grid goes off. this is to ensure you DONT feed back into the grid to protect anyone who may be working on the grid.

If you want up-time in a blackout, you need to have a good size battery, designed to be charged and supply your home. This battery system is meant to
a) Provide any short full in what your home wants V solar generation.
b)The battery is also meant to take any excess power from the solar (i.e. to charge the battery, or deal with the excess somehow)
c) If the battery is full but their is excess solar then the off grid monitor will turn of the solar inverter such that their is no excess. Older system will just be shutdown, newer integrated can tell the solar inverted to output less.

Full off-grid systems should also have batteries for the short fall, while at the same time be able to redirect excess into a load (e.g. a water heater)

Either way, the grid must be isolated if you intend to run your home when the grid is down; a good battery system will have a gateway to do that.

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Hi Michael

Missed that little bit. I was thinking of trying to power other houses. I do know linesmen have been hurt y people powering from a generator during an outage without switching the main switch off. Getting slow in old age.
Cheers Bob

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Thank you all for spending time to reply my post. I feel i should buy hybrid inverter than playing with these.
Its very unfortunate, day time no power supply due to load sharing and its summer here more solar power is generated, its summer full sunlight temperature up to 45degree celcius but no grid supply, complete waste of setup.

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And a whole lot safer too. The “hybrid” inverter would probably have the facility to isolate the grid when there is enough solar built in. You need to check. I just don’t know.

I would personally advise talking to the Solar specialists explaining your problem as they would probably have the best solution.

Yes a shame to waste all that good sunlight. It would be good to gather it all up and hold it until needed. Battery “storage” ???. But that is also pretty expensive but could be the complete “off grid”, or partially anyway, system.
Cheers Bob

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Hello, when you say you want to power other houses, then you are effectively becoming your own utility. I believe the law will prevent that? One fatality and you are history.

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Hi Bryan

Nobody said “want to”. I said if you don’t isolate the grid while powering your own house locally (solar or generator??) this is likely to happen.
Cheers Bob

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My miss interpretation of the wording. This forum is most likely not the correct platform for such a subject. Electrical Power Engineering?
Regards
Bryan

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Hi Bryan
Thought that might have been the case. All OK.
Cheers Bob