4 day power cut – rehearsal for the apocalypse

While working on my solar project I’ve joked with my boys that I’m preparing for the apocalypse. Three days into a 4 day power outage, they’ve joked with me that I am poorly prepared!

My off-grid circuit with a 2S4P 195W solar array and 8S 24V 304Ah battery is fine for getting me through the regular 6 hour evening peak tariff period, but it is inadequate to get us through this extended power outage.

Adding to the frustration as we stand in a cold shower is the fact that we have 6.5KW of solar panels on the roof. We can’t use them for legal reasons, because they are part of a grid-tied system on which we received a large subsidy, and we can’t use them for practical reasons because they are wired for a high voltage inverter and my off-grid system is all low voltage.

I’d be very grateful if anyone can suggest ways of tapping the power from my larger solar array without either breaking the law or spending a fortune.

Diagrams of both systems are shown below.

Are you in Australia ?

RandyP said:

Are you in Australia ?

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Yes, Western Australia - the most backward and restrictive state.

mcart117 said:

While working on my solar project I’ve joked with my boys that I’m preparing for the apocalypse. Three days into a 4 day power outage, they’ve joked with me that I am poorly prepared!

My off-grid circuit with a 2S4P 195W solar array and 8S 24V 304Ah battery is fine for getting me through the regular 6 hour evening peak tariff period, but it is inadequate to get us through this extended power outage.

Adding to the frustration as we stand in a cold shower is the fact that we have 6.5KW of solar panels on the roof. We can’t use them for legal reasons, because they are part of a grid-tied system on which we received a large subsidy, and we can’t use them for practical reasons because they are wired for a high voltage inverter and my off-grid system is all low voltage.

I’d be very grateful if anyone can suggest ways of tapping the power from my larger solar array without either breaking the law or spending a fortune.

Diagrams of both systems are shown below.

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View attachment 107079

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So you have eight 195W panel = 195W x 8 = 1560W, so 5 hours typical of usable Sun = 7.8kWh you are producing per day, but it will not be that much in the real world.
So how do you come up with "we have 6.5KW of solar panels on the roof."? It is confusing to me.
How kWh per day are you using?

What is your off grid inverter? Is it capable of AC coupling and using frequency shifting to control the big PV inverter?

The easiest solution would be to disconnect from the grid and tie your two systems together. But then you need a battery big enough to sink all 8kw of solar, at 24v that's a lot of amps.

Bud Martin said:

So you have eight 195W panel = 195W x 8 = 1560W, so 5 hours typical of usable Sun = 7.8kWh you are producing per day, but it will not be that much in the real world.
So how do you come up with "we have 6.5KW of solar panels on the roof."? It is confusing to me.

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He has two installations: his off grid one (with the 195W panels) and the on-grid 6.5kW one.

Bud Martin said:

So you have eight 195W panel = 195W x 8 = 1560W, so 5 hours typical of usable Sun = 7.8kWh you are producing per day, but it will not be that much in the real world.

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This is the off-grid system, using discarded panels (top diagram)

Bud Martin said:

So how do you come up with "we have 6.5KW of solar panels on the roof."? It is confusing to me.
How kWh per day are you using?

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This is the on-grid system (Bottom diagram)

upnorthandpersonal said:

He has two installations: his off grid one (with the 195W panels) and the on-grid 6.5kW one.

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Thanks upnorhtandpersonal, well explained.

mcart117 said:

I’d be very grateful if anyone can suggest ways of tapping the power from my larger solar array without either breaking the law or spending a fortune.

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Not without a grid forming (off-grid) inverter and battery system with the capability to manage and control the output of the Fronius grid-tied system using frequency control. You will also need a way for the Fronius inverter to be completely isolated from grid connection.

By law need this would need be installed by a qualified electrician and unless your off-grid system has the capacity to be a grid forming AC coupled system then it would require an inverter with those capabilities. I would be pretty surprised if a 24 V off-grid inverter would have that capability. This is how a Tesla Powerwall 2 operates (and it would work well with a Fronius 5 kW inverter) but obviously it's hideously expensive.

In short it's neither cheap nor easy. It's certainly not a DIY job in Australia.

mcart117 said:

Yes, Western Australia - the most backward and restrictive state.

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From the perspective of grid tied solar PV systems WA have tighter limits on grid exports but otherwise the requirements are fairly harmonised across Australia.

mcart117 said:

battery is fine for getting me through the regular 6 hour evening peak tariff period, but it is inadequate to get us through this extended power outage.

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Off-grid you have
8 x 195 W panels = 1.56 kW
24 V x 304 Wh battery ~= 7.3 kWh

During outages you need work on restricting energy consumption to essentials only. It requires a rethink of what you absolutely need to power. 7.3 kWh should be more than ample to get you through a night and back to solar production hours. Lights, communications, a computer/TV, a fridge. Should be able to do that with 2 kWh.

mcart117 said:

Adding to the frustration as we stand in a cold shower

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Depending on what sort of HW system you have it may not help you anyway. If it's an instant gas HW then that's all good, maybe a heat pump unit would be sort of OK to run in the daytime after batteries have been recharged but it's not likely you would want to power an electric resistance element hot water tank with a small off-grid inverter.

I also have both grid system (11 kW PV, 10 kW Fronius inverter) and a smaller off-grid system (2.22 kW PV / 20 kWh SLA battery / 4 kW inverter). Obviously I have more battery than you but in the end a run of really lousy solar days may result in a loss of power (I'm powering 2 homes and one outbuilding).

Which is why I have a backup generator which can be used for a 2-3 hours to recharge the batteries should it be required.

400bird said:

What is your off grid inverter? Is it capable of AC coupling and using frequency shifting to control the big PV inverter?

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XIJIA SwiPower 5kW. I am only using a fraction of its capacity right now, but it seems to be performing well.

400bird said:

The easiest solution would be to disconnect from the grid and tie your two systems together. But then you need a battery big enough to sink all 8kw of solar, at 24v that's a lot of amps.

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I have thought of ripping 8 panels of the house roof and making another 2S4P set, but I don't think I'm allowed to do that. Under the terms of the subsidy, they all have to remain tied to the grid, but there is no restriction on how much I am allowed to consume. I have thought of buying a big battery charger (or in extremis an EV) and just sucking power out of the on-grid system during the day.

But I am looking more for a replicable solution which takes power from a solar array and pumps it into a LiFePO4 battery before passing it on to the grid-tied inverter.

Maybe I just need to find a low voltage grid-tied inverter which will accept input from a 24 or 48v battery/charge controller.

Thanks for the reply @wattmatters. Funnily enough I was just reading your thread on your system. It has similarities with my off-grid system, except that yours is more professionally wired and set up. Mine is still a work in progress.

wattmatters said:

This is how a Tesla Powerwall 2 operates (and it would work well with a Fronius 5 kW inverter) but obviously it's hideously expensive.

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If I could steal the electronics from the Tesla Powerwall, and connect them to my own battery, it would be great.

wattmatters said:

8 x 195 W panels = 1.56 kW
24 V x 304 Wh battery ~= 7.3 kWh

During outages you need work on restricting energy consumption to essentials only. It requires a rethink of what you absolutely need to power. 7.3 kWh should be more than ample to get you through a night and back to solar production hours. Lights, communications, a computer/TV, a fridge. Should be able to do that with 2 kWh.

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You are quite correct, it is not enough. I have ordered 8 more cells to make a second battery, and before next winter I'll add more panels to the off-grid system.

wattmatters said:

Depending on what sort of HW system you have it may not help you anyway.

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I have a large electric HW system, which, when the grid is working, is an excellent energy sink for my on-grid system.

So this post is not just about my own system. I am looking for more general solutions, which make more sense than what is being offered.

Like in Australia, where single phase systems are only allowed a 5KW inverter, why would you put the inverter before the battery? Why not take all you can from the solar panels, and then put what you can't store into the inverter?

Or better still separate the 5KW choke and safety switch from the inverter. Or if you can't do that, two inverters: one "offgrid" inverter feeding straight into the house, and a second "on-grid" inverter feeding anything you can't use or store into the grid.

mcart117 said:

why would you put the inverter before the battery?

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I'm not really sure what you mean by this.

mcart117 said:

Why not take all you can from the solar panels, and then put what you can't store into the inverter?

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Inverters can't store energy, but I think you meant battery.

You certainly can do this - by having a grid tied battery system, either an AC coupled battery (like a Powerwall 2) or by having a hybrid inverter (like a Fronius Gen 24 hybrid) which connects to a suitable battery. Any inverter output which is not consumed by the household is then stored in the battery, only solar PV energy beyond what the household and battery can consume is exported to the grid.

mcart117 said:

I have a large electric HW system, which, when the grid is working, is an excellent energy sink for my on-grid system.

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Yeah as an energy storage they are great (I have a smart PV diverter for ours), but the problem when off-grid is the power demand.

Typical heating elements are 3.6 kW, some are 4.8 kW. If you expect to power an electric hot water system while off-grid then you'll need a LOT more solar PV and storage and a much more powerful inverter.

Depending on the tank's capacity and your HW consumption you may get a couple of days out of it before it runs out.

Our electric resistance HW system is not powered at all from off-grid, but we have alternative HW supply if that scenario occurs (my mancave has an instant gas HW unit and a shower).

wattmatters said:

I'm not really sure what you mean by this.

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In the graphic above DC from the panels goes into an inverter. The inverter is before the battery in the energy flow.

In my modified graphic I've put the battery before the inverter. Any energy not stored by the battery flows through to the inverter for distribution wherever.

The Tesla Powerwall is an AC coupled battery - it has its own built in inverter. The output from the Powerwall is AC, not DC.

Put another way - it's operating in parallel with the solar PV inverter and grid.

wattmatters said:

During outages you need work on restricting energy consumption to essentials only.

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Good practice for the apocalypse. ?

mcart117 said:

While working on my solar project I’ve joked with my boys that I’m preparing for the apocalypse. Three days into a 4 day power outage, they’ve joked with me that I am poorly prepared!

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You can certainly have a hybrid grid tied inverter which connects to a DC coupled battery. If you replaced the Powerwall in your second diagram with a straight battery (no inverter) and moved it as an input/output to a hybrid inverter, this becomes a DC coupled system.

Like this:



The difference in WA are the Western Power connection limits for AC coupled and DC coupled battery systems are different (unfortunately). With DC coupled you are still limited to a 5 kW hybrid inverter, same as if you didn't have a battery. With an AC coupled system (i.e. your first diagram) you are permitted 10 kW of inverter capacity, being 5 kW solar PV inverter and 5 kW for the battery inverter.

wattmatters said:

The Tesla Powerwall is an AC coupled battery - it has its own built in inverter. The output from the Powerwall is AC, not DC.

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Sorry, I was being lazy. I was using the Tesla graphic, but I don't want the Tesla battery in the circuit, I want my battery.

I'll post a better illustration of what I'd like to do later.

mcart117 said:

Sorry, I was being lazy. I was using the Tesla graphic, but I don't want the Tesla battery in the circuit, I want my battery.
View attachment 107184
I'll post a better illustration of what I'd like to do later.

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The missing link in that diagram is a solar charge controller. It goes better the PV and battery. I doubt there's a restriction on this size, so you could have as many KW as you want, it's just charging a battery.

Then the inverter takes that DC from the PV/battery and turns it into AC for your house or to push out to the grid.

400bird said:

The missing link in that diagram is a solar charge controller. It goes better the PV and battery. I doubt there's a restriction on this size, so you could have as many KW as you want, it's just charging a battery.

Then the inverter takes that DC from the PV/battery and turns it into AC for your house or to push out to the grid.

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Yes, I needed time to construct my own diagram, and here it is.

The solar array connects to a battery (which will have to be bigger than my current one – maybe 32S 96V) and the battery takes what it wants. The Fronius inverter is connected in parallel with this circuit, and it does its thing. In the event of a power outage, the changeover switch on the main power board allows the XIJIA Swipower inverter emergency access to the home circuit.

Wait, what? No, not like that

You're battery would be 24 or 48 volts, I assume.
I'm pretty sure the Fronius inverter is a grid tie PV inverter, it isn't meant to connect to a battery. I could be wrong, but a quick Google came up with only PV and AC connections.

If you want to charge the battery from solar, an MTTP charge controller is a good way to do it. But, I don't think you've got the right idea on how to connect everything.

I assumed you would be disconnecting your Fronius when the grid goes down and plugging those panels into the charge controller. You can't modify the system, but when the grid is down it isn't going to do them any good.