What's the best Home Energy Storage option for retaining domestic stored solar power in preparation for power cuts?

  • 20 October 2021
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Due to inept official planning and misguided political priorities there seems to be a consensus that power cuts are more likely than usual this coming winter. In fact. preparation for power cuts was a strong motivation to me for installing extra storage batteries to keep systems operating during outages.

However, I am looking for some help in how best to achieve this.

I live in Cheshire in the NW and have 28 JASolar PVMono 345 south-facing panels, nominal output 9.5kW.

4 Pylontech 3.5kWh batteries = nominally 14kWh capacity.

Pre-solar my domestic load averaged around 10 kWh per 24 hours and is still similar.

I don’t fully understand the relationship or the interface between the AC grid and my installation as it was all put in by a professional team.

The main inverter is Solis 5G Single Phase.

Batteries are controlled by Growatt SPA3000TL BL (AC Coupled inverter)

(BTW, does anyone really know what SPA means in this context? There are vague possibilities among the over 200 meanings listed in “thefreedictionary.com”: Switching Power Amplifier, or Serial Port Adaptor, or Solar Panel Assembly, and several more.)

On any reasonably bright day between April and September and especially with some sunshine my batteries become fully charged, the HW cistern is heated and the grid receives all my excess. The smart meter doesn’t move from one day to the next and I’m basically using solar for everything, which is gratifying.

As the community will be aware, October to March can be rather different. Much less solar input so that a lot of my power is drawn from the grid and the batteries are down to less than 10% capacity each morning.

They do sometimes fill right up after a few hours of winter sunshine, but are used up quickly once the day darkens.

I would like to explore 2 possibilities in preparation for if power cuts start happening:

  1. Some way to manually isolate the batteries once they are filled so as to preserve the stored energy to be available when the lights go out.
  2. When there is very little daylight and not much chance of charging the batteries from the panels, some way to manually charge them from the grid while it is live and then reconnect then during the next power cut.

Does anyone have experience of this, or do you think it could be done?

Thanks, David 101


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Howdy @dnshorto !

This is definitely something that a few forum volunteers know about. I’m not one of them myself, but no doubt they’ll stop by soon.

In the meantime, do you have the wiring diagram and a list of all components in your system handy by any chance? It’ll almost certainly prove quite useful if you’re OK with sharing that.

Thanks!

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Thanks @blastoise186

Good idea. I will attached the schematic which was prepared recently by the actual Oct 2020 installers, (not the deceptive Sales company who claimed the installers were their own team), plus some photos. I’m not sure all the isolating switches shown on the schematic are there in reality. 

 

 

 

David101

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Not an expert either, but it’s something I’ve been interested in. The battery back-up was the main reason I’d consider the Tesla Powerwall.

It looks like Victron offer an inverter with a grid disconnection relay. Some info from Joju Solar:

https://www.jojusolar.co.uk/batteries-smart-grids/using-home-battery-power-cut/

The main issue will be meeting the strict safety rules to make sure you don’t fry anyone working on the network during the power cut!

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Mind you, I’d heard the backlight on the Landis+Gyr E470 was horrible, but I never thought it’d be so terrible that you need to supplement it with a lamp to see the display! XD

That’s a pretty nice looking Geo Trio II IHD you’ve got there mind you.

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Thanks nealmurphy

That is a very useful link, as I find that juju solar have some other highly relevant articles.

David101

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Hey @Transparent @PeterR1947 , I’ve got a present for you two! It’s your favourite solar inverter style too! XD

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As it so happens, I’ve just started trying to decipher the workings of a similar installation owned by a Forum Member in Scotland.

If you look at the photos of the kit and the schematic diagrams which we’ve been exchanging, it seems that his Solis inverter differs from yours @dnshorto. But we may need to clarify this.

As I currently understand his set-up, his 4x Pylontech batteries are directly connected to a Solis RHI-3.6k-48ES Inverter.

Moreover the specification for that inverter series, which I’ve obtained from the Chinese manufacturer, shows that it has a mains (240v?) output for devices that must be kept live in the event of a power-cut.

I don’t yet know if that feature is present on the models available for sale in the UK, nor how they could pass the G98/G99 certification for grid-connection. A key feature of those standards is that there must be no possibility of power being sent back onto the house supply when there is an outage on the Distribution Grid.

A UK manufacturer of Storage Batteries, PowerVault, also used to have an auxiliary mains-out connection of their hardware. But this was removed a couple of years ago in order to better comply with the UK safety requirements.

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@dnshorto - you might also want to look at the suggestion I made on the topic about Heat Pump Installation in order to avoid using electricity during the evening period of peak demand.

Rather than store the energy as heat, which would require a massive tank, I proposed a grid-connected battery feeding an off-grid inverter.

You might think that this somewhat pushes the subject of safety. However, that off-grid inverter could not be grid-connected because it has no inbuilt facility for synchronisation to the 50Hz grid frequency. It can only work when its output is regulated by its internal clock.

If grid-connection were attempted, then the clash of the two 50Hz frequencies would blow the output fuses on the inverter. However powerful your inverter is, it’s no match for your nearest commercial power-station!  :thinking:

This goes some way towards answering the other part of the original question you posted above about having power during an outage.

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Having survived the power cuts of 1976, as the only house with a gas fire in the family, the house was kept warm by the gas far and family body heat; candles or torches provided the light and we enjoyed family time together, much easier to arrange than all these teccy solutions!

Old timer Peter

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Very much intrigued by this discussion, @dnshorto.

 

As I’m by no-means the expert in Home Energy Storage I’m happy to be learning too!

 

 

I don’t yet know if that feature is present on the models available for sale in the UK, nor how they could pass the G98/G99 certification for grid-connection. A key feature of those standards is that there must be no possibility of power being sent back onto the house supply when there is an outage on the Distribution Grid.

 

I’m confused by this point, @Transparent - so would this mean any grid-connected battery can’t be used to supply your house in case of a power cut?

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It does seem that way. I’m nowhere near an expert capable of giving a proper answer, but I would assume that it might be something to do with preventing an overload or imbalance somewhere.

Off-grid batteries could be considered to be a bit like an Uninterruptable Power Supply, (or even just a giant powerbank) which can kick in and flip to battery power in the event that the mains supply goes down for any reason. Usually, this is designed to either allow you to ride the storm through a short outage, or give you enough time to safely perform graceful shutdowns on all protected equipment and take it all offline properly, so that nothing gets lost or damaged. How much load a UPS can handle and for how long very much depends on multiple factors, such as the capacity of the batteries, type of UPS and the load that’s depending on it.

But apparently, grid-connected batteries and microgeneration has to shut down if there’s a power outage on the grid.

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would this mean any grid-connected battery can’t be used to supply your house in case of a power cut?

Yes, that’s correct. It’s part of the specification for G98/G99/G100 devices.

If a generation device such as a Storage Battery or PV Inverter is grid-connected for export, then it is required to de-couple its output within a few mS of a power-outage.

Moreover, when the mains supply is restored to the house, all export devices must remain disconnected for a random amount of time before automatically connecting. This ensures that there will be no surge-currents in an area when the DNO switches the Grid back on.

This may better explain why I feel that there is a lot to be gained by having at least some off-grid capability for any house with Storage Batteries.

In light of this new information @Jess_OVO you may now want to re-read the story about the low grid voltage at March End. :slight_smile:

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This may better explain why I feel that there is a lot to be gained by having at least some off-grid capability for any house with Storage Batteries.

 

Sounds like off-grid storage might be the best fit for use as a back-up power supply then, @dnshorto

 

 

  1. When there is very little daylight and not much chance of charging the batteries from the panels, some way to manually charge them from the grid while it is live and then reconnect then during the next power cut.

Does anyone have experience of this, or do you think it could be done?

 

@Transparent, does this mean the possibility of filling batteries from the grid for use during a power-cut wouldn’t currently be allowed?

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No @Jess_OVO - in fact the opposite should be the case.

Look at the diagram I posted above. Charging a Storage Battery from the mains is permitted, whether or not you export any of that electricity back to the Grid.

If that off-grid battery is to run a Heat pump or EV charge-point, then I still think that the Low Carbon Technology application should be made to the regional DNO in the usual way.

Moreover, such a configuration of Storage Battery could provide major benefits to the Distribution Grid.

a: if that area of the country currently has an excess of renewable energy generation, then it makes more sense to store it in customers’ homes than to pay compensation to the producers to remove some of the available capacity.

b: If there are losses due to phase-imbalance at the local substation, these could be reduced by adding a Smart-load, such as a Storage Battery or immersion-heater, to the phase which is drawing least current from the Grid.

c: Customers in energy-poverty could be provided with a Storage Battery by a local Housing Association, and linked to a Time Of Use tariff. That enables them to avoid taking power when it’s most expensive. There is a strong ethical dimension to this approach.

 

All this technology is available, but needs to be welded together using a predictive Smart-network such as the Flex Platform from Kaluza, or Kraken-Flex from Octopus.

What’s needed is a trial-site with a Storage Battery in an area with abundant generation from Grid-connected commercial Solar and Wind farms which can’t always be fed back to the National Grid. They’ll need to be put on a variable ToU tariff ahead of other customers so that the relevant algorithms can be developed.

Oh look!

I just happen to be in the ideal location with a PowerVault Battery which can’t be used for the original intended grid-support trial!  :smiley:

I do hope the Smart Home Team at OVO are seriously looking into this.

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No @Jess_OVO - in fact the opposite should be the case.

 

Well now I am confused, @Transparent !

 

 

Rather than store the energy as heat, which would require a massive tank, I proposed a grid-connected battery feeding an off-grid inverter.

 

So by having a grid-connected battery but an off-grid inverter would this allow the battery to charge up from the grid but the inverter would still operate during power-cuts even though the battery isn’t able to? Does the Inverter act as another store of energy then or is it able to access the energy stored in the battery somehow? 

 

Hope you’re following this better than me, @dnshorto!

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That 100A Off-grid Inverter draws energy solely from the battery stack @Jess_OVO

How and when the battery is re-charged is a separate issue. The householder will want to achieve that in the cheapest and greenest way possible.

An off-grid inverter continues to operate in a complete power-cut.

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Great chat above, I think we’ve narrowed it down and we have an answer for you, @dnshorto:

 

An off-grid inverter continues to operate in a complete power-cut.

 

Although I’m aware you have two inverters in play here. The Solis inverter and the Growatt AC Coupled inverter.

 

So where does that leave you? 

 

Imagine me as the future viewer of this thread who’s keen to understand their options! 

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Thank you to the various contributors who responded to my question; I have been away in Scotland until today and found it frustratingly impossible to log in to my Ovo account on my cell phone in order to reply. Back on my laptop today which has given me access once more!

In particular, thank you @transparent, as before, for highly relevant answers.

When planning to install solar panels and batteries, one of my primary motivations was to make provision for future power cuts. In a 1980s house with zero provision for alternative domestic heating that will work without mains input (no flues, no fireplaces) and with household members suffering from long-term disabilities I knew I had to think ahead. All this was made abundantly clear to the solar panel company who won the contract.

Imagine my shock on learning a few days ago that the batteries as set up in the approved system CANNOT be used during a power cut. I suspect that this would have been news to the solar panel salesman too, as he had given not the slightest hint that my plan was unworkable. I had originally planned to install a diesel generator for back-up power but chose the solar-powered battery bank as a much greener (and quieter) option.

@Transparent has written of an “off-grid” battery, which could be charged from the grid when available. 

Thinking of the parallel situation on board a boat which these day may well have many of its domestic systems running on 230v ac, powered when in dock from a shore line and when at sea from batteries  via an inverter, is there a possibility here to replicate a similar system for a house?

Via a suitable changeover switch or switches, could the house be disconnected from the grid during a power cut and the battery bank take over the load? 

An “off-grid battery” suggests a whole separate item, which inevitably means extra cost.

During normal times the present battery bank is a great asset and reduces our grid consumption brilliantly, so that for much of the year I would want to keep it connected. What I need to work out is some way to utilise that stored power for those coldest, darkest days and nights when this coming winter’s power cuts are most likely to catch us out.

David

 

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Correction: “ In a 1980s hours”, should have been  “In a 1980s HOUSE” of course.

Also, having now re-read some earlier posts on the laptop (instead of the cell phone!) I see that @Transparent is clearly discussing an off-grid INVERTER, rather than a separate battery bank. My mistake. 

Would that separate inverter be able to use the same domestic distribution system within the house, or would completely separate circuits have to be run?

The primary concern would be the gas CH boiler, which is within the same integral garage as the batteries, inverters etc, so a separate feed to that would be quite feasible.

Cooking could be by portable “camping’ type gas ring, and lighting is also easily obtained. It’s the ability to keep the house warm in  a really cold winter that concerns me.

David

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To be honest, I think you’ll find that a backup diesel generator would be a terrible option for other reasons too. Not least the fact that the ones which are suitable for use as a UPS (Uninterruptable Power Supply) solution of any kind are generally pretty expensive and not easy to maintain, so they’re not really a viable option for domestic use.

This may also come as a surprise to everyone who decided to panic buy petrol and diesel recently, but there’s another flaw too! Petrol and diesel literally have use by dates! XD

Seriously, I’m not joking. If you leave that stuff just sitting in a tank for too long, it goes off and becomes unusable… And that’s assuming it hasn’t somehow evaporated before then! Because yes, that’s also a thing which can happen as well. You can literally lose an entire tank of petrol and diesel into the atmosphere. :D

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@dnshorto wrote:

Would that separate inverter be able to use the same domestic distribution system within the house, or would completely separate circuits have to be run?

 

This question suggests that I’m still not adequately explaining the overall concept of an off-grid inverter and why you can still run it during a power-outage.

There’s a YouTube channel with a guy called Will Prowse who is an expert on solar power and Storage Batteries in large motorhomes (in the USA). Thus he well understands the need both to be independent of the electricity grid, and yet be able to plug in when visiting a static trailer-park.

Here’s a relevant video he’s published recently:

 

The off-grid Growatt inverter Will uses in this video-tutorial is the SPF3000TL LVM which is for 120v AC output.

The equivalent for use in UK/Europe, running on 230vAV 50Hz, is SPF3000TL HVM. There are other models in the series and an option for 24v or 48v for the (Lithium ion) battery.

Will speaks fast, but everything he mentions is shown, in similar fashion to the Royal Institute Christmas Lectures. Nevertheless, I still prefer to watch his videos with my finger ready on the pause (K) key!

If @dnshorto is happy with this I’ll then go on to draw a diagram of how this type of inverter can be legally and safely installed in a UK home.

If you’re still finding the video incomprehensible, please say what you’d like me to expand on first. There’s little point in me drawing installation diagrams while you’re unsure about the overall concepts - such as being able to input power from either mains or PV Panels.

I also rely a lot on @Jess_OVO telling me when we’re going too deep technically, which is very useful :slight_smile:

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Thanks @Transparent . I understood Will’s video, which was quite relevant to my quest although I had to repeat some of the footage. It seems that the Growatt inverter he was using has a mains charging input which would be vital in a winter UK setting. His average 5.5 hours of sunshine might well be available in Nevada but not here, especially in the power cut season.

I would be very grateful if you could draw an installation diagram but would hope to be able to ask for clarification if I don’t fully understand it.

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OK…. so here’s a basic diagram of an off-grid mains & solar battery, @dnshorto :

 

The two Distribution Boards both deliver power to mains appliances within the home.

In the event of a power-outage, devices on circuits connected to DB1 will fail, whereas those connected to DB2 will continue to operate (whilst you still have energy in the battery).

I have more things to add to this diagram, but let’s stop there for the moment. Do I need to clarify anything?

 

Error in diagram: The mains connection to the Off-grid inverter via its isolation-switch should have come from a trip in DB1 and not directly from the Smart Meter.

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Thank you @Transparent, that’s very clear as a new installation and easy to understand. I’m aware that you have more additions to include but, just to take this basic diagram to start with, `I have a question. Where the Off-grid inverter has a mains input, obviously that could be subject to controls so that the main battery input would be the solar panels, with grid input available for dark days. How automatic could that control function be?

Moving on to the actual situation I am in, it is obvious that if I had known that my system as installed a year ago would be incapable of maintaining supplies during power cuts I could have used a version of your off-grid diagram to make provision for them. So naturally I immediately try to envisage how what I have could be modified to achieve that. A number of other questions immediately start to occur, but maybe I should step back and wait for the next layer of detail in your excellent diagram. I greatly appreciate your help in this, and hope it may also be useful to other members.

@dnshorto 

 

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I have your existing installation mind, @dnshorto  as I present the second diagram:

 

In this scenario there are now two solar inverters. One is grid-connected, and must therefore be export-limited to 3.68kW (16A per phase) as is normal for applications approved by the regional Distribution Network Operator (DNO).

The rooftop solar panels are now divided into separate arrays and connected via a String Combiner box. This contains separate safety trips and fuses for each array, together with any lightning suppressors you may wish to include.

The trips in the String Combiner allow arrays to be directed either to each Inverter separately, or else added together to feed just one inverter. It provides the flexibility for changing how much energy is sent to either system.

 

Here’s my own String Combiner box which has inputs from three separate solar arrays on the roof.

 

And here are the rooftop panels, showing how they are divided

 

Arrays A, B & C connect to the String Combiner above. Arrays D & E feed different off-grid systems which will both be altered/upgraded next summer (2022).

 

You wrote:

Where the Off-grid inverter has a mains input, obviously that could be subject to controls so that the main battery input would be the solar panels, with grid input available for dark days.

Absolutely. The control system is most important.

At the moment mine is very manual.

I had been hoping that OVO would use this Trial Site to understand how the Flex Platform can apportion the available solar energy for the day so as to optimise the storage facilities. After all, Flex has weather forecast input and would therefore ‘know’ what proportion of the generation to send through each route.

However, as matters currently stand, my site is no longer on the Storage Trial because Flex ‘assumes’ that full export to the Distribution Grid is always available.

Since Grid voltage in my area is high (about 246v), this assumption may not be true. Kaluza are only monitoring the PowerVault Storage Battery, and not the grid-connected inverter, so they could not know why export may be constrained when their system attempts to force it.

I am now looking outside of OVO for a solution.

We are therefore both in the same situation…. and there are a growing number of houses which will have more than one Grid-connected Export Device. Flexibility of control systems is of increasingly greater importance.

We can return to the Control logic a little later.

 

Let’s pause for a moment before I further add to the diagram. Do you want me to clarify any of the above?

We also need @Tim_OVO and @Jess_OVO to catch up with this weekend’s posts. Both have been asking relevant questions, and last week Tim wrote:

Imagine me as the future viewer of this thread who’s keen to understand their options! 

I don’t think we’re quite ready for a discussion of options yet. I have one further concept to introduce.

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