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

Thanks for the very clear pictorial layout drawing @Transparent; that’s very helpful in working out which hardware item does what. There are actually 2 consumer units, DB1 & DB2 one above the other but I guess the mains connection into one inevitably connects both.

Given that the Growatt inverter connects to the grid at the small metal distribution board (let’s call that DB3?), how does it only use solar power to charge the battery? Is there a diode in that circuit to prevent grid power going the other way, or is that incorporated in the inverter itself? Hope that is not too dumb a question.

Another question. I know (now) that if grid power fails the inverter(s?) must disconnect from the mains immediately. Is that built in to the inverters? Will the potential off-grid inverter be a different type, or specification, or simply with adjustable settings to enable it to function without grid input?

Regarding the DIN enclosures; is it possible to be confident that all the needed components fit on to a DIN rail? No extra “non-DIN” space needed for incompatible items?

It’s all highly useful information, thank you

@dnshorto 

There’s no diode @dnshorto  - We’re talking about Alternating Current, so a diode would only stop it flowing in one direction.

You’ve got a couple of Current-clamps on the Live-Feed to the house.

I’m uncertain where they connect to, but I would guess one for the Growatt and one for the Solic Load Diverter. The current-clamp enables those devices to first take what solar-generation you have available for storage, and only then allow the remainder to be exported.

The mains-disconnect system is inbuilt to every export device which is permitted to be connected to the UK Grid. They must be tested and certified to the G98 Standard.

It doesn’t apply to an off-grid inverter because it’s not an export device.

Please may I redraw your attention @Transparent  to earlier questions about batteries? I am extremely grateful for all the time and effort you are putting in so am hesitant to make this request, but to me some way of using the existing storage capacity during the winter is an essential part of my original question

I now understand the necessity for an off-grid system to cope with power cuts, but which would also have year-round value for some dedicated AC loads such as gas boiler pump and fridge motor plus DC lighting etc. You have made that very clear, thank you.

I also understand the steps so far taken towards an off-grid inverter, powered by a variable number of panels via a string combiner. No doubt there are more details yet to be revealed but the basics are now clear and it has all been extremely useful info.

Obviously a battery will be needed too.

On your pictorial diagram of my installation you show the battery connected directly to the Growatt inverter and I can confirm that the heavy red and black cables go straight there. I have 4 Pylontech 48volt Li-ion units in parallel. each with a usable capacity of 3200Wh according to the user manual, and a recommended charge/discharge current of 37 Amps, maximum 74 A.

Could a new off-grid inverter be directly connected to the battery instead via a DC changeover switch, so that either inverter could be linked to the battery but not both?

Thanks,

Dave

@dnshorto 

All highly relevant and useful thoughts thank you @Transparent. That’s a very helpful picture-drawing of a possible solution (only the off-grid consumer unit needs to be DB4 as DB1 and DB2 exist already, along with DB3) but I would 100% want to keep the battery in it’s present role for the sunnier half of the year. The capacity is enough to run everything easily through the night if it has had a reasonable charge during the day, so that we use zero grid energy for much of the time. The meter stops for days on end! Plus I am sold on your plan of a string combiner so that i can have flexibility of input from the sun for different priorities and changing seasons.

It’s a different story in winter though, which is why my preferred plan is to have the entire battery switchable between inverters. The storage is little use in the winter for the full domestic load in the evenings and I am concerned that the battery never gets a full charge, whereas it would do much better only powering DB4.

I had wondered about a Contactor for switching the battery between inverters and there are 48v coil versions available, but a manual switch such as this one would cost around £70:

or similar rated at 100 amp for around £80.. Would this suffice?

The new off-grid hybrid inverter would ideally be capable of handling up to 21 of the 28 panels via the string combiner, so 21 x 340 = 7.14kW. If only 14 panels then 14 x 340 = 4.76kW.

Although it is “off grid” I see your drawing shows the inverter connected to DB1 via an isolator and hope that this implies that the off grid inverter could also be used to charge the battery if needed in low solar input times. Is this last paragraph making sense?

Once again, huge thanks for the detailed and valuable knowledge you are sharing.

@dnshorto 

A fascinating thought @Transparent and a very economical and ingeneous way to repurpose the existing Growatt inverter for off-grid use. However, since I want to retain my present grid-connected system, to enjoy the benefits of solar for my whole domestic load and reduce my reliance on the UK’s dwindling mains power I need my Growatt for that.

I can’t go fully off-grid without a sizeable back-up generator but what I have realised that I can do (with the invaluable help of this forum) is to prepare for future power shortages by installing a separate off-grid inverter variably fed through a string combiner and feeding a dedicated distribution board as already discussed.

I had an opportunity to test the makeup of the solar panel strings and it seems you were right @transparent to hesitate, and doubt your initial guess that the 28 panels had been configured 14 -7 -7.

Remembering your warning about high DC voltage I disconnected the 6 terminals at night, and was surprised that there was a small but steady input. The inverter has 6 terminals, number 1 on its own, number 2 as two pairs, so for clarity I refer to them as A for terminal 1, and B and C for the other two.  The A pair showed a night-time average of 4.3v. B and C an average of 3.2v each.

In very weak sunlight the next day A gave 390v, B & C 352v each. Slightly brighter light put A up to 400v, B & C up to 370v each, from which I calculate that terminal A has 10 panels, while B & C have 9 panels each; 28 in total. Would you concur?

Turning back, if you don’t mind, from interesting but possibly illicit tinkering inside the Growatt to the perhaps boring topic of batteries, my preferred plan is to have the entire battery switchable between inverters. The storage is little use in the winter for the full domestic load in the evenings and I am concerned that the battery never gets a full charge, whereas it would do much better only powering DB4 (off-grid appliances) 

I had wondered about a Contactor for switching the battery between inverters and there are 48v coil versions available, but a manual switch such as this one would cost around £70:

or similar rated at 100 amp for around £80.. Would this suffice?

The new off-grid hybrid inverter would ideally be capable of handling up to 19 of the 28 panels via the string combiner, so 19 x 340 = 6.46kW. If only 10 panels then 10 x 340 = 3.4kW. Some off grid hybrid inverters have a TOU function for topping up batteries at cheap rate times which I would need in winter. How about SKU: SOL-5K-RHI-48ES-DC Categories: Hybrid Inverter, Solis,, or there is a 6kW version?

Once again, huge thanks for the detailed and valuable knowledge you are sharing.

@dnshorto 

Thanks for the latest ideas @Transparent. I have a few more questions too, if you don’t mind. Hopefully the answers may be useful to other readers in due course

I realise that buying a third inverter may seem wasteful but I still lean towards that solution in my particular situation. The system as installed works well and is under warranty. If I monkey about with the Growatt I might regret it, so I am prepared to splash out on a separate off grid inverter to be ready for power cuts. A big enough generator would cost much more and I’d rather harness the sun as far as possible.. 

My previous post included a photo of a rotary switch to changeover the battery connection between inverters. The photo vanished so here is another.

<<I had wondered about a Contactor for switching the battery between inverters and there are 48v coil versions available, but a manual switch such as this one would cost around £70:

or similar rated at 100 amp for around £80.. Would this suffice? Or would a specifically DC type be better?» 

 I assume that I would only need to divert the positive battery cable, and that the negative could remain connected to both inverters?

The 6 pole rotary isolator which you suggested for the incoming 6 PV leads is proving slightly puzzling. There are several offerings, but the suggested wiring layouts are all somewhat confusing. I picture a simple 6 incoming terminals paired with 6 corresponding outgoing terminals, but the accompanying drawings of wiring diagrams make it all look much more complicated. Any ideas?

Lightning protection. I understand your proposal that the surge protection devices are incorporated in the DIN rail enclosure of the string combiner. Due to lack of adjacent space this item will be several metres away. Would it be better to introduce stand-alone lightning protection right where the 6 cables from the roof arrive; before or after the 6 pole isolator?

Solar PV cabling. Preparing to extend and supplement the existing cabling I need to establish the size. I suspect it is all 4mm as this seems to be the most common, but the OD by micrometer is nearer to 5mm. Does this sound like 4mm to you?

Solar panel topology. Thanks; next time the sun shines I will measure the voltages again to try to be sure the panels are arranged 10 : 9 : 9. The new inverter should be sized to accept input from up to 18 panels, so realistically the present grid-connected set-up would have 10 panels permanently connected, and the off-grid 9, with the 3rd string of 9 switchable between them. Simpler than first proposed.

Off-grid domestic loads. Thanks for the CH boiler peak-load estimate of 500w. When choosing the 3rd inverter I would rather over specify than under. While the boiler is #1 priority as the house has no other form of heating and I have vulnerable occupants to protect in 2 separate households, I would also want to build in the capacity to run the 2 fridge freezers if the battery was up to it. The rating plate on the larger one states 2 amps but I realise there will be higher start-up loads.

As winter advances I am now anxious to get something set up urgently, with bells and whistles added later. Once we have the new inverter, fed by 9 or 18 panels and storing energy in the full battery of nominally 14Ah capacity I can run 230v extension leads to the appliances in an emergency.

Once again, massive thanks for the time and brain-power you are spending on this.

@dnshorto 

Let me pick off another couple from the list:

Solar cables from the roof are most likely to have a cross-sectional area of 4mm² with an outer diameter of 5.4mm. It gets abbreviated to 4mm because none of us can be bothered to say ‘square millimeters’ all the time.

Lightning protection. See the tutorial first because it’s full of pretty graphics.

There must be nothing between the physical cables from the Solar Panels and the surge suppressors  – no switch and no trip!

The whole point is for the high-current surge to pass through the suppressor and thence to ground. If it has to pass through anything else first then it will vapourise the contacts in that device. That leaves the suppressors no longer physically connected to the Panels which they’re trying to protect!

We can possibly ‘allow’ a fuse because it has no contacts to arc and there’s little chance of it melting in a few nS. But it must be the correct HRC (High Rupture Capacity) and totally enclosed in its DIN-rail mounted carrier.

Many thanks @Transparent  for your 3 separate nibbles at my stack of questions. Please may I seek further clarity? I’m well aware that certain aspects of the technology are of more interest than others, which I am therefore hesitant to keep asking you about. Despite the arguable advantages of the more adventurous solutions which you would perhaps prefer, as you will have gathered I have decided to retain my present system as installed, but to ADD the off-grid facility and utilise my present battery with it in the appropriate season. I am keen to get the equipment soon because the winter cometh, and my home is not prepared.

On “dumb” LI battery arrays, connected alternatively to my current Growatt on-grid inverter, or else to the proposed off-grid one (OGI for short); you say <<Provided that there is no data-transfer required between an inverter and the batteries it is connected to, such a sharing arrangement can work. Each inverter only works out the capacity and State of Charge (SoC) by reading the combined cell-voltage for itself.>>  The Growatt has data connections for battery management which seem to work efficiently. If instead, the battery is connected to the OGI, without data transfer, are you saying it can still be kept charged by the OGI working solely on the combined voltage; by PV solar when available, or by TUO at cheap rates by timer? Is this correct?

Lightning Protection; thanks for the very helpful tutorial. I understand the desire to protect against strikes, but the cost seems to be prohibitive. My 3 incoming circuits would require 6 surge compressors, and the Phoenix Contact items seem to be around £300 each with VAT. This is a high insurance premium, when my household cover already protects me against lightning strikes. I get what you write about fire hazard, and I know strikes happen even in urban areas; our own son had lightning strike through his TV antenna, but his insurance put it all right and the danger zone is not in our house but in the garage. I’m inclined to take the risk, unless I have misunderstood and there is a more affordable method of protection such as the HRC fuse? The photo you kindly supplied of your own installation shows 4 suppression units. How does that work with the multiple arrays you have?

DC rotary isolator switch. With all those separate arrays, how do you manage with a 4 pole switch? These are relatively inexpensive, but the 6 pole versions are much dearer. The CPC Farnell 6 pole DC rotary isolator switch is £45 and the CEF offering is over £100. Plenty of 3 or 4 pole versions at £15 to £30. Do the incoming negative PV leads need isolating? Would a 3 pole isolator for the 3 positive leads be one solution?

Battery changeover switch. Yes, I still want to follow this method if possible. I understand your point about DC use of an AC rated switch and eventual points pitting, but if the changeover were to be done intelligently, when load was low for instance, this could be reduced. Is this a valid argument?

Solar panel topology. I checked the voltages again in full (winter) sunlight but the sun was so low that next door’s roof partially blocked one row of panels. A = 408v, B = 357v, c = 367v; so still I believe A = 10 panels, with B & C having 9 panels each.

Multiple questions again I fear, but I hope these exchanges will prove useful to other Forum readers.

Thanks again

@dnshorto 

Thanks for that very useful reply @Transparent.  I have been “thinking Chinese” now for several hours! The variety of interesting products on the AliExpress site is fascinating, but even though I have spent virtually all evening searching I still can’t find a suitable 6 pole isolator; not even the Yaming Electrical one you spotted “in less than a minute”. Infinite numbers of 4 pole and less, and some higher pole-number selectors, but 2 position isolators have eluded me. The “cam switch” description helps, but still not enough. I have registered with AliExpress, and left a query so await a reply. Although they seem to offer a bespoke service. in reality it is only for significant orders; I only want one!

This seems very lame, but I also need help in selecting the lightning suppression modules. You are absolutely right to say that they are very much more affordable on the FEEO site but it is very comprehensive and there is just so much choice. What parameters should I be applying in choosing surge protection? Or better still, if you were looking for 6 reasonably priced suppressors for my 6 incoming PV leads, which of the multitude available would you choose?

Meanwhile, thanks for your “thinking time” over the Pylontech data link and for all the other time you donate so generously to steer this project to an efficient outcome.

@dnshorto 

Once again @Transparent  you have found a relevant product which I have been unable to discover. I still don’t know how you do it! The Yaming Electric LW26-32/6 could be used although not ideal as it has 3 positions, and with shipping is up to nearly £18 with delivery by month end. Farnell’s standalone isolator at £45 with UK stock still might make sense.

I had a reply from FEEO on AliExpress messages.

“Hi friend, we not have 6 pole isolator.”

So I asked another AliExpress messages question:

<<Yaming Electric have a cam switch LW26-32/6, 6 pole 3 position changeover. It does not feature on AliExpress list. I need it to isolate 6 PV solar feeds at max 410V DC at 10A. I need all 6 switches to changeover simultaneously, (not 3 on position 1, and 3 on position 2).>>

Response awaited.

I tried Yamin Electric’s own website but buying direct the minimum order is for 100 items, and the LW26-32/6 isn’t listed there either! Where did you uncover it?

Any further thoughts on the suitability of the inverter I proposed?

SKU: SOL-5K-RHI-48ES-DC Categories: Hybrid Inverter, Solis,

333mmx505mmx249mm. 17kg. £1200

3D printer eh! That’s worth remembering!

@dnshorto 

I don’t think that switch diagram is correct @dnshorto 

The part we were looking at is a 3-position switch, but their diagram shows only 2-pos.

Here’s the diagram from their website for the switch we’re interested in and then adapting:

So this shows 6 stacked wafers. I’m a bit surprised they did it that way because a cam (moving contact) is surely able to connect either to one fixed contact or a different one as it turns. You don’t need two wafers to achieve the same result.

So the way I’d expect them to do it would look this this:

Even if they don’t have wafers which do this, they should at least understand that the diagram shows a change-over switch.

My diagram shows 6 wafers, each with three numbered terminals.

Terminal 2 connects to terminal-1 when the switch is in position-A;

and to terminal-3 when the switch is in position-B.

Now it’s my turn to be confused.

I thought you were looking for a switch which could connect a ‘source’ to one of two possible options.

There are two locations where one might be required to switch a ‘source’ either to the Growatt grid-connected inverter, or else to the proposed off-grid inverter:

• PV Panels
• Storage batteries

In any event, the device I’ve described in the table above has a centre-off position and will therefore act as an isolator as well.