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

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.

If you chose the required specification for the new Hybrid inverter, then you can achieve everything you want from that. Simply allow it to feed back to the grid in summer.

The only problem with the existing Growatt is that it has no capability for DC-input from Solar Panels. So I’m suggesting it’s moved to a section of the supply system where that isn’t required.

There are alternative ways to achieve this. But I’m starting from the position that you already have the Growatt.

One other possibility is to sell the Growatt and simply buy a standard 48v off-grid inverter  the required power-rating. 500w is a typical peak-load for a central-heating system with a gas boiler. So the decision rests on precisely what you require to be run from 240v AC (pure sine wave) during winter.

Solar panel topolgogy: It seems most likely that the layout is indeed 10-9-9 based on your measurements.

It’s slightly tricky to be certain of this because

(a) you’re measuring open-circuit voltage of those panels (41.36v max.) which will vary more than the specified voltage under operating conditions (MPPT in action).

(b) there’s not a great deal of difference between 9 and 10 panels

If it were me, I’d be taking a further measurement in brighter light to check if the same voltage ratios were consistent.

I need a bit more time to check the specs of the Hybrid inverter you suggest. Its MPPT inputs have to match the open-circuit voltage at the moment of switching panels to it.

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 

Sorry, there’s a lot to take in from your last message @dnshorto 

May I just pick off one point in an attempt to simplify issues:

In the diagrams I drew, I have placed the Storage Battery assembly between the two inverters in such a way that they do not have to be switched!

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.

So having ‘dumb’ Lithium Ion arrays can be advantageous.

That still means we need safety isolators between the battery-stack and the inverters they connect to. But that’s just in order to allow them to be removed for ‘maintenance’ as necessary.

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.

DC rotary isolator switch: This is the 4-pole device I used for my own String Combiner. I bought direct from FEEO via AliExpress. You’ll probably find the same item on ebay or Amazon at a higher price if it’s already in the UK.

A red/yellow rotary switch is the normal, accepted ‘standard’ for a safety isolator on renewable-energy installations. Each ‘pole’ is effectively a disc on a central shaft. The more poles you have, the deeper the switch will be.

The switch depth dictates whether it could still be fitted alongside trips in a DIN-rail enclosure. 6-pole switches therefore tend to be in separate boxes.

Take care that the switch really is capable of switching DC at the required current. The DC capability will be less than that specified for AC. If only one current-rating is stated, then the isolator cannot be used for DC because the contacts will degrade and become pitted.

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 

@dnshorto  wrote:

I have decided to retain my present system as installed

Ah… that’s the bit I hadn’t understood!

I was attempting to use the existing devices but reconfigured differently rather than “as is”.

I need a bit of thinking time! It’s the data connections between the Pylontech batteries and the Growatt which make this slightly more tricky.

Lightning protection. My present arrangement is being used as part of the OVO PowerVault Storage Battery Trial. As this has only two solar MPPT inputs I’ve (temporarily) got the three rooftop arrays strapped together within the String Combiner Box, and therefore sharing just 2 pairs of suppressors.

The enclosure does have space set aside for the third pair, but I just haven’t got around to fitting them yet.

Costs: Yes, Phoenix Contact units are the top-end of the market. They get more thorough testing and intensive product design than their rivals. But my house is positioned on top of a hill in full view of the prevailing SW winds off the Atlantic!

You can purchase much cheaper units made by manufacturers in the Far East, including FEEO, who make the DC rotary isolator. Buying a ‘named brand’ is better because they won’t want to risk adverse publicity if the product should fail to provide protection.

You should feel free to message these Chinese Manufacturers with any questions. Many can reply in English, and others use the inbuilt AliExpress auto-translator which is pretty good! They are extremely polite and their culture puts a high value on customer service.

(We have so much to learn from them!).

4-pole & 6-pole DC rotary isolator switches: It is usual practice to keep each pair of Solar Panel drop-leads separate and not try to combine the -ve between arrays. Thus each array is treated as ‘floating’.

Remember, you do not know how the MPPT inputs are wired within the inverter. There is no ‘standard configuration’ which suggests that the -ve wires are connected internally. It is permissable for the inverter manufacturer to keep each MPPT input channel isolated and floating, with no common reference point.

The prices you are quoting for rotary isolators are high because you’ve only considered the usual UK electrical suppliers. If you think ‘Chinese’ then the prospects are far more encouraging. Here’s a 6-pole unit I found in less a minute:

These Chinese suppliers will put together whatever combination of contacts and switch handle you require. They even say so in their product descriptions:

These rotary isolators are often called ‘cam switches’. Just go to AliExpress and type “isolator cam switch” into the search box at the top of any page. It really is that easy.

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 

I wonder, @dnshorto if you’ve used the generic Alibaba Request-to-Quote system, rather than directly communicating with a particular supplier on AliExpress.

Here’s another 6-pole switch from Yaming. It has two-positions and centre-off, and is rated 32A, which is ample for your needs. Just check with them that it’s OK to use those contacts at DC 410v 10A (the open-circuit rating of the longer array).

All you’d be asking for is

a: all 6 switches to change-over between 2 contacts simultaneously. Ie you don’t want 3 connecting in switch position-1 and the other 3 in position-2 

b: a change to the front-end. You’re seeking a yellow bezel and a red lever.

And if they ‘can’t do that’ I’d be very surprised.

If it were me, I’d probably buy this switch ‘as is’ and use my 3D printer to make myself a nice chunky red handle!

But it might be going a bit overboard for you to start ordering a 3D-printer just for this one job!

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