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

That is very much appreciated @Transparent , thank you. The video clip makes that switchover sequence very clear.

Referring back to my own installation, the situation is actually worse than is at first evident. What looks like a tiny junction box is in fact a bundle of cables.

The interior of the box they are routed into (P V System. Main AC Isolator) 

is as below:

I would go along with your recommendation to fit instead a String Combiner box to allow flexibility and to intelligently divide up the incoming PV energy. I can make room for it and the off-grid inverter to the right of the present clutter of items.

Presumably, if this String Combiner box were to be designed for the purpose, a greater or lesser number of strings could be diverted into the off-grid inverter depending upon seasonal PV generation, domestic load and the likelihood of imminent power cuts! The inverter would be chosen to deal with a maximum input from say 3 of the 4 strings. Is this feasible?

Would the battery also be independently divisible between the existing 4 units?

Once again, many thanks for a very helpful discussion.

Dave

@dnshorto 

Thanks for those close-up photos @dnshorto 

I no longer believe that the small metal 4-module enclosure is the one where the panel cables enter. I can see a ‘mains’ earth wire, and the trips are clearly AC rated.

I think the red 100A ‘Switch Disconnector’ isolates all of the new installation for both inverters. The two blue MCBs are most likely one for each inverter.

The mandatory safety label is therefore correct.

On closer examination of your circuit diagram, you actually have three AC breakers, which I’m now labeling AC1, AC2 & AC3

If I’m right, then AC1 is the rotary one beneath the Solis. This style is specified within the MCS accreditation for approved solar panel installations.

As I can’t see another rotary switch, I’m inclined to believe that the 4-module enclosure is what the diagram refers to as AC2 and AC3.

The cables entering the Solis inverter at bottom left have MC4 connectors, which you’d only find on the single-conductor cables used for solar panels.

That means we still haven’t located the DC isolator referred to on the diagram. Is it possibly in the attic rather than the garage?

I rather think @Transparent that some of the breakers and isolators shown on the schematic diagram may be mythical. The drawing was done 12 months after the installation by a director of the sub-contractors who hadn’t visited site, and probably represents an ideal layout rather than one based on reality. Certainly, there is no wiring in the loft, everything is as photographed in the garage.

The installation was done in two phases. On 26 Oct 2020 26 JA Solar 345 panels were installed, and 3 Pylontech 3.5kWh batteries. Then three weeks later a further 2 identical panels were installed (space for them having been achieved by the removal of an unused cement flue pipe), and a fourth battery once I had observed how the panels easily charged the first 3 batterie. The enthusiastic but technically challenged salesman had warned that 2 batteries were probably all that the panels would keep topped up, whereas I wanted maximum storage capacity for future power cuts! :-)

So the inaccuracies of the schematic caused by lack of direct knowledge makes me question the legend: “4 strings of 7 panels”. There are 6 identical cables coming down from the roof and they all feed directly into the bottom of the Solis inverter as you have arrowed in blue. I don’t think the DC isolator exists. Is this a serious omission? 

I guess if a String Combiner box is to be installed the 6 incoming MC4 connected DC cables will have to be rerouted through it so siting of that box will need some thought. 

 Your wisdom in all this is very useful, thanks.

 Dave 

 @dnshorto 

OK, so I think we can assume that the circuit schematic you have isn’t entirely correct.

To better understand what the real situation is, I’ve started by downloading the Installation Manual for your Solis 5G-8k PV inverter from the Chinese manufacturer here. Here’s part of the specification table towards the end:

This tells us the maximum parameters for the panels which can be attached. It’s slightly complicated by the fact that there are three pairs of input sockets, but only two MPPT (max power-point tracking) systems.

As I currently understand this, it means that MPPT system-1 can take 520v at 12.5A in peak sunlight.

MPPT system-2 can take two strings of panels (in parallel) totalling 520v at 25A max.

You have 28 panels, which we believe to be rated 340w each (total 9.5kWh). We can download the specification sheet from the Indian manufacturer, JA Solar. These are the maximum parameters for a new unit:

This shows us that the maximum number of panels in series would be 14 if you are to remain within the input voltage range of the Solis inverter (14 x 34.63 = 484.82v).

So my first guess is that you have a series-string of 14 panels connected to MPPT-1, and two series-strings of 7 each connected to the pair of inputs to MPPT-2.

How does this sound to you?

It sounds spot on @Transparent. I should have looked under the Solis inverter before, as we may have found the missing DC Isolator too. 

There are 4 DC cables from the panels connected to the 4 terminals marked DC2, and 2 cables connected to the 2 terminals of DC1.

I think you are quite correct. The system has been working well for over a year, so if the installers had overloaded DC1 by connecting more than 14 panels we would have had trouble before now.

@dnshorto 

Well I don’t want to jump to too many conclusions about the conguration of your Solar Panels yet. The three pairs of downleads could equally well be connected to arrays of 10, 9 & 9 and still be within the specifications.

The roof layout does indeed suggest 14, 7 & 7. But it also depends on how those last two Panels were connected in when the flue was removed.

Nevertheless, let me present the following diagram as a ‘working example’:

The left-side of the diagram shows the three strings in a 14, 7 & 7 arrangement.

I like using unique colours for each separate array. I mark the downleads and trips to match. Purple, orange, pink and light-blue are the least common colours in PVC electrical tape, but available separately on ebay. So I won’t get these DC leads confused with anything else in the house installed by a qualified electrician!

To right of the panels is the basic layout of fuses and trips which I would expect to have installed. This allows strings with faulty panels to be isolated and some measure of protection against surges.

Yes, I’m suggesting both fuses and MCB trips. You shouldn’t be opening the fuse-carrier on a ‘live’ circuit. That’s what the manual lever on a trip is for!

We here have basis for a design of a String Combiner box, albeit without any two-way switching or lightning protection at the moment.

To the far right is the DC isolator. Yours is black and not easily accessible beneath the Solis inverter. It’s a safety device to switch off all electrical input. As such it should be readily identifiable… by a fireman for example.

I’ve drawn a 6-pole rotary switch which needs contacts rated 20A min.

The one on the inverter may be a more common 4-pole version if Solis have decided that this is the point where the two strings connected to MPPT2 get placed into parallel.

(As you know, firemen are more familiar with a single pole)

I’m hoping that this gradual series of steps in the analysis will allow others to follow the discussion in future.

The litmus test for this is whether @Tim_OVO and @Jess_OVO are still keeping up!

Your step-by-step approach @Transparent is clearly the most likely to be useful to most people and I am finding it very easy to follow, thank you. I’m sure the admin team will too if they have time to read it. Other users will have allied or similar questions in the future and will find this discussion helpful, even though it is specific to a particular installation.

To establish whether the 3 strings are actually arranged as 14, 7, 7, would it be feasible to measure the incoming voltage on each pair? I was thinking that the MC4s could be disconnected after nightfall and then measured with a multimeter early the next day.

I like the idea of clearly differentiating the separate circuits with unique colours and wiring, and marking the switchgear etc with matching tape. I also favour fitting a 20A 6 pole rotary switch directly on the incoming DC leads.           In practical terms this could be mounted next to the Solis inverter without the need to extend the 6 leads.

From this new switch the appropriately identified leads can pass into the string combiner box, though the lightning protection etc and on to the two-way switching elements of the box. I revisited the earlier page where you included a photo of the box you designed and assembled and am of the opinion that I could probably tackle the sourcing of bits and putting them together but would need assistance with getting the design and components correct. I have used RS at odd times in the past for not dissimilar projects and you may well have found other suppliers to be useful.

We haven’t yet discussed the possibility of also dividing the battery modules but am confident that the step-by-step approach will get there eventually!

Thanks again

@dnshorto 

Yes, I would certainly start by disconnecting the MC4s and measuring the voltages. Just remember to mark the cables unambiguously before you start disconnecting.

MC4 connectors are polarised so there is no danger of reconnecting a pair the wrong way around. But you could inadvertently get two negative leads the wrong way around, for example. Each positive and negative must remain as a pair.

I don’t think you need to wait overnight to do this test. Just choose a time of day that’s not full bright sunshine.

If you turn the DC isolator to ‘off’ then no current is flowing on any of the six cables, so you could disconnect and test a pair at a time for its voltage.

Remember that you are measuring this ‘open circuit’, without any load being imposed by the MPPT electronics in the inverter. Open-circuit voltage is specified at 41.36v per panel, so even a 7-panel string would really hurt!

DC voltages are actually more dangerous to us than AC ones because the contact ‘sticks’ to the skin when it zapps you.

I will, of course identify where you can buy components to put together a String Combiner. I buy most of these on AliExpress direct from China. They simply clip onto a standard 35mm DIN rail, which most often comes fitted within a box, but can be bought by the metre.

The enclosure itself needs to be as large a DIN-rail box as you can fit somewhere on the wall! Due to its size you want to source this closer to the UK.

It doesn’t need to be made of steel, like the mains consumer units must be. That means there’s still a possibility of picking up a plastic one which a distributor has got left on the shelf.

Start looking in the Clearance Bargains of suppliers such as CPC, Rapid Online, and even ebay. (I’ll need to change that last link to something more generic later).

@Jess_OVO - I’m aware that there is a lot of practical content emerging here which others might want to find if they’re reading the general topic on PV Panel Installation.

Let us know if you want things moved, or whether we periodically insert links to direct people here.

The largest enclosure I can site @Transparent  in a reasonably accessible position would be 600 high, 400 wide X 200 deep, with a hinged door. There are a few plastic examples available in the UK but generally just with a steel backing plate for attaching the rails, rather that with DIN rails incorporated. More flexibility of layout that way and easier to fit non-compatible items. I am assuming that most or all switchable items would only be operated inside the box, rather than by external switchgear.

Looking again at the rotary breaker you have labelled AC1;   by pulling aside the 6 DC cables laid across its top I can see the AC output from the Solis inverter just above passing into AC1 on the left, but cannot see where the R/H cables go

Would they be routed up to the Growatt inverter and thence to the white “Main AC isolator” box above? I don’t yet fully understand the interaction or relationship between the 2 inverters. I know my system was too much for a single hybrid, and believe that the Growatt is to send and receive 48(ish) volt DC power to and from the battery and pass 230 (ish) volt AC on to the domestic consumer units and then the grid, while the Solis receives the DC from the panels and converts it to 230v AC.  But is all the Solis output channelled through the Growatt? On a sunny day the Solis becomes almost uncomfortably warm to the touch and on really bright days it hums. The Growatt seems to stay cooler.

@dnshorto 

I know the sort of enclosure you’re thinking of @dnshorto - I use them for 24v distribution boards in my house.

For your purposes you really need the trips and the lightning suppressors to be visible. If there’s a fault you should be able to see it without opening a door and then having direct access to all the high-voltage wiring!

I did a Google Image search for “DIN rail Enclosure” and found these two:

Such a format would enable you to dedicate a row to each of the three arrays.

The CEF one is 500 x 355 x 140mm  (H x W x D) and can accommodate 16 modules per row. CEF is a nationwide electrical supplier.

The Thompson/ebay one is 470 x 307 x 100 and holds 12 modules/row.

A DIN-rail module is 18mm wide.

You need four modules-width to accept each panel-array incomer (2 fuses and a 2-pole MCB trip), plus lightning suppression. A double-pole changeover switch occupies 4 modules. So the space soon goes!