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

Thanks again @Transparent; that is very clear and I see the advantage of the String Combiner box for allocating where best to direct the PV input. You have an impressive spread of panels!

Did you design and assemble the String Combiner box yourself, or is this something that can be obtained ready made? There are so many variables that I suspect each one would need to be tailored to the application.

I have a number of other questions but will suppress those for now while you develop the diagram.

@dnshorto 

You are quite correct @Transparent, this thread opens up so many interesting peripheral topics, but I must stick to the main subject and explore those later. 

The present set-up works so well, say April to September that I would be reluctant to change it during those months; the meter hardly moves!

In these 6 winter months, it’s obviously useful to collect what solar energy there is available but the reduction in grid input is much less valuable and provision for potential power cuts becomes more of a priority. The battery rarely gets a full charge and is quickly depleted in the evening so is far less useful in the winter months.

Setting up a separate Off-grid inverter to run a few essential circuits (CH boiler, fridge/freezer etc) sound a perfectly workable idea but is there a way to use the same battery for both purposes, or maybe alternate the battery’s function every 6 months?

@dnshorto 

All perfectly clear, thank you @Transparent.

I am very familiar with low-voltage DC lighting systems. More than 40 years ago, long before LED became so cheap and efficient, I installed 12v back-up lighting throughout my whole house during the 1979 “Winter of Discontent” when power cuts were frequent, and retained the system for decades afterwards as “night-lights”. 

Your diagram and tables are fully understandable and as drawn would work on a year-round basis, so that the off-grid, pure sine wave-dependant appliances would always use that inverter. 

Is my idea for a 6 monthly changeover workable, and how could it be accomplished? 

In the 6 sunny months my system would remain as installed; the battery supplying power when the sun wasn’t and excess solar energy passed on to the grid.

In the 6 darker months, when there was little spare battery power available for evening domestic loads, the battery would be isolated from the on-grid inverter and would be kept topped up by occasional sunshine, with the option of manually charging from the grid when needed. It would then only power the CH boiler and the fridge via the off-grid inverter, plus optional DC circuits as outlined by you. 

Is this achievable?

@dnshorto 

Just slightly amended this topic title and added some tags in the hope this will help others find it.

What do you think of making this more of a discussion topic than a question with one single best answer? (Feel like  all of your comments deserve equal consideration here, @Transparent and @dnshorto)

So this brings us to the next step:

How to select the required inverter.

Each solar-inverter specification shows the voltage and current range of the PV panels which can be connected to it. Here’s the relevant portion of the spec for the Growatt SPFx000TL range which featured in the earlier YouTube video

Looking at the right side of the table for an inverter connected to a 48v Li-ion battery, there are three parameters which we need to note:

1. The inverter can only use its Max Power-Point Tracking (MPPT) electronics whilst the operational voltage range is between 60-115vDC.
2. The maximum permitted (open circuit) voltage must not exceed 145v when I first connect those panels, otherwise the inverter may be damaged.
3. The maximum current which the panels can deliver in sunlight is 30A

Those three parameters dictate which solar panels can be used and in what arrangement of series and parallel.

For example, here’s the specification for the high-end Panasonic HIT panels I have on my main (upper) roof area, compared with a more common Longi panel of similar overall power:

So if I were to put two of my Panasonic panels in series (71.3 + 71.3v) they are only just within the maximum 145v permitted for connection to the Growatt SPF-series inverters. But their peak current of 5.7A falls a long way short of the 30A allowed.

If I’m going to install solar panels which can be switched between a grid-connected inverter in summer and an off-grid one in winter, then I need to start by choosing an arrangement which gives me the required flexibility.

The above arrangement shows three solar arrays. A & B will be permanently linked to the two MPPT inputs of a grid-connected inverter. Array-C can be switched between an off-grid inverter or added in series to Array-B.

The alternative is obviously to keep two completely different sets of solar panels for grid-connected and off-grid use. That depends on available roof-space and how much you’re prepared to spend.

Yes I think this topic has now reached the point where we need to consider details of your own installation @dnshorto 

I’ll assume at this stage that you have no approved Load Diverter or export limiter at your house. That may not be the case because your initial post for this topic said:

On any reasonably bright day [….] the HW cistern is heated

Leaving that point aside for the moment, let me just dissect your diagram (above) and show the current/power of these different elements, based on a nominal mains voltage of 230v AC.

To have both inverters grid-connected requires the installer to have applied to your Distribution Network Operator (DNO) for a G99 certificate (see explanation here).

The default situation is that a household may not export to the Distribution Grid more than 16A per phase, which equates to 3.68kW.

The G99 export assessment process adds together your export/generation devices on the assumption that solar-output and battery export is simultaneous. So you should have a G99 certificate which grants permission for 8kWh + 3kWh. That is considerably in excess of the ‘standard’.

I think that Solic 200 Solar Immersion Heater Controller falls within the group of devices which we’re calling Load Diverters here on the Forum.

At the moment the DNOs only allow Load Diverters which send energy to immersion heaters. We need to make representations to the Electricity Networks Association (ENA) requesting that Storage Batteries can similarly be tested and approved to act as diversionary loads.

Tell us the size of your immersion heater, which I guess is going to be the usual 3kW.

The Load Diverter should be referred to in the G99 documentation. Even so, you’re still above the normally permitted export capacity.

8Kw (solar) + 3kW (Pylontech batteries) - immersion-heater

Let’s take this analysis of @dnshorto ‘s installation a bit further.

It is a reasonable proposal to have one or more of the PV strings switched from the existing Solis inverter to another off-grid one, especially during winter when you’re less likely to want grid-export.

I don’t know the exact specification of the 340w panels in the diagram, so let’s make a sensible guess that their peak output is  34v @ 10A.

A series string of seven can therefore supply 238v @ 10A max.

It shouldn’t be too difficult to find an off-grid hybrid inverter with an MPPT input which could take Solar Panel export at that level. It would probably need to be rated at 350v minimum in order to allow for the higher ‘open-circuit’ voltage when the panels are first connected.

This would be easier to achieve if the four rooftop arrays arrived in a String Combiner box. But the original photos at the top of this thread indicate that they enter a relatively small metal box, just 4-modules wide.

That tiny enclosure doesn’t have room for a pair of fuses and a double-pole trip for each of the four PV strings. Whatever is in that box won’t allow a string to be isolated from the others in the event of a single panel being faulty.

I’m sure that can be changed to a more flexible arrangement. But Dave mentions there being only six wires (not eight), so I’m guessing that 2 strings share a -ve cable between them.

If so, then it would be necessary to switch a pair of PV strings from the Solis to an off-grid inverter rather than just one at a time.

The video clip shows a double-pole change-over trip in my own String Combiner box during construction. The mechanism permits only one pair of contacts to be ‘on’ at a time.

It’s important to note that this is a DC-trip. Each of the four sections has a spark-quenching ‘ladder’ to prevent the switching-arc from welding the contacts together! For that reason the terminals are marked + and - so that the arc gets drawn into the quencher by a magnet.

Polarity is important when switching high-voltage DC.

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?

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?

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!

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.

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!