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

The cable which exits the AC rotary switch below the Solis inverter goes to the small metal distribution board we discussed earlier.

I’m pretty sure that the 8kW Solis goes to the blue 32A trip, whilst the Growatt is fed via the 20A trip to its left. They’re in parallel.

Thus the 2-pole red disconnector isolates both inverters from the mains.

I can’t yet comment on whether there is or isn’t a hybrid inverter with adequate capability for the 9.5kW of PV Solar Panels on your roof.

I haven’t yet researched this, but I intend doing so because a similar solution would be required by participants in the Zero Carbon Heating Trial if they wish to run their Heat pumps off-grid.

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 

Great minds, @Transparent! I think there’s a real value in keeping this as a separate discussion but great point about including a link to this thread on our Solar PV guide, I’ll get that added now! 

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.

Spot on as usual @Transparent . The left hand cable clamp is to the Solic 200, the right hand one I had to reposition early on in the saga as the original (dumb) meter readings were not being reduced by the solar input.

In a previous interchange about a year ago on this forum you pointed out that the clamp was too close to other cables and was thus suffering from interference.

The panels were churning out power and the phone app showed how much I was exporting to the grid but the daily meter readings had hardly changed from pre-solar days, and the bills are based on what the meter records. So, from what you have just written, it wasn’t that the meter was influenced by moving the clamp but that I wasn’t using the incoming PV power optimally for storage nor for domestic load. Makes sense. @dnshorto 

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 

Thanks for keeping me on track @dnshorto 

Yes, in theory your 48v battery pack could be connected either to one supply (existing Growatt) or another (new off-grid inverter).

It would probably need to be achieved with a Contactor (relay) rated at 80A+ because it would be a hefty manual switch to be specified for use at 74A current.

[Aside] You might wonder why you couldn’t have just two of the four PylonTech batteries switched across. In theory this is also possible, thus dividing your storage assets. But it would need checking with Pylontech because re-joining the battery packs would occur with them at different charge levels.

One of my present thoughts is that you might as well replace the existing Growatt inverter with a hybrid one which could take input from either the mains or a  Solar array. This would be permanently connected to the same four batteries, but only used for charging (not discharge). There’s then no requirement for any switch.

The existing Growatt then gets repurposed for export only.

Depending on the characteristics of the new hybrid inverter, that might leave you with no export capability from the Pylontech batteries, either to the Distribution Grid or back into the home.

Is that a critical issue?

It could later be overcome if you had an EV with a V2G charger.

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 

Let’s leave aside the battery characteristics and their switching for the moment @dnshorto and have a look at your Growatt grid inverter:

Unlike an off-grid inverter, yours has no on-board reference for voltage and frequency. As it can be grid-connected, it isn’t allowed to!

The Growatt senses the mains voltage and frequency, and then adjusts its output accordingly.

So to repurpose it for off-grid use it will require a small off-grid inverter in parallel to provide those two references:

I’ve never tried this before! The important points are the small inverter must :

• provide a full sine-wave output (not a stepped pseudo sine-wave)
• supply negligible current to the output (that’s the job of the Growatt)
• not go into oscillation as the Growatt kicks in after a couple of minutes

I’ll bet there’s someone else on the internet who’s done this and uploaded a YouTube video or a blog. See if you can find one.

If I were doing this myself, I’d be very tempted to open up the Growatt’s case and separate out the Grid sensing circuitry. I could then use the small inverter to supply just those two references:

But I couldn’t possibly suggest on the Forum that you get out your soldering iron and start cutting and joining 240v connections. So I’ll avoid doing so.

Here’s a typical 48v pure sine wave inverter which might do the job. I found this on AliExpress ‘Black Friday deals’ and that price includes shipping!

I doubt it would provide the specified 1kW with any appropriate level of stability. But for your purposes it doesn’t even need to supply 1w

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