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

So great to see such valuable knowledge sharing going on here - @Transparent and @dnshorto.

Can’t say I’m 100% following, but I’m sure those researching their solar storage options will find this thread a great help.

We’re always keen for a separate more detailed guide on any energy subject (but off-grid lighting does sound like a winner!), @Transparent - let us know if you’d like us to move any existing comments over or you’d rather start the topic afresh...

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)

OK with me @Jess_OVO . Whatever is most useful to most people. @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.

Thanks @Transparent. I have to confess that I struggled at first to comprehend your latest post, but repeated readings helped and I now see generally what you mean.

For these chats to be relevant to other enquirers in I understand the need to keep the information general, but of course I also value help with my specific installation. For comparison with the table of specs for the Growatt inverter you mentioned, here is the plate on the side of mine:

How does this compare?

You also mention the possibility of dividing up the output of the solar panels, or adding more. That isn’t an option for me as all my south-facing roof area is taken up with the 28 existing panels, but they are apparently in 4 strings of seven, even though there are 6 cables coming down from the roof, which is a puzzle.

Any personalised advice you are able to offer specific to my particular installation would be greatly appreciated, including a suitable inverter and how I might redirect the available solar input to achieve the off-grid power I’m seeking.

I hope that this too might be useful to others with similar problems.

@dnshorto 

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’.

You put your finger on a long-standing problem there @Transparent . Last year you asked if I had a G99 certificate. I asked the Solar Panel company, who assured me that they had the certificate and would forward it to me. I naively believed them.

Many months later, after repeatedly but unsuccessfully requesting the G99 I learn that they have closed up shop. Thankfully a company called Flexi-Orb have picked up the cudgels for me as I have been in and out of hospital rather a lot recently and they seem to be making progress with the DNO, so I hope to have it soon, albeit a year late. The surveyor has apparently already checked the situation and sees no problem in granting the certificate.

There is no “load diverter” or export limiter, but a Solic 200 Solar Immersion Heater Controller diverts power to the water heater when there is enough PV energy to spare (a solution which I remember you described as slightly “selfish” in a community setting)!

@dnshorto 

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

Quite correct @Transparent, 3kW immersion heater, but of course only drawing power while the water is cooler than the thermostat setting, so on a sunny day the load diversion is intermittent.

On the vexed topic of my G99 I finally received the relevant paperwork this morning, so my installation is officially recognised and approved, even though it has been sending energy into the grid for over a year.

However, the immersion heater diverter was fitted only a few months ago, so is not referred to on the G99, an omission which I will now take steps to correct, thank you.

@dnshorto 

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.