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

Apologies @Transparent. I didn’t even see the second half of your reply, listing the further reasons why 2 Solis inverters might cause problems. Trying to do too many things at once.

So maybe I need to find a different inverter without grid-export capability.

One comment of yours which I can respond to is <<However, it might constrain your choices if you decide in future to operate both on-grid and off-grid storage simultaneously.». Who knows how needs and ideas might develop in future, but my plan if you remember is to use the off-grid facility seasonally; basically in the winter, and to revert to my original system when the sun shines, when power cuts are less likely and when the need for central heating is not critical.

@dnshorto 

@juliamc  - having a straightforward off-grid storage system is simple.

There are two issues with @dnshorto ‘s site which have made it sufficiently complex that I’ve felt the need to write about the details in this topic.

a: We’re modifying an existing grid-connected installation, which is already complicated by the installer’s use of two separate inverters.

b: We’re trying to use the same set of batteries for either on-grid or off-grid.

Dave, the main/obvious reason that Solis state you can only use one hybrid inverter on site is that their design doesn’t allow parallel operation in order to handle greater capacity.

Almost all off-grid inverters do have parallel operability, which is why it’s important for Solis to state this limitation for their product.

However… if the software doesn’t allow for parallel operation on the DC side, then is it OK to assume that two Solis inverters may be installed in parallel on the AC (grid) side?

It’s the absence of such information which I’m flagging up!

I see, but it sounds highly likely that buying an ‘off the shelf’ PV + battery system would result in the same problem. I’m glad I didn’t go with the Solar Together option recently quoted for via Surrey County Council as that wasn’t designed for off-grid, at least I don’t think so.

For @juliamc and others whose life will forever be simpler than @dnshorto’s, let me present a more typical off-grid installation using a hybrid solar inverter:

Note the arrows. The hybrid inverter can import electricity from the Distribution Grid during winter when there is insufficient solar energy to support the house.

However, the inverter does not export back to the grid and therefore doesn’t require a G98 grid-connection certificate.

The only reason that the hybrid inverter is required to create 240v 50Hz ‘mains’ is in order to operate appliances which can only be used with AC power. Typically these would contain synchronous motors, such as water pumps.

There is no point in running WiFi routers, DECT landline phones and LED lighting from this off-grid mains supply because they simply use a ‘switch-mode’ transformer to convert it back to DC again!

At some point I need to create a new topic on running DC devices and home lighting directly from a Storage Battery. They will use various voltages, but there are cheaper, safer and more efficient ways to overcome this using DC-DC Converters. You do not need to go all the way up to 240v AC and back down again!

Running household devices off-grid has tended to be the domain of enthusiastic amateurs. They freely help each other using YouTube videos and online forums, and have built up a wealth of shared expertise.

When most people say that they’re ‘having solar installed’, it is generally assumed that this is grid-connected and will create an income stream when it exports.

But this shouldn’t be the case any longer.

The need to combat Global Warming and the present high price of gas should persuade more people to consider running as much as possible off-grid.

This Forum is ideally suited to such discussions.

We can be sure that the Moderators would encourage such topics despite them working for an Energy Supplier. After all, we’re not advocating customers to sever their Grid connection. But we do want to make best use of the precious energy resources available in the UK.

@dnshorto  - I’m re-posting inverter specifications similar to that which you published above so that I can comment on a few points. I’m actually getting this from here on the Ginlong website which has “off-grid” inverters without the 5G suffix.

Hopefully this will put you (and others) in a better position when trying to track down a hybrid off-grid inverter which does what you want.

DC / PV input spec

Apart from the fact that your total array size is slightly larger than their recommended figure, everything else seems a reasonable match for your arrays using the 340w panels if we consider the string of 10 panels

However, this inverter could not support the two strings of 9+9 panels in parallel. The resulting peak output voltage of 19.6A is far more than the 11A max rating of the MPPT input.

Battery storage:

The specified maximum battery capacity of 2000Ah gives us 9.6kW at the nominal 48v of your Pylontech batteries. So this inverter cannot handle the 14kWh capacity of your present batteries.

I’m unsure how it ‘knows’ this. Perhaps it’s advisory, based on the low charge-rate of just 3kW… just one-third of what your PV panels can deliver.

AC output:

The AC output side is ok to operate a gas-boiler heating system, but you couldn’t simultaneously boil a kettle.

It’s also too little to operate a Heat pump, in case anyone else is interested in that!

The switch-over time suggests that this inverter normally drives the AC-output directly from its mains-input. When this fails it draws on the battery instead; similar operation to a UPS for a computer. The 0.02secs lag is equivalent to one mains-cycle at 50Hz.

Thanks @Transparent. I hugely appreciate all your helpful input over many exchanges of information, and trust that others who may read through what we have discussed will benefit; if only by avoiding the mistakes I have made!

I hope to return to the task of setting up an off-grid element to my solar installation but have had to shelve it for now. As you know, my primary motivation all along has been to prepare for power cuts and I have come to realise, with a great deal help from this forum, that even with my rather OTT solar array and battery capacity converted to off-grid use it would leave my home vulnerable to prolonged outages. I hope they don’t happen, but fear they might.

Therefore I have reverted to Plan A and invested in a 5kVA inverter generator to be quite sure of being able to keep my vulnerable family warm with gas central heating. Not a solution that is likely to appeal to many members but it has a number of advantages, including availability. 

I plan to return to protecting my solar system with surge suppression in the new year and would value your help in sourcing the components, maybe on a separate thread.

Many thanks again

@dnshorto 

5kVA inverter generator

a diesel generator set?

I’m not complaining about the power-source. It’s a logical approach to an immediate need.

But I’m intrigued how this provides the electricity to run the central heating when there is a power-cut. Have you installed an automatic transfer switch (ATS) on the mains supply to the boiler?

If not, then what prevents the generator from sending power back to the substation?

And I’m just going to make a note  to the moderators: @Tim_OVO and @Jess_OVO 

I don’t think we should regard this subject as ‘dead’.

In this case it has proved more difficult than @dnshorto expected to utilise his existing grid-tied Storage Batteries installation.

But the desire to have such Batteries able to deliver electricity in a power-cut is a more widespread issue. See the comment above (a fortnight ago) from @juliamc and the diagram I posted as a response.

Let’s remember that she is a participant in the Heat pump Trials and we have also identified a need for off-grid Battery Storage at those sites. They require storage in order to time-shift when they draw mains power, whether or not they have PV Solar Panels.

The technology to achieve this is already available, but there is a dearth of knowledge required to attain it. That’s where the Forum can help.

We should consider a fresh topic where this can be discussed (without the complexities of @dnshorto ‘s existing installation!).

Perhaps I should copy across the earlier diagrams on this thread where I introduced the underlying concepts. That leads directly to this diagram which I created above for JuliaMC:

Those early concept diagrams are here, here and here. They’ll benefit from a bit of simplification and tidying.

Off-grid storage still a big subject so I’ll need to count it as satisfying all of my 2022 New Year Resolutions!

Wise words @Transparent.  There is clearly a need for a workable solution to using stored energy in a power cut situation and a new, uncluttered thread would make sense. One inherent weakness in what I was trying to acheive is that power cuts are more likely in winter, when solar input is low, so a dedicated battery kept full in preparation would be more likely to be of practical use. Hence my wish for a seasonal switchover capability.

A friend introduced me to the archives of the defunct US magazine “Home Power” (https://0964fed1-9106-4800-b313-32ef2cf225aa.filesusr.com/ugd/d49ff9_3cdac33096894ce88b2743937bd6a6c7.pdf)

which is full of fascinating info, if predictably American biased.

You asked how I have connected the generator power to the boiler in such a way that it doesn’t then send power back to the substation. Building on the wisdom gleaned from this forum I inserted a 4 pole changeover switch between the generator, the grid, the boiler and a double “generator only” socket (to run the fridges). The boiler receives power from either the grid or the generator and all seems to work happily. In fact with boiler and 2 fridge freezers drawing power there is still ample capacity to run a 2 kW heater. For “running-in” the engine prefers a half load for the first 20 hours, so 10 hours have already been clocked up in this way.

Also utilising the tips and hints so generously shared here I have now received direct from China a very nicely made 6 pole isolator for the 3 solar strings, made to order with amazing customer service! Surge suppression wisdom sought soon please!

@dnshorto 

I agree this is a fantastic thread specifically about using energy storage in the presence of power cuts. 

@Transparent this topic you’re suggesting for energy storage and heat pumps sounds fantastic. Feel free to simply create a new thread to get the ball rolling on that. I have a feeling it won’t be the last time that subject is covered, what with the community of heat pumpers we already have!