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

@dnshorto wrote:

Would that separate inverter be able to use the same domestic distribution system within the house, or would completely separate circuits have to be run?

This question suggests that I’m still not adequately explaining the overall concept of an off-grid inverter and why you can still run it during a power-outage.

There’s a YouTube channel with a guy called Will Prowse who is an expert on solar power and Storage Batteries in large motorhomes (in the USA). Thus he well understands the need both to be independent of the electricity grid, and yet be able to plug in when visiting a static trailer-park.

Here’s a relevant video he’s published recently:

The off-grid Growatt inverter Will uses in this video-tutorial is the SPF3000TL LVM which is for 120v AC output.

The equivalent for use in UK/Europe, running on 230vAV 50Hz, is SPF3000TL HVM. There are other models in the series and an option for 24v or 48v for the (Lithium ion) battery.

Will speaks fast, but everything he mentions is shown, in similar fashion to the Royal Institute Christmas Lectures. Nevertheless, I still prefer to watch his videos with my finger ready on the pause (K) key!

If @dnshorto is happy with this I’ll then go on to draw a diagram of how this type of inverter can be legally and safely installed in a UK home.

If you’re still finding the video incomprehensible, please say what you’d like me to expand on first. There’s little point in me drawing installation diagrams while you’re unsure about the overall concepts - such as being able to input power from either mains or PV Panels.

I also rely a lot on @Jess_OVO telling me when we’re going too deep technically, which is very useful

Thanks @Transparent . I understood Will’s video, which was quite relevant to my quest although I had to repeat some of the footage. It seems that the Growatt inverter he was using has a mains charging input which would be vital in a winter UK setting. His average 5.5 hours of sunshine might well be available in Nevada but not here, especially in the power cut season.

I would be very grateful if you could draw an installation diagram but would hope to be able to ask for clarification if I don’t fully understand it.

OK…. so here’s a basic diagram of an off-grid mains & solar battery, @dnshorto :

The two Distribution Boards both deliver power to mains appliances within the home.

In the event of a power-outage, devices on circuits connected to DB1 will fail, whereas those connected to DB2 will continue to operate (whilst you still have energy in the battery).

I have more things to add to this diagram, but let’s stop there for the moment. Do I need to clarify anything?

Error in diagram: The mains connection to the Off-grid inverter via its isolation-switch should have come from a trip in DB1 and not directly from the Smart Meter.

Thank you @Transparent, that’s very clear as a new installation and easy to understand. I’m aware that you have more additions to include but, just to take this basic diagram to start with, `I have a question. Where the Off-grid inverter has a mains input, obviously that could be subject to controls so that the main battery input would be the solar panels, with grid input available for dark days. How automatic could that control function be?

Moving on to the actual situation I am in, it is obvious that if I had known that my system as installed a year ago would be incapable of maintaining supplies during power cuts I could have used a version of your off-grid diagram to make provision for them. So naturally I immediately try to envisage how what I have could be modified to achieve that. A number of other questions immediately start to occur, but maybe I should step back and wait for the next layer of detail in your excellent diagram. I greatly appreciate your help in this, and hope it may also be useful to other members.

@dnshorto 

I have your existing installation mind, @dnshorto  as I present the second diagram:

In this scenario there are now two solar inverters. One is grid-connected, and must therefore be export-limited to 3.68kW (16A per phase) as is normal for applications approved by the regional Distribution Network Operator (DNO).

The rooftop solar panels are now divided into separate arrays and connected via a String Combiner box. This contains separate safety trips and fuses for each array, together with any lightning suppressors you may wish to include.

The trips in the String Combiner allow arrays to be directed either to each Inverter separately, or else added together to feed just one inverter. It provides the flexibility for changing how much energy is sent to either system.

Here’s my own String Combiner box which has inputs from three separate solar arrays on the roof.

And here are the rooftop panels, showing how they are divided

Arrays A, B & C connect to the String Combiner above. Arrays D & E feed different off-grid systems which will both be altered/upgraded next summer (2022).

You wrote:

Where the Off-grid inverter has a mains input, obviously that could be subject to controls so that the main battery input would be the solar panels, with grid input available for dark days.

Absolutely. The control system is most important.

At the moment mine is very manual.

I had been hoping that OVO would use this Trial Site to understand how the Flex Platform can apportion the available solar energy for the day so as to optimise the storage facilities. After all, Flex has weather forecast input and would therefore ‘know’ what proportion of the generation to send through each route.

However, as matters currently stand, my site is no longer on the Storage Trial because Flex ‘assumes’ that full export to the Distribution Grid is always available.

Since Grid voltage in my area is high (about 246v), this assumption may not be true. Kaluza are only monitoring the PowerVault Storage Battery, and not the grid-connected inverter, so they could not know why export may be constrained when their system attempts to force it.

I am now looking outside of OVO for a solution.

We are therefore both in the same situation…. and there are a growing number of houses which will have more than one Grid-connected Export Device. Flexibility of control systems is of increasingly greater importance.

We can return to the Control logic a little later.

Let’s pause for a moment before I further add to the diagram. Do you want me to clarify any of the above?

We also need @Tim_OVO and @Jess_OVO to catch up with this weekend’s posts. Both have been asking relevant questions, and last week Tim wrote:

Imagine me as the future viewer of this thread who’s keen to understand their options! 

I don’t think we’re quite ready for a discussion of options yet. I have one further concept to introduce.

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 

Yes, that’s my own design/assembly of a String Combiner.

You can buy them off-the-shelf, but even when sourced straight from a Chinese supplier they are not well thought out, and still pretty expensive for what’s included.

The one I’ve picked to show here has a pair of fuses for each string array, but only one overall mcb/isolator.

Despite the label, the fuses are not ‘Circuit breakers’. You can’t disconnect a ‘live array’ by opening the fuse-drawer because you’ll get a huge spark and risk welding the fuse to its contacts!

And if you look carefully, the Supplier has incorrectly labelled the 2-pole isolator as ‘Lightning protection device’, which doesn’t inspire confidence!

I bought the bits I wanted from (Chinese and German) component suppliers and assembled them into a DIN-rail enclosure myself.

We can return to this topic later. At the moment we haven’t even concluded that you’re going to need the ability to switch where PV arrays are sending their energy!

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 

And I’ll now be able to pick up that last point @dnshorto by presenting the 3rd iteration of the diagram, which has now considerably grown in complexity:

The appliances and devices to the right are illustrative of the technologies which can be operated off-grid.

Some off-grid equipment must use the Inverter to generate 240v with a pure 50Hz sine-wave. These predominately have pumps/motors which rely on the mains frequency for their (smooth) operation. Under no circumstances should you attempt to run these using the much cheaper pseudo-sinewave inverters.

The rest of the off-grid devices can be run directly from the battery. After all, they would normally have a cheap-and-nasty transformer plug to power them using DC derived from the mains!

Many of these devices are likely to require a small DC-DC converter to create the required voltage. We’ll return to that later in the discussion.

Here’s a table showing appliances which are most likely to be operated using the three categories of power now available in the home:

Once again, these are illustrative. You may have your own ideas either because your priorities differ or possibly because you can’t face the technological barriers to achieve that mode of operation.

I’m well aware that we will need to discuss further how and why I’m able to put LED-lighting in the off-grid category. It’s a comprehensive subject in its own right and I’m tempted to start a completely separate topic to address this.

What do you think @Tim_OVO and @Jess_OVO ?

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