Your solar array must be positioned perfectly.

Between Dec23 and Jan24 I had 30 days with production below 5kWh (worst day was 0.4kWh) and that's with an 11.4kW array.

We have 6kW of PV but our neighbours have large trees that the sun can’t get over in winter, and a 9.5kWh battery. I assumed we would get another battery when the heat pump went in. But some simple sums show it would never pay back it its lifetime. 
Nevertheless we just might buy another 9.5kWh battery just for the pure satisfaction of being independent of the grid and only ever buying the cheapest electricity.

But there is another limit for us which is getting to fully charged within the cheap periods (we’re on COSY) since our AC3 inverter can only charge at 3kW so it takes just more than the 3hours to fill it (if empty) so that means a further AC3, not just an extra battery. Right now the GivEnergy software can’t reliably control parallel inverters, which they call a plant.

Your Huawei battery will have similar charge limits and do you know what they are? No manufacture supports mixed supply  of different makes indeed some are mutually compatible in their own product range

@irmartini Ive spread my bets with and additional East and West orientation in addition to the original South facing one 🙂

@judith Ive just had a quick look and I think its its saying 1.5 kW per 5 kWh module

So not being one to rest on my laurels, it occured to me that I should be able to get a more accurate prediction of my energy usage from Octopus using their csv download.

I have to say the results surprised me a bit. I excluded the electricity usage during the cheap go periods but found some quite high usage when I wasnt expecting it. After a bit of investigation I noticed this was Go using slots outside its normal range and also some free electricity slots during days when the grid was green.

After a bit more cleaning of the data, I finally got to the following graph which indicated my daytime import over the year was 1225 kWh (much more than I had estimated initially using my other data).

Calculating the impact of a second 5 kWh battery module came in with a saving of £214 over the year, giving a payback period of about 10 years. While still being borderline it is a lot more promising than I initially thought.

One thing that might have affected the outcome is that for the initial part of the year, I was using Octopus Cozy, not Go, so its possible charging the battery in the afternoon slot might have exagerated the benefit. I will do the analysis again when I have a full year of Go data. 🙂

Posted by: @judith

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 No manufacture supports mixed supply  of different makes indeed some are mutually compatible in their own product range

 

if course what I should have written was INcompatible within their own range!

Posted by: @bontwoody

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in the end settled for a 5 kWh Huawei Luna battery module

Can you expand on that choice please?
I'm particularly interested in security of supply.

Having visited the Luna product page, it appears that the extended guarantee is linked to a requirement to use their cloud-based software for monitoring and system diagnosis.

Do you have App preferences to prevent Huawei taking your battery off-line, for example to install an over-the-air upgrade?

Posted by: @bontwoody

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If we assume I was to purchase a second battery module then this could hold another 5 kWh. [....]

Given the cost of an additional battery module is about £2000, the payback period would be about 67 years

[bigger snip]

Calculating the impact of a second 5 kWh battery module came in with a saving of £214 over the year, giving a payback period of about 10 years.

The price of battery-grade lithium carbonate has fallen to 20% of what it was 2 years ago.
That's primarily due to Western Countries putting back the date by which they will no longer allow ICE vehicles to be sold.

That has an immediate effect on the price of importing LiFePO4 cells from agents in China.
I'm being offered 16x 280Ah cells for around £1500.
Some agents have stocks already in Poland, Germany or Turkey to avoid the 7-weeks marine shipping delay around South Africa.

Add to that a Seplos Mason steel enclosure, incl BMS and cabling, for around £500.

The DIY route gives you 14kWh for the same £2000 price as another 5kWh from Huawei.

@transparent At the time, it was very difficult to get a solar installer and the one who did reply only fitted either Solar Edge or Huawei. The 10kWh SE quote was out of my range so I settled on the modular Huawei. Annoyingly you can buy a Luna module in Germany for £1400.

I do use the online monitoring platform and apart from the fact I havent been able to combine its data with any of my other monitoring systems, Im reasonably happy with it. Im not too worried about the security of supply (barring war with China!) and the system as a whole works well. I havent investigated the app preferences in that regard.

With respect to the DIY route, I am open to doing most things DIY but I do admit to being a little scared of the home battery scenario. The thought of totaling the battery through a mistake is haunting!

If I could use a DIY battery with the Huawei inverter with a confidence of success, I would certainly consider it, I could then sell on the original Luna module and perhaps knock another £1000 off the price. At present my inverter and battery are outside but I hope to install them in an extension next year.

Posted by: @bontwoody

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If I could use a DIY battery with the Huawei inverter with a confidence of success, I would certainly consider it

You have a high expectation of success with the assistance you'll get on this forum!

Tell us the model number of the inverter and I'll look at the specification.

It's likely that the Huawei App is picking up data from both the inverter and the Luna battery simultaneously.
But I doubt you're spending any time looking at the cell-voltages obtained from the CAN-bus link between the two units.

Once a battery has been configured to work with a particular inverter, most end-users are only interested in what data comes from the inverter.

If you really want to know what's happening within a battery, most BMS units generally their own App which allows you to check.

The one piece of data which is invariably incorrect is the State-of-Charge.
That's notoriously difficult to calculate unless you regularly calibrate the monitoring software by completely discharging the battery and then re-charging to 100%

Most home-battery users wouldn't consider doing that.
We tend to use batteries in the middle of their capacity range.

@transparent It was really the SOC I was interested in 🙂 but with a bigger battery that wouldnt be so critical.

I suppose it wouldnt be the end of the world if I had to replace the inverter too, and I could sell than on with the battery 🙂

Photos attached

Let me quickly address the State-of-Charge (SoC) issue,
and I'll come back to the inverter once I've read the specs.
[aside] today's plan is to replace the floor joists on one my rooms, and it's not progressing fast!

A BMS unit would normally be expected to undertake four roles:

1. protect the cells against over-charge by continuously checking for the one with the highest voltage
2. ditto for too much dis-charge
3. balance the cells to attain an even voltage across them whilst in charge-mode
4. calculate the SoC

It's pretty good at the first three, but you can't measure the SoC properly whilst there's current flowing.

The best method is to deploy a separate coulometer which measures current and voltage on the main connection cables.

The market leaders in this technology are Victron in Holland and Batrium in Australia.
Each of them has developed a finely engineered coulometer which costs several hundred pounds.
The price tells you a lot about how difficult this is!

However, there's a third alternative from a Chinese manufacturer called Peacefair.

I've been using their stand-alone Coulometer PZEM-015 for over a year, measuring current via a shunt rated at 300A.
It costs about £18 incl a shunt, and beats the socks off the three different BMS units I have here on test.

I spoke to Peacefair about 8 months ago because I would've liked to have a data-out port from their unit, rather than just the inbuilt LCD.
At that time they couldn't offer such a device.

Last month they emailed me a newsletter which offers an alternative with a data-out

The PZEM-017 is a dedicated coulometer which provides data using Modbus across an RS485 connection.
The downside is that it doesn't have any LCD.
You can only extract the data, which you then take to whatever computer system you want.

There's also a dedicated coulometer from another Chinese company called JuncTek, costing around £60-ish.

I've used one of their KH140F units for about a month.
But tests were curtailed because one of the front-panel buttons wouldn't work, which limited what we were trying to do with Modbus.