@batalto if you can do the batteries and replace my existing set then id love to have 10kw of storage, hell id even pay for you to come up to the highlands to do it ! 

@boblochinver given you've already got the inverter, couldn't you just replace the batteries? is you inverter programmable to other battery outputs? If you can its basically a straight swap and you can get LiFePO4 batteries at really reasonable prices.

EBAY.CO.UK "https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=lifepo4+280ah&_sacat=0"

you can get them even less on AliExpress if you are willing to wait. Are they going to be amazing batteries? Probably not, however for the cost they will last years so who cares if they only pull 95% of the rated capacity.

Thank you all for your helpful responses.

It seems that my initial estimate, based on Julian's actual usage, is probably nearer the mark say 2,000kWh per year.

I'm not sure about batteries. The least daily electricity we've ever used is about 15kWh (in July, 10 in the day, 5 overnight). That's more than the projected peak output of the panels. I've been quoted for Q-cells (hanwha).

The more I learn about this the more I understand why there isn't wider acceptance of all things renewable heating wise. This forum excepted it is so hard for the average customer to get good impartial advice. For most consumer products we rely on specialists (who are usually the vendor) but this is a field in which it's really difficult to know whether or not your "expert" really does know what they are talking about and is impartial. I know we all have a responsibility to inform ourselves when buying a product but if I had to know as much about cars as it seems I need to know about renewable energy, I would never have got behind the wheel. But they are a very long established product with pretty well understood performance parameters among the average customer.

I worked in the City for many years and despair when I hear about people who have got into totally unsuitable financial products. I shout at the radio or TV, "why didn't you ask this or check that first". But then it's obvious to me because it's my area of expertise. This is definitely not.

Great to continue these dialogues here.

Mike

@batalto I look at the ebay sales but don't honestly know what im looking at ? how many of those battery packs do I need to make 10kw ? and is it simply put a bunch together and it all works ? don't I need some kind of intelligence for them altogether ?

@mike-patrick I think your numbers are slightly off are you saying your house only uses 2000kw per year ? 

@mike-patrick 

Hi Mike,

I forgot to mention that you may be able to get a system with a little more output, it all depends on the available roof space.

Most commercially available solar PV panels have an efficiency of around 20%, the different output ratings are therefore down to the panel size. Larger panel, more output.

My Particular solar array (now the best part of 10 years old) consists of 16 panels covering 24 M2, giving an overall output of 4kW (250W per panel). More modern panels are slightly more efficient, but high efficiency panels are more expensive. It is a compromise between available roof space and budget.

You also need to try to ensure that the panels will not be in shade for efficient operation. In the depth of Winter the Sun is only 13 degrees above the horizon at its zenith each day.

@mike-patrick I think your usage will be nearer 5,000kw over the course of the year. If you are using 15kw in the summer, you will 100% be using more in the winter due to darker days, more time at home etc. The best way would be for you to check your power bills for the year.

On panels and batteries, you are confusing kilowatts (kW) and kilowatt hours (kWh). Kilowatts is the maximum output of your system, kilowatt hours is the amount of kilowatts you can store and release. Think of panels as a flowing stream and batteries as a bucket you can catch water in. It doesn't matter how big the flow or the bucket is. You can fill the large bucket with a small flow, it just takes longer. Likewise batteries (buckets) have a maximum fill opening. Even if you are producing a lot of power, all of it cannot be pushed into the batteries at once. I have 14.2 kWh of batteries, but my max charge is 3.2kw. even at full charge it will still take just under 4hrs to fully charge them. Likewise my max discharge is 3.2kw so even with a demand of 4kw the most I can service on batteries alone is 3.2kw the other 0.8kw I have to bring in from the grid or my panels.

@boblochinver you need three things.

1. An inverter which can charge (I believe you have this)
2. A battery management system (BMS) for the batteries. Your current batteries have this already built in
3. The batteries you want to charge

With those batteries I'd advise you watch several of the YouTube channels where people build battery packs. Essentially you are going to want to build them in series and parallel as they are 12v and I'm assuming your battery system is 48v. 

Power = Ah x V / 1000 (to get to kw).

So 280 x 48 = 13.4kwh

Off the top of any head you'd need 16 of these. 4 in series and then 4 of each of those in parallel. Plus a BMS around 200A to charge them. All in, probably around £2k if you have an inverter which can work with them. A used Sofar ME3000SP (the same as I have) is on eBay for £270 and I know that would work as it has battery profiles including "custom". However I have Pylontech batteries and I got 14.2kwh for around £2600 so I don't need any more. My only temptation would be a second inverter and battery to give me more power in output e.g. another 3.2kw to take me up to 6.4kw peak output from my battery. 

I'll consider that after winter as with Octopus Go I could fill maybe 30kw for £1.50 overnight and that would basically mean I never bought peak power except a few days a year. In the summer it would give me substantial carry over on a gloomy day.

@boblochinver No, we use about 12,000 kWh p.a., the 2,000 was the estimate for the solar PV.

Mike

@mike-patrick sorry for slow response.  No I'm not able to give you a monthly breakdown - I don't track it.

I do get more in the spring, when sun is bright, but it is still cool.  I would say I get 70% from April to Sept.  I would support what Bob says above - use of micro-inverters if you have any shading issues.  I don't.  And they were a lot more expensive 9 years ago.  But definitely the way to go now.

I am less of a fan of a home based battery set up.  I prefer an Electric Vehicle and a solar diverter to use the "spare" electric.  One day soon we will have Vehicle to Grid (V2G) charging units (they are currently expensive and can only be used with Chademo EVs - Nissan Leaf).  But CCS chargers are being developed.  This will mean charge up overnight when it is cheap or via PV and use electric from your car battery during the day to run your home.

@boblochinver I found these today on Aliexpress 14.88kw for £359 delivered. The cells might be a bit rubbish for that price, but even if they were only 90% of capacity, who cares!!

All you would need is something like a 200amp BMS and cables; maybe 32mm2 (2 gauge) and you would be good to go. Frankly I might buy some and have a play for this price.

Our solar production numbers for 2020 and 2021: 6.16kW array. How’d your array do this year?