On-grid domestic battery storage

michaels

Question

I could switch to an octopus tariff costing 15p/5p or stay on my current tariff at 12p.

Is it possible to get battery storage at a lifetime cost (excluding interest costs for simplicity) at 7p per kWh (12-5) stored for a 20kwh store (our daily usage)?

Suppose need 25kwh batteries to give 20kwh daily for 15 years, system would supply 20kwh x 365 days x 15 years = 109500kwh x 7p per unit = £7665. Am I right in thinking this is probably about half of the current cost?

Reed_Richards

@michaels, are you thinking of pure battery storage without associated solar PVs? Although easy in principle, I don't think persuading a battery to charge overnight and discharge during the day is all that simple. I have both solar PVs and a battery and I'm not sure if I could persuade my battery to charge from the mains at night. Normally the inverter maintains a battery minimum State Of Charge at 10%. But this is a setting I can alter manually, in principle. If I stayed up until after midnight then altered the minimum SOC to 90% I think the inverter would charge from the mains up to 95%. I would then have to wake before the day rate kicked in (7 am?) to change the SOC setting back and allow the battery to discharge.

The sort of battery capacity you are thinking of might be found in an electric car. So perhaps what you are seeking is an electric car that you don't use very much that could use it's battery to supplement your mains supply when it is plugged-in at home?

Solarchaser

Taking the Ebay listings into account, I'd imagine you could get 24kwh of pylontech batteries and a sofar me3000sp for around £8500, possibly less from somewhere like think renewables. That would give you the 20kwh you desire.

The sofar, as with all battery systems (as far as I know) can be set to charge overnight and discharge when required.

However it can only discharge at a max of 3kw, so based on the 20kwh/day you state... which is approximately what my house uses, you will probably find that some of your usage is way above 3kw, say for instance someone is in the shower at 9kw, or washing machine, tumble dryer and cooker / iron is on, well over the 3kw.

So then you would need to look at a parallel able system like the lux ac coupled, which you could have say 3 batteries on 2 systems and 4 on 1, and this would cover up to 9kw of output, but for the parallel system I'd think you would be around the £10k mark.

So not quite double, but not really too within 15 years either.

However, add in £2-3k of diy solar, and imo the figures change quite a bit.

As a full picture let's say you spent 3k on DIY solar, getting around 6kw of solar on a garage roof.
You spend £1k on a thermal store for hot water.
And you replaced the electric shower with a thermostatic which will run off your thermal store.

Then let's say you spend 9k on 2 parallel systems and 18kwh of batteries.

You have spent £13k, but you will be saving around 3-4mwh of your usual 7mwh you are paying a year (going on 20kwh a day and rounding), as well as saving another maybe 4-5Mwh on no longer heating hot water with a combi boiler.

In the summer and shoulder months you will only be buying gas for cooking, and most of your electric needs will be free.

In the winter you set the inverter and thermal store to charge/heat overnight at the 5p/kwh rate and so most of your electric will be at 5p/kwh and gas will only be for heating/cooking.

Add to this you move onto the octopus tariff that pays you for actual export and you might find you actually earn money in the summer months to offset some of your winter usage.

At the next level you add an EV which can almost charge fully for free in summer, and is on overnight in winter, and assuming your in an ice car now doing 10k miles /year at around 15p /mile so £1500/ year on fuel, the ev will do around 1.25p/mile, so £125 a year if charged overnight, considerably less if free with solar.



Ooft I went off on one a bit there.
The figures a quote are mostly solid, some a little woolly, but I think they stand up to mild scrutiny.

I've had solar for 5 years, and extended it this year, an EV and batteries for around 9 months, and will definitely be moving to thermal store, and thermostatic showers in the coming months, so some is my data, and some is what I'm aiming for.

Hope that helps

Martyn1981

Solarchaser wrote: »

Hope that helps

Yes, very much.

The theory side (no offence intended) is looking ever more solid and reasonable, so just the economics, which I have no doubt will work eventually, and 'eventually' is hopefully not too far away.

As an aside, just rambling thoughts really, an off-gridder gave me figures on FLA's (flooded lead acid) batts a few years ago, that came in at around 2p/kWh based on a 40% DoD (so for 20kWh, you'd need to buy 50kWh of batts), but was based on he batt costs minus their scrappage value, which is not an unreasonable argument I suppose.

Not suggesting that's the right route, as FLA's need some love and attention, and venting of any H2 gas. But since then I've seen arguments (perhaps discussions/chats is a better description) as off-gridders start to consider Lithium.

Everything going in the right direction slowly

EricMears

Solarchaser wrote: »

Taking the Ebay listings into account, I'd imagine you could get 24kwh of pylontech batteries and a sofar me3000sp for around £8500, possibly less from somewhere like think renewables. That would give you the 20kwh you desire.

I've just looked at eBay too. One seller is offering a new sofar me3000sp for £653 inc delivery although there are other inverters available secondhand (but allegedly working) from around £250. In the vehicles section you could buy a complete Nissan leaf 24kWH with owned battery (be careful : some that age have leased batteries) for under £6k. If you bought the complete Leaf and took the batteries out you could probably sell the rest of the parts - maybe even at a profit.

Zarch

What is stopping the battery controller exporting and importing at your command?

Software within the controller? Are they in essence 'dumb' ie, charge till full, discharge when required?

ie, what's stopping you track pricing from Agile Outgoing and then exporting a full battery between 4pm and 7pm at 10p or 11p per unit for maximum profit?

Solarchaser

Yeah a second hand leaf is not a bad shout, and something I have pondered myself a few times.

There are some things to consider though.
Removing the leaf batteries and dividing them into useable cells is a decent undertaking.
I've watched a few YouTube videos of folk doing it, its certainly not impossible, and would offer good value for money, but not quite the *plug and play* solution of pylontech batteries.

The pylontech have a patch cable type arrangement where the batteries can talk to each other and the sofar system, which I believe helps with balancing cells etc, using leaf batteries would be more like what Mart suggested about lead acid batteries, and so there would be a bit more maintenance involved.

If you look for a user called Nowty on the Navitron forum, he has done this sort of thing, but with lithium cells from growatt batteries which come up on ebay quite often, and is very pleased with the result, but certainly his expertise is far above mine.

It's worth noting the pylontech 2.4kwh battery is about the size of a VCR or DVD player and can be stacked horizontally or vertically, which makes it a pretty compact solution.
The leaf batteries could potentially be arranged in the same way, it's just a bit of hassle factor.

The other thing is this, if the leaf is at 6k (unless its accident damaged) then it will be a gen 1 that's done alot of mileage... or more importantly, it cant do alot of mileage.

My 15 plate low mileage leaf is at 91% of the 24kwh battery, I'd imagine the 6k leaf will be closer to 50%, and so the 24kw battery becomes more like 12kwh, so only 10 useable. (Though you could check this before purchase with a bluetooth obd2 dongle and leafspy)

I'm in no way saying dont do it, it's still something I consider and troll salvage markets occasionally, and could be very cost effective, I'm just urging a little caution over the things I tend to get caught out with.

Solarchaser

Ahh excellent timing Zarch, I was hoping you would appear, as your website would give a good reason to do just that.

I thought it may add too much complication to my earlier post, but YES, exactly as you say, most controllers I've come across could be set to discharge on command, so if you knew the unit price on export was at 30p/kwh between 4pm and 7pm, you could set your batteries to discharge to the grid at that point, and then recharge overnight, you could effectively make decent money on exporting from your batteries.

Some caveats in there about your own use 4pm to 7pm as well obviously.
No point exporting at 30p to import at 40p etc

Zarch

Solarchaser wrote: »

Some caveats in there about your own use 4pm to 7pm as well obviously.
No point exporting at 30p to import at 40p etc

https://www.energy-stats.uk/octopus-agile-outgoing-export-yorkshire/

Just be careful with those figures from Outgoing, for example.

16:00 to 16:30 today:
import 20.79p
export 9.76p

Whilst the graphs and figures/prices track each other between import and export all day, the difference between the two between 4pm and 7pm is quite large.



I still think you'd be able to use the data to your financial advantage if you can wangle the way the battery works. But no one is going to be giving you 30p per unit for your spare energy!

Solarchaser

Haha wishful thinking I guess.

Must have been mixing up import with export