How much off-peak charge can you store in 4hrs?

r8sso80 · Mar 23, 2022

I'm considering buying a home battery storage solution and read that with 3kWh feed you can only charge 12kW during the 4 hour Octopus Go off-peak window, which makes sense.

Is it the mains supply, battery or inverter that bottlenecks this 3kWh? I've read some batteries (or was it inverters?!) allow 6kWh charge/discharge so would this allow 24kW to be charged in 4 hours?

Thanks in advance
Ross

marconi · Mar 23, 2022

Are you mathematically adept? It will help if you are to do the analysis of your question.

In fact I will not answer your question because it is simply yes, yes and yes. One needs to be careful in the domestic setting about schemes to store electricity in useful amounts in batteries because it has high capital costs. Thus one has to be convinced that the return on this capital investment is better than something else you could do with the same expenditure. Folk often do not factor in replacing the batteries, housing the installation, equipment defects and repair, under utilisation ie you do not take advantage often enough of the saving you have attempted to make by your investment worthwhile.

On the green agenda I have not seen any scheme to sustainably shift towards batteries by recycling or reusing them. You can search on line and find firms which will offer you batteries previously loved in ev cars but their performance, quality and reliability is vague.

Unless you really can demonstrate it is a good investment then I would caution against storing mains power during off peak periods for use during expensive periods. The analysis I did last month for my home did not convince me - but I am fallible.

If you have solar power in excess there is some merit in storing some of it if you can reliably use it in quantity later and thereby avoid that consumption from the mains and the economic saving justifies the capital expenditure.

A great combo for solar pv is with air conditioning - which is the West Coast of USA experience.

I like others moan about the daily standing charge which has gone up. I think it is about 20p for us each day. But put in perspective, for that 20p I have access to a supply of up to 12kW every second, every hour, every day........I'd find that impossible to achieve for 20p a day at a reliability of something like 99.9%. My mum and dad in a Northern town have never in 20 years had a power cut. We get about one 5 minute power cut a year in winter. You would find it hard to invest yourself for your requirement to achieve those levels of availability and reliability and capacity of electricity. You could invest in being largely off-grid but be prepared to change your behaviours and life-style in your consumption - these have costs/implications too.

Food for thought. I do not have time I am afraid for a discussion.

r8sso80 · Mar 23, 2022

marconi said:
Are you mathematically adept? It will help if you are to do the analysis of your question.

In fact I will not answer your question because it is simply yes, yes and yes. One needs to be careful in the domestic setting about schemes to store electricity in useful amounts in batteries because it has high capital costs. Thus one has to be convinced that the return on this capital investment is better than something else you could do with the same expenditure. Folk often do not factor in replacing the batteries, housing the installation, equipment defects and repair, under utilisation ie you do not take advantage often enough of the saving you have attempted to make by your investment worthwhile.

On the green agenda I have not seen any scheme to sustainably shift towards batteries by recycling or reusing them. You can search on line and find firms which will offer you batteries previously loved in ev cars but their performance, quality and reliability is vague.

Unless you really can demonstrate it is a good investment then I would caution against storing mains power during off peak periods for use during expensive periods. The analysis I did last month for my home did not convince me - but I am fallible.

If you have solar power in excess there is some merit in storing some of it if you can reliably use it in quantity later and thereby avoid that consumption from the mains and the economic saving justifies the capital expenditure.

A great combo for solar pv is with air conditioning - which is the West Coast of USA experience.

I like others moan about the daily standing charge which has gone up. I think it is about 20p for us each day. But put in perspective, for that 20p I have access to a supply of up to 12kW every second, every hour, every day........I'd find that impossible to achieve for 20p a day at a reliability of something like 99.9%. My mum and dad in a Northern town have never in 20 years had a power cut. We get about one 5 minute power cut a year in winter. You would find it hard to invest yourself for your requirement to achieve those levels of availability and reliability and capacity of electricity. You could invest in being largely off-grid but be prepared to change your behaviours and life-style in your consumption - these have costs/implications too.

Food for thought. I do not have time I am afraid for a discussion.

Thanks Marconi - you're not from Chelmsford are you, the birthplace of radio?

I've done the math on Solar PV, Solar PV + storage and storage alone. For me, Solar PV is pointless if I can't use the energy during the day, selling back is pennies. Adding batteries increases the cost but you get to charge them for free which is great if you use 31p a unit for your ROI calcs, but as you can charge the batteries off-peak for 7.5p a unit I don't think this is an accurate way to do it. Which made me think, why don't I sack of Solar PV and just go for a pure battery storage solution.

I use 558.0kWh per month or ~19kWh per day. At 16.1p a unit that's ~£90 a month. When that jumps to ~31p a unit I'm looking at ~£173 a month. If I get an 18kW battery, and charge it at 7.5p a unit, that's ~25% of what I'm paying peak rate so ~£43.25 a month. That's a theoretical saving of ~£129.75 a month, ~£1557 a year. I'd like to think that ROI could be recouped in 5 years. I've seen a complete 16.4KW GivEnergy kit for £6138 inc VAT. That's a 10 year warranty and 6000 cycle discharge which is ~16 years if you fully charge/discharge once a day.

For me, taking the green/carbon aspect away, battery storage seems to be the most efficient way of getting a ROI.

marconi · Mar 24, 2022

When I did the calculation I factored in depreciation in order that after the assumed life one could renew the capital equipment especially the battery, and installation costs. And net present value to account for inflation. Would you keep us informed on how you get on please? Pictures are always good to look at.

Octopus seem to offer the most innovative tariffs at the moment. I had a quick look at Givenergy; they appear teamed with Octopus which is promising.

I will do my sums again using Givenergy products - thank you

r8sso80 · Apr 10, 2022

marconi said:
When I did the calculation I factored in depreciation in order that after the assumed life one could renew the capital equipment especially the battery, and installation costs. And net present value to account for inflation. Would you keep us informed on how you get on please? Pictures are always good to look at.

Octopus seem to offer the most innovative tariffs at the moment. I had a quick look at Givenergy; they appear teamed with Octopus which is promising.

I will do my sums again using Givenergy products - thank you

For anyone who finds the thread and would like to know how it pans out... I've pulled the trigger on a Solar PV & Battery Storage solution.
12x Hyundai 410kW panels (25yr product & efficiency warranty!)
12x Tigo optimisers
LUX 5kW Inverter
12kW Storage
Comes in at £10,150 installed.
I'll post again once I've had it installed and again when it's run for a month or two.

marconi · Apr 10, 2022

Thank you for telling us. What is the predicted performance over a year and month by month?

WallaceP · Apr 10, 2022

Brilliant, I am looking forward to seeing this set up

DPG · Apr 10, 2022

I'm also interested. I need a temporary solution until my over-unity machine is finished.

Jim_e_Jib · Apr 10, 2022

DPG said:
I'm also interested. I need a temporary solution until my over-unity machine is finished.

What depth of sea water and how much will the air expand in the buckets?

DPG · Apr 10, 2022

Jim_e_Jib said:
What depth of sea water and how much will the air expand in the buckets?

Ooh, he must be due to reappear soon! He's probably trying to find an infinite number of buckets

r8sso80 · Jun 14, 2022

WallaceP said:
Brilliant, I am looking forward to seeing this set up

marconi said:
Thank you for telling us. What is the predicted performance over a year and month by month?

So it's been a month since install so I'll give a quick update.

In the end I had 395W panels delivered not the 410W which I had agreed. They knocked £300 off which I was happy with as it it's not a major drop in total W. They also gave me 14kW of storage instead of 12kW at no extra cost; 4x 3.5kW batteries instead of 5x2.4kW (think that was a supply issue on their side). They also installed bird protection for free after I asked for it post install which again I thought was a nice offer from the supplier.

The weather has been great this last month so the system is working really well. I've yielded 711kWh in total and fed 183kWh back to the grid, which is a shame as I'm not on a SEG yet!

Batteries are generally fully charged by early afternoon. My biggest surprise was that even by 9am the PV is enough to cover my consumption (~600W) with SSW facing panels. I've had 3 poor days where I produced 4/5/9kWh and other than those it's been 10+kWh. It will be interesting to see how it fairs over the winter months.

Not the best photo of them but they blend in with the grey tiles nicely.

MisterB · Oct 31, 2022

Hi everyone, i am not an electrician (i served my time (C&G) as a plumber) and i found this forum after searching for info on home battery storage systems, which i am looking to have installed in the very near future. one of my questions is actually the subject of this topic and i am not sure its been answered - i also want to know whether a 3kw inverter could charge up a sizeable battery bank in the 4 hour octopus go period.

my initial thoughts are that as a general 'comment' lithium batteries shouldnt be discharged below a certain level (%) or above a certain level (%) - the actual percentage varies depending on where you get the info from .... but i am looking at not discharging below 15% or charging above 90% - can anyone offer any advice on those settings please?

i intend to have a battery initially installed of 13.4kw - so effectively would only ever need to charge up 75% or 10kw which i am sure is well within an inverter with a 3kw charge rate. However when i add additional battery storage of another 13.4kw - that would need 20kw of energy (or would it?)

it seems to make sense to me therefore to have an inverter with a 5kw charging rate installed or should i go for 2 x 3kw inverters, which also adds resilience in case one inverter becomes faulty? i think the cost difference in buying two smaller inverters as opposed to one bigger inverter is only about £300, so not a massive additional cost long term.

additional to the above, i have a 4kw PV system installed (under the FIT scheme) so dont want to affect those payments, so will be looking for AC coupled inverter(s) that have the facility to utilise the excess PV to charge the battery bank when it is producing a surplus to what is being used at any one time and only goes back to the grid when the battery bank is full. of course i wouldnt have the current PV connected direct to the new inverter(s) and would have to harvest it when its been converted into AC - i know its not the most efficient way but as i said, i dont want to lose my FIT payments.

if anyone can offer any advice on my proposed 'plan' in terms of glaring errors or can suggest which inverters i should consider, i would be very grateful. in terms of the batteries, they will be 'home brew' built using 16 x 280AH EVE cells (lithium) and a suitable BMS (JBD)

Thanks and best wishes

kropaske · Nov 1, 2022

MisterB said:
my initial thoughts are that as a general 'comment' lithium batteries shouldnt be discharged below a certain level (%) or above a certain level (%) - the actual percentage varies depending on where you get the info from .... but i am looking at not discharging below 15% or charging above 90% - can anyone offer any advice on those settings please?

Depends of battery manufacturer but 85% DOD is fairly safe. As for charging you can go 100% as battery will start discharging straight away. Its not recommended to store them at full charge for long periods.

MisterB said:
i intend to have a battery initially installed of 13.4kw - so effectively would only ever need to charge up 75% or 10kw which i am sure is well within an inverter with a 3kw charge rate. However when i add additional battery storage of another 13.4kw - that would need 20kw of energy (or would it?)

LiFePo4 can ba charged 0.3c - 1c but higher you go shorter the life of the cell will be. Usually up to 0.5C is OK. Also factor some loses, 10kw ac won't convert to 10kw dc and then back again to 10kw ac. I would say 10-15% will be lost.

MisterB said:
it seems to make sense to me therefore to have an inverter with a 5kw charging rate installed or should i go for 2 x 3kw inverters, which also adds resilience in case one inverter becomes faulty? i think the cost difference in buying two smaller inverters as opposed to one bigger inverter is only about £300, so not a massive additional cost long term.

I would use 1 charger to keep it simple. They are quite robust

MisterB said:
additional to the above, i have a 4kw PV system installed (under the FIT scheme) so dont want to affect those payments, so will be looking for AC coupled inverter(s) that have the facility to utilise the excess PV to charge the battery bank when it is producing a surplus to what is being used at any one time and only goes back to the grid when the battery bank is full. of course i wouldnt have the current PV connected direct to the new inverter(s) and would have to harvest it when its been converted into AC - i know its not the most efficient way but as i said, i dont want to lose my FIT payments.

Just factor some more loses through the conversion process.

MisterB said:
if anyone can offer any advice on my proposed 'plan' in terms of glaring errors or can suggest which inverters i should consider, i would be very grateful. in terms of the batteries, they will be 'home brew' built using 16 x 280AH EVE cells (lithium) and a suitable BMS (JBD)

Estimate first how much storage do you really need. No point to put 30kw batteries if you using 10kw energy per day. Batteries are still quite expensive. If you building your own system factor all the extra bits and bobs like cables, connectors, good quality charger for balancing the cells, all the labour and time you need to build this system, also if you ordering directly from China it may take months before the cells arrive. What if you make mistake and fry 1 cell etc, more time and money will be wasted. All this needs to be off set against bit more expensive out of the box system that will start working straight away.

MisterB · Nov 5, 2022

kropaske said:
Depends of battery manufacturer but 85% DOD is fairly safe. As for charging you can go 100% as battery will start discharging straight away. Its not recommended to store them at full charge for long periods.

LiFePo4 can ba charged 0.3c - 1c but higher you go shorter the life of the cell will be. Usually up to 0.5C is OK. Also factor some loses, 10kw ac won't convert to 10kw dc and then back again to 10kw ac. I would say 10-15% will be lost.

I would use 1 charger to keep it simple. They are quite robust

Just factor some more loses through the conversion process.

Estimate first how much storage do you really need. No point to put 30kw batteries if you using 10kw energy per day. Batteries are still quite expensive. If you building your own system factor all the extra bits and bobs like cables, connectors, good quality charger for balancing the cells, all the labour and time you need to build this system, also if you ordering directly from China it may take months before the cells arrive. What if you make mistake and fry 1 cell etc, more time and money will be wasted. All this needs to be off set against bit more expensive out of the box system that will start working straight away.

Thanks for responding. I am buying the EVE 280 cells from a reputable UK supplier, Fogstar. They will be in the UK in the next couple of weeks. I had a site visit from my electrician yesterday and all being well it should be up and running mid December