On-grid domestic battery storage

Zarch

Solarchaser wrote: »

If you were to get the sofar I'd recommend 3 batteries as a minimum (7.2kwh) as this let's it do the 3kw charge and discharge.

I won't be getting batteries anytime soon as I just can't make the figures remotely work. Here's my thinking.

Looking at my import figures...... between March to August i've rarely imported more than about 3kWh per day, Solar seems to cover most of my needs.

So why do I need to get double 3kWh when through the summer half the battery wouldn't be required?

Granted, through the winter I would use a bigger battery as would be generating less / importing more.

But anything over 3kWh would seem a waste for me for 6 months of the year, especially considering the extra cost of double the battery?

Surely better to use 3kWh each and every day rather than 3kWh of 7kWh for half the year. ie, only pay for what you're going to use 100% of the time?

Does that make sense? Or have I not understood or misread the benefits of a bigger battery?

Happy to stand corrected.

EricMears

Zarch wrote: »

. . . Surely better to use 3kWh each and every day rather than 3kWh of 7kWh for half the year. ie, only pay for what you're going to use 100% of the time?

Does that make sense? Or have I not understood or misread the benefits of a bigger battery?

Happy to stand corrected.

It's not good for any battery to be run down to zero EVER ! Hopefully there's some sort of battery management system to ensure that battery never drops below 10% but there may not be.

So if you're often (or even only occasionally) taking 3kWh out of the battery in Summer you're in line for dropping battery to minimum occasionally and would do so frequently in Winter. Having a bigger battery would probably extend its life.

Exiled_Tyke

EricMears wrote: »

It's not good for any battery to be run down to zero EVER ! Hopefully there's some sort of battery management system to ensure that battery never drops below 10% but there may not be.

So if you're often (or even only occasionally) taking 3kWh out of the battery in Summer you're in line for dropping battery to minimum occasionally and would do so frequently in Winter. Having a bigger battery would probably extend its life.

I'll be posting my experiences with my new battery when I have the time. However in response to this, the inverter for my battery stops discharging at 10%. It also has settings to manage the discharge rate. This was set at 3.5Kw but the battery specs recommend 2.5Kw. The installer said that short bursts (e.g. to boil a kettle) would not be a problem and has set to 2.9Kw for now - although I wonder whether this should be lowered.

Solarchaser

Your thinking is correct.
Generally speaking, most systems run with 80% of the battery, so 7.2kwh equates to 6kwh useable.

I very much feel that if you are a low use household, then batteries are a no go.
If you only use 3kwh a day, I cant see how it would make sense.
My house uses around 20kwh a day.

Theres been only 2 days sofar that I've still had a bit of battery left (of the 6kwh) in the morning when the sun comes up.

And really, what would be nice is in the winter, having 20kwh useable of batts and charging the batts at e7 etc with 5p/kwh and using during the day, this would more than half my winter leccy use.

I'd say if you use less than 3mwh a year, you are gonna struggle to justify batteries.

But dont underestimate the satisfaction you get from not buying a single watt of electricity all day, even though the house has continued as normal, not going and switching things off, not staging loads apart.... making the system work around you.

Zarch

Solarchaser wrote: »

Your thinking is correct.
Generally speaking, most systems run with 80% of the battery, so 7.2kwh equates to 6kwh useable.

I very much feel that if you are a low use household, then batteries are a no go.
If you only use 3kwh a day, I cant see how it would make sense.
My house uses around 20kwh a day.

Theres been only 2 days sofar that I've still had a bit of battery left (of the 6kwh) in the morning when the sun comes up.

And really, what would be nice is in the winter, having 20kwh useable of batts and charging the batts at e7 etc with 5p/kwh and using during the day, this would more than half my winter leccy use.

I'd say if you use less than 3mwh a year, you are gonna struggle to justify batteries.

But dont underestimate the satisfaction you get from not buying a single watt of electricity all day, even though the house has continued as normal, not going and switching things off, not staging loads apart.... making the system work around you.

Apologies, maybe I wasn't clear.

Our daily consumption is around 10kWh..... its the import that is only around 3-4kWh through the summer thanks to the Solar and trying run most things through the day.

Most of the time, its just baseload outside of Solar that we import.

Reed_Richards

Purely by way of comparison, I have a battery with a 6.5 kWh storage capacity. Between 10th March and 20th August I have imported 127.5 kWh, 0.78 kWh per day. Hitherto my rate of electricity consumption was about 8 kWh per day.

Although my battery was fully charged at 18:00 yesterday (when the PVs were generating 400W) it had got down to 60% by midnight and reached a low of 31% around 8:30 this morning. So in principle that's about 4.4 kWh of electricity I did not have to import. I would say this is fairly typical at the moment.

I would be self-sufficient in electricity (for at least much of the year) if it were not for the output limits of my battery (not enough to boil a kettle on its own) and the inertia in responding to a sudden increase in demand. So I don't need more battery storage capacity now - but winter remains to be seen.

joefizz

I bumped my battery up from 4.8kw to 9.6kw nominal for a couple of reasons. Firstly to enable the full 3kW output (mentioned above) for immersion use or use of different appliances at the same time as oven etc. Also mentioned above to spread the load across the batteries and lower the charging cycles. Plus it would also give me a bit of redundancy (one of the pylontech batteries could go offline and yet still give me 3kw output etc).


In an in depth discussion previously in this thread I mentioned that having the battery could increase usage or change of use (petrol lawn mower to electric, use of immersion instead of oil hot water, use of ASHP for heating instead of oil heating etc). None of these are battery related per se but the battery does give you that wee bit more choice and opportunity.



Importing 3kw per day probably isnt enough to justify buying a battery alone in current financial terms but that ignores opportunity costs, future costs vs maintenance etc etc. Most of us with batteries will still import something per day (figures from about 0.3kw to 1kw have been mentioned here) and indeed I import more when Im not at home on minimal load than I do when Im here working the battery to the fullest.


My previous long points in this thread were to try and open peoples minds to the fact that say a usuable 6kw battery isnt your usage plus 6kw but your usage plus how many times a day you can charge/discharge the battery. People dont think of that and I can understand its a difficult concept for people to understand unless you actually experience it.
Of course if you just think of it in current monetary terms then its a very simple calculation but even moderate price increases compounded over the life of a battery could put it a close run thing.



Its really more about how you use it rather than comparing what you do now unless you plan to not bother changing anything. Most of us changed our behaviour to reduce electricity usage down through the last decade or so, most of us then changed our time of day operations or usage when you get solar panels, the battery aspect will also allow a different change in behaviour.


One point I will make about increasing battery for winter. I think thats a non starter (certainly for me) as there will be long periods where the battery will be at 20% and stay there. With modular systems there is a case to argue to reduce the number of batteries online during the winter.
Effectively the size of battery increase will mean that it will take a longer time at the start of say December to discharge batteries to the minimum then the extra capacity will not be worth much again until the uptick in feb/march.
Where the increase does make a huge difference (above the available output level mentioned above) is in cases of increased usage and crappy weather during summer or if you do increase the load.
To give you an example of this Im currently measuring my ashp and immersion usage in terms of percentage of battery. So roughly 10% of battery is enough to run the immersion for a sinkfull of hot water and enough for a shower, similarly 10% of battery is enough to run the ashp for a couple of hours in the evening to bring the temp up or run a/c or run dehumidifier depending on conditions.

So run both and thats 20% of 9.6nominal. That would be 40% of the battery I had this time last year (if you ignore it wouldnt deliver the juice for the immersion). So effectively only leaving me around or below my normal evening usage.

Of course Ive only started doing both since I upped the battery so costs dont really apply to replacement power usage but it will probably save me at least 300 litres of oil this year. The price of that has varied over the last few years so ymmv on physical/actual costs.


Its one of those things you would probably need to see in action or go and talk to someone who actually has it installed. For me certainly its been more of a mindset thing rather than a straight figures on paper but ymmv.

Solarchaser

Most of that I agree with completely Joe, however I think you missed my point about increasing the batteries in winter.... it was so I could buy e7 electric overnight and use it during the day to more than half my electricity cost.
Not that I thought they would be charged by the solar.

1961Nick

Solarchaser wrote: »

Most of that I agree with completely Joe, however I think you missed my point about increasing the batteries in winter.... it was so I could buy e7 electric overnight and use it during the day to more than half my electricity cost.
Not that I thought they would be charged by the solar.

I’m moving to the Octopus Go tariff before winter so that I can charge my batteries @ 5p/kWh overnight. After factoring in the round trip loss, I should save about £90 Nov - Feb as we import at least 10kWh/day during that period. I might also go for a partial overnight charge in Oct & March as this has the potential to save another £20.

JKenH

It may not happen for a few years yet but as the take up of EVs and domestic batteries increases with attendant sophistication of charge management and time shifting there will presumably be more use of night time electricity and the cheap night rates may disappear.