On-grid domestic battery storage

1961Nick

zeupater wrote: »

Hi

Long term maybe, but you're also bringing the battery into play ... you can (for example) either measure consumption directly by having the battery & PV connected to a separate consumer unit and then accurately metering all energy flow into the main home consumer unit, or you can accept that the variance in battery charge levels creates inaccuracy & uncertainty within the methodology you're looking to adopt and this increases as the time between readings decreases due to the effect of charge variability, for example (excluding efficiencies etc) ...

10kWh(TGM)+5kWh(Metered)-3kWh(Unidirectional clamp Monitored Export) suggests household consumption of 12kWh ....

Add a battery system ...

If the above is applied to battery system which is full the day before the same (/similar) result may apply ... however, if the battery was depleted a proportion of the TGM reading would have been diverted as charge ... 10kWh(TGM)+5kWh(Metered)-3kWh(Export)-5kWh(change in charge status) suggesting 7kWh household consumption ...

You have to ether measure the battery input/output accurately (which you are already looking to do), or measure & include the opening & closing battery charge levels into your calculation .... your post contents so far suggest that it's not the case, but can you confirm you are not doing this ....

Another consideration regarding your clamp based monitor .... does it continually monitor the mains voltage for variance and apply that to the energy it records or is the voltage a selectable variable? - mains voltage changes related to local grid demand & can vary by around (/up to) 5% over a day, which is normally okay if you're simply monitoring spot power, but can cause significant errors in metering energy ... furthermore, how does your clamp monitor handle meter side power factor - many appliances have a considerable affect on the apparent:real power ratio, for example, our microwave on standby consumes 2.5W, however in having a PF of 0.05 the apparent power (as measured by a clamp sensor) would register this as 50W, a potential metering error of over 1kWh/day caused by just one appliance! ... and yes, because of the effect on our clamp-based monitor readings (that's power monitor, not energy meter!), we do turn the microwave off at the wall ...

Regarding the battery system inefficiencies ... yes, it does make a considerable difference to the economics of using E7, that's effectively what I've been trying to convey for some time ... in moving to E7 there are two main considerations ... the usual increase in daytime tariff rates & the efficiency loss through the battery on the cheaper night-time rate ... if the losses are 18%, then you're effectively paying the equivalent of 22% (100/0.82) more per kWh used than the actual billed rate, which makes quite a difference to battery justification economics based on E7 or any other form of demand based reduced tariff periods ...


I really am not trying to sound like someone that is anti-batteries, however, you must appreciate that when attempting to discuss the relevance of the variables which must be considered if looking to cost-justify a purchase, a realistic approach which may not at first seem to conform to the initial understanding of others may come across as negativity ... I can assure you, it's simply conveying the realism that marketing brochures & sales teams tend to skip over in the narrative (but often publish somewhere in technical specification tables) as well as helping to steer the though processes required to assess likely difference between maximum performance specifications & real world operation, for example, maximum charge efficiencies & the real-world impact of charge power on battery resistance energy losses as heat ...

HTH
Z

If we can go back to my original assertion that was that "batteries change behaviour in the same way that panels do when you first install them".

I backed this up with 5.9% & 9.8% reductions in household consumption during February & March.

My batteries had fully discharged before midnight on the 31st Jan & the 28th Feb. They were holding a charge on the 31st March but I decided not to correct for that because a) it would have been a guess & b) my point was adequately made without adjusting the raw figure for March upward.

I could also have corrected for battery losses to make a fairer comparison as these are not 'House consumption'. However, there was no need to in the context of the point I was making.

FYI, if I exclude round trip losses (18%) & assume a 4kWh carry over from March to April, the reduction in household energy consumption during March goes from 9.8% to 15.4%.

I get you point about "conveying realism & I fully endorse that, but all I am trying to do is demonstrate that there are things on the other side of the fence that require consideration when trying to "cost - justify a purchase".

It is my opinion that this change in behaviour will have an effect, and in some cases a very significant effect, on the overall energy usage of anyone adding battery storage to their PV system.

Off track & irrelevant but my import figures for March......

2015 - 571kWh
2016 - 509kWh
2017 - 462kWh
2018 - 568kWh
2019 - 268kWh

The import reduction for March was much greater than I expected, even after accounting for high solar output, and that's what made me delve deeper into what was happening.

zeupater

1961Nick wrote: »

If we can go back to my original assertion that was that "batteries change behaviour in the same way that panels do when you first install them".

I backed this up with 5.9% & 9.8% reductions in household consumption during February & March.

My batteries had fully discharged before midnight on the 31st Jan & the 28th Feb. They were holding a charge on the 31st March but I decided not to correct for that because a) it would have been a guess & b) my point was adequately made without adjusting the raw figure for March upward.

I could also have corrected for battery losses to make a fairer comparison as these are not 'House consumption'. However, there was no need to in the context of the point I was making.

FYI, if I exclude round trip losses (18%) & assume a 4kWh carry over from March to April, the reduction in household energy consumption during March goes from 9.8% to 15.4%.

I get you point about "conveying realism & I fully endorse that, but all I am trying to do is demonstrate that there are things on the other side of the fence that require consideration when trying to "cost - justify a purchase".

It is my opinion that this change in behaviour will have an effect, and in some cases a very significant effect, on the overall energy usage of anyone adding battery storage to their PV system.

Off track & irrelevant but my import figures for March......

2015 - 571kWh
2016 - 509kWh
2017 - 462kWh
2018 - 568kWh
2019 - 268kWh

The import reduction for March was much greater than I expected, even after accounting for high solar output, and that's what made me delve deeper into what was happening.

Hi

I agree that it's good to reduce consumption through both improving energy efficiency and behaviour modification ... however, as both can be achieved without resorting to installing solar PV or batteries it's largely a self justification issue ... in the early days of PV on these boards & elsewhere you'd be surprised how many posts attributed savings to solar PV which could easily have been achieved by installing an energy monitor and I would consider it a crying-shame if we went back to the 'bad old days' dodgy double glazing salesman like practices as the market for batteries starts to heat-up.

In this I'm not including things that should be considered valid, such as running high loads when generation is high or moving from parallel high load use to sequential, it's the idea of adding later efficiency measures such as A rated appliances, LED lighting & TVs into the mix & claiming it's related to solar, which it's not as it could have been done anyway ...

As a direct comparison to act as an example ... here are our March energy imports without a battery system to compare to your own ...

2015 - 571kWh - 92
2016 - 509kWh - 88
2017 - 462kWh - 103
2018 - 568kWh - 128
2019 - 268kWh - 107

... with the last time we had a reading over 400kWh being back in 2007!

Note from both sets of figures the difference between 2018 & 2017 ... yours +23% against ours at +24% .... that describes a weather & PV related anomaly which will heavily skew any savings attributed to the 2018 to 2019 reduction in your figures so maybe attributing a 53% import reduction to batteries should be held back for a while as we achieved 16% by doing nothing different ... whatever the case, it looks like there's plenty of room for electricity efficiency improvements yet ...

As an aside, but still ambient condition related, our gas usage this March was 63% down on 2018, but over 7x higher than 2017 ... but then again, our 12 month running total to the end of last month isn't much above 700kWh!


HTH
Z

1961Nick

zeupater wrote: »

Hi

I agree that it's good to reduce consumption through both improving energy efficiency and behaviour modification ... however, as both can be achieved without resorting to installing solar PV or batteries it's largely a self justification issue ... in the early days of PV on these boards & elsewhere you'd be surprised how many posts attributed savings to solar PV which could easily have been achieved by installing an energy monitor and I would consider it a crying-shame if we went back to the 'bad old days' dodgy double glazing salesman like practices as the market for batteries starts to heat-up.

In this I'm not including things that should be considered valid, such as running high loads when generation is high or moving from parallel high load use to sequential, it's the idea of adding later efficiency measures such as A rated appliances, LED lighting & TVs into the mix & claiming it's related to solar, which it's not as it could have been done anyway ...

As a direct comparison to act as an example ... here are our March energy imports without a battery system to compare to your own ...

2015 - 571kWh - 92
2016 - 509kWh - 88
2017 - 462kWh - 103
2018 - 568kWh - 128
2019 - 268kWh - 107

... with the last time we had a reading over 400kWh being back in 2007!

Note from both sets of figures the difference between 2018 & 2017 ... yours +23% against ours at +24% .... that describes a weather & PV related anomaly which will heavily skew any savings attributed to the 2018 to 2019 reduction in your figures so maybe attributing a 53% import reduction to batteries should be held back for a while as we achieved 16% by doing nothing different ... whatever the case, it looks like there's plenty of room for electricity efficiency improvements yet ...

As an aside, but still ambient condition related, our gas usage this March was 63% down on 2018, but over 7x higher than 2017 ... but then again, our 12 month running total to the end of last month isn't much above 700kWh!


HTH
Z

I posted those import figures as an aside because they are meaningless without knowing what the solar consumption was for those months.

I know our energy import is still very high, but you should have seen it before Solar PV, batteries, LED bulbs, ASHP, A** appliances etc. At today's prices we would have been paying over £3000 pa for electricity alone!

With your minuscule import, it'll be a very long time before you can make an economic case for batteries.

Not having to bother about sequential running of high loads is quite nice though.;)

zeupater

1961Nick wrote: »

I posted those import figures as an aside because they are meaningless without knowing what the solar consumption was for those months.

I know our energy import is still very high, but you should have seen it before Solar PV, batteries, LED bulbs, ASHP, A** appliances etc. At today's prices we would have been paying over £3000 pa for electricity alone!

With your minuscule import, it'll be a very long time before you can make an economic case for batteries.

Not having to bother about sequential running of high loads is quite nice though.;)

Hi

Understand, but the reason I mentioned it was to convey that March 2018 probably wouldn't be a good energy baseline to measure energy consumption reduction (with batteries) against as it was relatively high ...

£3000 for electricity would represent around 20MWh, around 50kWh per day or an average demand of well over 2kW .... that is high if it doesn't include heating or running loads such as a bank of servers or a small business .... we used to consume over 5MWh of electricity per year a couple of decades ago (which I consider high!) but finally convinced MrsZ to accept that turning at least some of the lights in the house off as well as not needing to use the dishwasher multiple times with just a few mugs & plates on some days (etc ...) made quite a difference, and all that was before efficient appliances & solar ... from memory, by the time we started to look at solar we were averaging around 2600-2800kWh/year and have made further inroads into that since ...

The reduction in gas usage is far greater ... we used to use in excess of 40MWh, but as highlighted in the previous post, that's also been subjected to considerable incremental improvements ...

Maybe our 'miniscule' imports are considered unbelievably low by some, but if you think about the average UK electricity consumption as 3300kWh, add in some efficiency improvements and tweak habitual usage this can realistically fall to around 2500kWh, then stir in around 4kWp of solar PV and you get to approx 1500kWh of grid imports or even below ... there are a couple of posters on these boards with solar PV that use roughly the same as us and quite a few more that use around the 1500kWh mark, so it can't be considered impossible or particularly atypical, it just takes a little time & effort in the form of incremental improvements ...

HTH
Z

mmmmikey

1961Nick wrote: »

With your minuscule import, it'll be a very long time before you can make an economic case for batteries.

Been following recent exchanges (re: round trip efficiency) with interest - thanks all.


I think Nick's point above about miniscule import is crucial - not just at Z's "import to be proud of" level, but to many others as well. The economic case for batteries is always going to be fundamentally limited by the capacity of the batteries or the amount of energy you can use from them or your ability to charge them.


If you take the example of a household using say 3000kWh per annum with for example a 4kW array generating 3000kWh per annum of which half is exported, you're left with import of 1500kWh per annum which puts a ceiling on your saving of an average of 4kWh-ish per day. But in the summer months in most cases you'll be importing less (both because the panels are satisfying more of your demand and your demand is lower) and in the depths of winter you won't be generating enough to charge the battery.


So how do you size the battery?


If you size the battery to satisfy your average daily demand then it's going to be significantly under-utilised in summer and winter. To some extent this is mitigated by longer battery life (as measured in years rather than cycles) but then you're pushing yourself well beyond the manufacturers warranty to see a return. For the economics to work the cells need to be inexpensive enough to have them sitting round under-utilised for a significant proportion of the time, and we're certainly not there yet.



If you size the battery system to ensure it's fully utilised in or close to the warranty period, then it's likely to be disproportionally expensive (i.e. a 2kWh battery system costs a lot more than half the cost of a 4kWh battery system because the inverter/charger and installation elements are not dissimilar).


So I just can't see this stacking up as a technology to be deployed alongside domestic solar panels for a long time, if ever, fo an awful lot of people.

.On a more positive note, if you can sell some of that under-utilised capacity via a grid-services contract, or if you take solar panels out of the equation and use the batteries to ensure you only ever buy energy at the lowest rate on a time-of-use tarriff then the economics change significantly. We're not really there yet with either of these mitigations (unless E7 works for you) but my crystal ball is telling me that these are on the near horizon.


My own prediction is that batteries will never be cost-effective for Z and many other solar panel owners but the growth in battery deployment will come through some innovatively funded scheme that sees them deployed in households without solar to take advantage of off-peak rates.

mmmmikey

zeupater wrote: »

Maybe our 'miniscule' imports are considered unbelievably low by some, but if you think about the average UK electricity consumption as 3300kWh, add in some efficiency improvements and tweak habitual usage this can realistically fall to around 2500kWh, then stir in around 4kWp of solar PV and you get to approx 1500kWh of grid imports or even below ... there are a couple of posters on these boards with solar PV that use roughly the same as us and quite a few more that use around the 1500kWh mark, so it can't be considered impossible or particularly atypical, it just takes a little time & effort in the form of incremental improvements ...

HTH
Z

...we crossed posts. I agree with this, I don't consider these levels to be miraculous or unattainable, if I take out my heating load I'm in the same ball park as Z.

zeupater

mmmmikey wrote: »

... My own prediction is that batteries will never be cost-effective for Z and many other solar panel owners but the growth in battery deployment will come through some innovatively funded scheme that sees them deployed in households without solar to take advantage of off-peak rates.

Hi

Good post with reasoning that pretty much reflects my thoughts over the last few years.

I think that the cost effectiveness for us changes if we seek to move more heat load from the log burner to extending heat-pump run times or maybe a second as originally planned, but as you've identified, we're at an energy efficiency level where almost whatever we do has diminished returns ... where we were burning a full load of logs each year, the last delivery has lasted 3 full winters & we've still got a stock just short of what we've used this last season left ...

My own thoughts are that when a system comes in at around £150/kWh there'll be a significant change in uptake and that represents a reasonable target price considering the predicted cell cost movements over the next few years (as detailed a few days ago) ... in which case something like an 8-10kWh system would probably suit our usage pattern & just about break-even with savings on imported energy costs within it's design lifespan .... we'll see in time, but just following developments at the moment ...

HTH
Z

1961Nick

mmmmikey wrote: »

Been following recent exchanges (re: round trip efficiency) with interest - thanks all.


I think Nick's point above about miniscule import is crucial - not just at Z's "import to be proud of" level, but to many others as well. The economic case for batteries is always going to be fundamentally limited by the capacity of the batteries or the amount of energy you can use from them or your ability to charge them.


If you take the example of a household using say 3000kWh per annum with for example a 4kW array generating 3000kWh per annum of which half is exported, you're left with import of 1500kWh per annum which puts a ceiling on your saving of an average of 4kWh-ish per day. But in the summer months in most cases you'll be importing less (both because the panels are satisfying more of your demand and your demand is lower) and in the depths of winter you won't be generating enough to charge the battery.


So how do you size the battery?


If you size the battery to satisfy your average daily demand then it's going to be significantly under-utilised in summer and winter. To some extent this is mitigated by longer battery life (as measured in years rather than cycles) but then you're pushing yourself well beyond the manufacturers warranty to see a return. For the economics to work the cells need to be inexpensive enough to have them sitting round under-utilised for a significant proportion of the time, and we're certainly not there yet.



If you size the battery system to ensure it's fully utilised in or close to the warranty period, then it's likely to be disproportionally expensive (i.e. a 2kWh battery system costs a lot more than half the cost of a 4kWh battery system because the inverter/charger and installation elements are not dissimilar).


So I just can't see this stacking up as a technology to be deployed alongside domestic solar panels for a long time, if ever, fo an awful lot of people.

.On a more positive note, if you can sell some of that under-utilised capacity via a grid-services contract, or if you take solar panels out of the equation and use the batteries to ensure you only ever buy energy at the lowest rate on a time-of-use tarriff then the economics change significantly. We're not really there yet with either of these mitigations (unless E7 works for you) but my crystal ball is telling me that these are on the near horizon.


My own prediction is that batteries will never be cost-effective for Z and many other solar panel owners but the growth in battery deployment will come through some innovatively funded scheme that sees them deployed in households without solar to take advantage of off-peak rates.

As I've said before, the economic case for having batteries is marginal in my case, but it had reached the extremities of the ball park.

The figures for last year were:

Import 5768 kWh
Generation 3926 kWh
Solar use 997 kWh

So in my case there is plenty of solar generation being exported & plenty of scope for deploying it to the household via batteries instead.

The removal of the 'use it or lose it' mindset is an additional gain I hadn't anticipated. The potential size of the round trip loss is higher than I expected, but I'm still hopeful that it's lower than the 18% Z suggested. Taking these 2 things together, I'm probably all square with the original calculation - or hopefully slightly ahead.

Based on the March numbers, I should be able to discharge 2150 kWh pa which will mean break even in year 14. I've assumed RPI at 2.5% & ignored the fact that I may have to add another battery to the stack to compensate for degradation.

The batteries have been running for 66 days & have discharged 340 kWh ~ 365/66 x 340 = 1880 kWh. Based on that, 2150 kWh for the full year looks achievable with the best months to come.

Martyn1981

I was going to quote somebody, but too many people and posts that cover what I have to say, so I agree with all.

Regarding consumption - Our import was 3,000kWh pre-PV. It then dropped to around 1,500kWh post-PV, and I assumed a consumption of 3,000kWh still due to a switch from gas hob to induction balancing out steady switch of CFL to LED's and LED TV.

But, given that it's now running at 3,000kWh pa for the last 2yrs (I can now measure export accurately) and during that same period we had the small ASHP installed, it must have been a bit lower.

Regarding economics of battery - with a leccy import of 1,500kWh, and an export of around 3,000kWh, a 4kWh battery would let us save 900-1,000kWh of import (not enough PV gen in the bottom 4 months). So that's roughly our cap, around 1,000kWh, let's say £150, or perhaps £200 if combined with E7 as the higher day rate would not be an issue as the bulk of the import would be nightime.

£200 pa, is a total be-atch. Not enough to justify a battery, and not enough to rule one out, completely.

But, if we (as planned) add 4 panels of steep south facing PV at the same time as a battery (60% increase in Dec generation), and get an EV (eventually) to mop up summer excess, then things improve, but a larger battery is then needed, probably 10-15kWh to allow for 5-10kWh of almost daily EV charging.


As this thread is showing, there's an awful lot to think about, but so long as we are rational and sensible, none of it is complicated, just a new learning curve, and a fun hobby.

Thanks to all for the detailed input.

mmmmikey

1961Nick wrote: »

As I've said before, the economic case for having batteries is marginal in my case, but it had reached the extremities of the ball park.

Yes - same for me. In my case the 2 things that give batteries a fighting chance are being on an E7 tarriff anyway so being able to take advantage of a 16p difference between peak and off peak rates year round, and the availability of a grid services contract to create additional income. To be honest the grid services contract is hugely speculative with doubtful economics, but the fun-factor of playing with the batteries makes it worth the risk.