On-grid domestic battery storage

orrery

Exiled_Tyke wrote: »

....And of course extending further into the future really eats into any meaningful NPV calculation.

... with the risk that batteries will come down at some time during that period and new purchasers will gain payback before you do! That happened with solar PV - two people who had it before me were still waiting to get to payback long after I had.

zeupater

Hi All

4800Wh battery
700W Charge
500W Supply

3300kWh/year demand
4000Wp Array
820kWh/kWp

No Constraints (Storage or Performance)


Std - £590
PV - £450
Stg - £336
E7 - £378
E7(+Smart/AI charging) - £356

Note Using Storage/E7 combination adds to purchases to meet demand!

HTH
Z

Martyn1981

Exiled_Tyke wrote: »

Then after 10 years the system has paid back a staggering £1,200 so a wonderful investment.

Great to see that you and Z both came to almost exactly the same figure.

It could be that we are all wrong, but I suspect we have a reasonable handle on the economics now. But I still want a batt ..............

zeupater

Martyn1981 wrote: »

Great to see that you and Z both came to almost exactly the same figure.

It could be that we are all wrong, but I suspect we have a reasonable handle on the economics now. But I still want a batt ..............

Hi

However, Z's results from approximately 6million individual calculations (at this level alone!) embedded into around 2million logic branches ....

What is particularly noticeable is the effect of such poor charging & supply when operated with solar PV, effectively the solution seems to have been designed to charge on E7 overnight at 700W (7x700=4900W) and to discharge either constantly at ~280W (4800/(24-7)) to cover baseload or to supply a 500W afternoon/evening load for ~9.5hrs(4800/500), both with round-trip losses excluded ....


Anyway, the figures show that a typical household with 4kWp of PV could reduce their imports by ~£114/year(450-336) if the system only soaks-up spare PV, whilst that is reduced to ~£72(450-378) if the battery is topped up by E7 overnight ... note, this assumes an average 92.5% efficiency on charging & 92.5% efficiency on the combination of storage(standing losses & inverter), so 85% round-trip.

The savings are particularly bad because of the inability of this particular system to support 500W+ loads, for example in a household with an existing 130W load (say TV+light+baseload) when switching on a 2700W kettle only ~14%((500-130)/2700) could be supplied from the battery ...

The above is mirrored by the poor charging rate hindering the ability to soak-up excess PV during the day, for example, a 4kWp system generating ~3.2kW with a 130W household load would still be exporting 74%((3200-130-700)/3200) of the PV generation as opposed to storing it for later!


In all, a poor match ...



HTH
Z

Coastalwatch

Martyn1981 wrote: »

Great to see that you and Z both came to almost exactly the same figure.
It could be that we are all wrong, but I suspect we have a reasonable handle on the economics now. But I still want a batt ..............

.........and so do I. But like you, with the current costs and the disappointing charge/discharge rates mentioned here it's going to be a while yet before I take the plunge.

Presumably a sizeable chunk of the cost is for the system itself, not to mention transport and installation. So just how much of the remaining cost accounts for the battery...50%...40%...30%...20%!
One would have thought then that a larger battery would offer better value per kWh, not to mention more useful charge and discharge rates. With electric showers consuming circa 9kW's then I wonder if a system and battery shouldn't be capable of withstanding this level of discharge. Having said that I've no idea what minimum size of battery might be required to cope with this!

JKenH

zeupater wrote: »

Hi All

4800Wh battery
700W Charge
500W Supply

3300kWh/year demand
4000Wp Array
820kWh/kWp

No Constraints (Storage or Performance)


Std - £590
PV - £450
Stg - £336
E7 - £378
E7(+Smart/AI charging) - £356

Note Using Storage/E7 combination adds to purchases to meet demand!

HTH
Z

I was quite interested to see the charge/discharge rates quoted. Which battery was this? I had been told that the Moixa 4.8kwh battery charged at up to 750w and was capable of discharging at up to 850w.

I presume the figures don’t take into account any Gridshare payment?

I would be interested to see how the figures worked out for someone with a larger array and higher demand. I know a battery doesn’t work financially for me as my 7.8kWp system is hampered by its orientation and I have an IBoost but with my pre solar consumption of 6900kwh pa I wonder (with no IBoost) what the ROI might be if that 7.8 kwp faced south? I do have a higher than average baseload of around 230w.

I should add that I happen to be retired and looked at solar power/battery storage as an investment alternative to an annuity which perhaps makes me more tolerant of a lower (albeit index linked) ROI.

zeupater

JKenH wrote: »

I was quite interested to see the charge/discharge rates quoted. Which battery was this? I had been told that the Moixa 4.8kwh battery charged at up to 750w and was capable of discharging at up to 850w.

I presume the figures don’t take into account any Gridshare payment?

I would be interested to see how the figures worked out for someone with a larger array and higher demand. I know a battery doesn’t work financially for me as my 7.8kWp system is hampered by its orientation and I have an IBoost but with my pre solar consumption of 6900kwh pa I wonder (with no IBoost) what the ROI might be if that 7.8 kwp faced south? I do have a higher than average baseload of around 230w.

I should add that I happen to be retired and looked at solar power/battery storage as an investment alternative to an annuity which perhaps makes me more tolerant of a lower (albeit index linked) ROI.

Hi

Charge/discharge specs are from 4800Wh version on Moixa site ... https://www.moixa.com/solar-battery/features-benefits/ ...

Power Inputs -
Mains AC . Fused input (3A) between 85 – 263V AC / 120 – 373V DC PV panel inputs, standard MC4 PV connections, up to 15A, max PPT 35V rated panel
1 x PSU input, rated at max 700W

Power Output -
1 x internal connector to micro-inverter, rated at 17A DC
1 x AC output (micro-inverter version), rated at max 500W

Regarding Gridshare, if you look at the standard payment it's a simple £50pa ... the issue is that whatever's in the battery when they call on it then they've used it so it's no longer there for you to use when you may need it ....

Looking further into the Moixa site I've just found a table describing the savings from the battery when added to an existing PV array as being £115/year ... https://www.moixa.com/compare/ ... which is surprisingly close to the calculated £114 .... however, they also show a £50 Gridshare income which is a little perplexing as any use of the system for Gridshare would deplete the battery and reduce the household available savings - in effect the £115 must logically be reduced in order to gain access to the £50, conceivably this could be by more than £50 if the scheme deprives the household of just under 1kWh of self consumption/day (£50/0.15/365) on average ...

Note, Moixa's analysis shows the billing excluding standing charges & therefore the figures do not reflect what they say, they are the value of units consumed, not a representation of the value of the bills received which has a considerable impact on the percentage savings as calculated & displayed - they're simply wrong! ... I also particularly dislike the attention paid to the proof-reading prior to publication of the table - there is no excuse for percentages to be shown as £!

Okay, go one further on this table ... just think of the 75% consumption column where 3700kWh/year is generated with 2775kWh used directly, leaving 925kWh to charge the battery ... in this case the grid import would logically be 1226kWh(4000-2774) - after battery the claim is that this would reduce to 433kWh, suggesting 793kWh(1226-433) diverted to the battery which is where the £115(793/145) saving is derived ... yet there's no appreciation of round trip efficiency or (more importantly) the generation profile of PV ... a PV system where there's already 75% in house consumption has a particularly high load which effectively means that the battery is unlikely to receive any charge in conditions other than very bright conditions - what would be needed here is the ability to soak-up as much excess generation as possible when these conditions exist because they're relatively rare, yet the charge is limited to 700W ... this raises the basic question of are there enough hours of excess generation available to divert 793kWh/year with such restrictions ?

HTH
Z

zeupater

JKenH wrote: »

... I would be interested to see how the figures worked out for someone with a larger array and higher demand. I know a battery doesn’t work financially for me as my 7.8kWp system is hampered by its orientation and I have an IBoost but with my pre solar consumption of 6900kwh pa I wonder (with no IBoost) what the ROI might be if that 7.8 kwp faced south? I do have a higher than average baseload of around 230w ..

Hi.

Modelled as :-

7.8kWp array / 820kWh/kWp
5kW limit (orientation/Inverter)

4800Wh battery as previous

6945kWh/year demand (3300kWh pattern model adjusted)

Daytime rates(No E7) - Approx (!)
Base - £1137
+PV - £850
+Battery - £687


How does the expected PV saving (£287/year) stack up against your expectation? ... anyway, with large array & large demand the revised estimate is ~£163/year saving on addition of battery. (although the accuracy of this figure could be improved by developing a dedicated high consumption profile & model!)

HTH
Z

JKenH

zeupater wrote: »

Hi.

Modelled as :-

7.8kWp array / 820kWh/kWp
5kW limit (orientation/Inverter)

4800Wh battery as previous

6945kWh/year demand (3300kWh pattern model adjusted)

Daytime rates(No E7) - Approx (!)
Base - £1137
+PV - £850
+Battery - £687


How does the expected PV saving (£287/year) stack up against your expectation? ... anyway, with large array & large demand the revised estimate is ~£163/year saving on addition of battery. (although the accuracy of this figure could be improved by developing a dedicated high consumption profile & model!)

HTH
Z

Thanks Z for working that through.

It is hard to say just what the PV saving will be for me as I have had the system for less than 6 months During the first three months I had a number of issues with outages which were eventually resolved by changing the inverters. I think I missed the best of the summer sunshine and have found as well that, October excepted, I have had a rough run of luck with the weather and am struggling to achieve PVGIS estimates. All other things being equal though, I would anticipate that your estimate of savings from PV is not far off.

However, I should add that since installing the panels I have become much more conscious of what energy we are using and we have made a number of lifestyle changes to ensure we make the best use of what PV is available. Last year our average daily consumption was 19 kWh and I am pleased to be able to say that we have only exceeded that once since installing the panels and that was when we had a house full of visitors. I will be very disappointed if we don’t reduce our annual consumption by at least 2000 kWh, hopefully more. A large part of the grid energy displaced to date however has been on E7 tariff, probably in the region of 5 kWh per day when that was available and that will increase in the summer as and when I succeed in getting energy diverted to our second hot water cylinder which is currently heated by our oil-fired boiler. So we will save on oil as well.

In this gloomy December weather we are averaging 10 kWh day usage and 6 kWh E7. In September we were using typically 6 kWh (4day/2night) on average but we had the 3kwh battery then (contributing an average of 1 kWh). Prior to September we had an old style meter which span backwards so 0 daytime and 2kwh E7.

I was hoping with our lifestyle changes we could get our average consumption to nearer 10 kWh per day which would project a saving of more than 3000 kWh pa but 12 kWh per day would save us 2500 kWh per year. I would suggest therefore savings of £230 to £300 per year are on the cards.

I would stress though that, other than E7 displacement and daytime base load savings, most of what we do save is because we are far more conscious about how and when we use our energy.

(Off topic, I know, but because we shower in the morning, putting 5 or 6 kWh into the tank during the day using the IBoost is probably only displacing 3 or 4 kWh of E7 with the other 2 kWh lost to cooling overnight. I think we will gain more benefit in terms of overall energy savings in the winter if we use ASHP in preference to the IBoost and let E7 take care of the hot water.)

Those discharge rates quoted for the 4.8 kWh battery are disappointing. I had not had a look at the specs on the Moixa website and was going from the figures quoted when I was invited to trial the new battery. I declined the trial because I didn’t think my installation (east/west and IBoost) would work with it very successfully. I did genuinely believe though with the higher rates I was quoted it would suit some installations and lifestyles.

Do other batteries designed for solar installations have similar constraints? I was told variable discharge rates were difficult to achieve because of how inverters work but I have no electrical knowledge so don’t know whether there is any truth in that. No doubt you or someone else on this forum can comment.

Martyn1981

JKenH wrote: »

Do other batteries designed for solar installations have similar constraints? I was told variable discharge rates were difficult to achieve because of how inverters work but I have no electrical knowledge so don’t know whether there is any truth in that. No doubt you or someone else on this forum can comment.

They all tend to vary. In some cases increasing the battery units, such as the offerings from the company we were 'chatting' with recently mean an uplift in the charge and discharge rate each time.

The Tesla Powerwall II previously stated a 5kW charge and discharge, with a 7kW brief discharge rate (30 secs I seem to recall), though a quick trawl today suggests it might be 3.68kW/5kW. This may reflect UK legislation on export limiting output, but I'm not sure, however, I'd suggest anything approaching 2-3kW (equal to a powerful kettle, or the heating cycle for dishwasher, washing machine etc), is probably enough to cope with almost all sensible demand at any given point in time.

Off the top of my head, I recall the Enphase offering was about 300W, but these were 1.2kWh units with a micro-inverter in each, so stacking them together, resulted in multiples of 300W, so 5x 1.2kWh = 6kWh = 1.5kW charge/discharge (etc).

Basically we have a multitude of different offerings, all with different prices, different life cycles, different warranties, different charge rates, different discharge rates ........... at least it's nice and simple for us.