On-grid domestic battery storage

JKenH

JKenH wrote: »

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.

I thought it worthwhile to check with Moixa about the charge and discharge rates and have been in touch with their Smart Battery Product Manager. This was his response.

“The 750W and 850W are the correct figures for the new battery. We are not actively selling the new battery yet so we will make sure to update the website before we start doing that.”


Thanks Mart for your comments on the discharge rates. Are you aware whether the discharge rate on the likes of the Powerwall are variable? Can it discharge at, say, 1kW and how rapid is the response to changing load. My old battery had a flat discharge rate and in the evening when I went to bed and the load dropped it would start to discharge the excess to the grid ( which incidentally the IBoost would then mop up). I am aware the new Moixa battery software has been upgraded to give a variable discharge rate and respond quicker to changing load but I wonder how many other batteries have similar issues.

Martyn1981

JKenH wrote: »

Thanks Mart for your comments on the discharge rates. Are you aware whether the discharge rate on the likes of the Powerwall are variable? Can it discharge at, say, 1kW and how rapid is the response to changing load. My old battery had a flat discharge rate and in the evening when I went to bed and the load dropped it would start to discharge the excess to the grid ( which incidentally the IBoost would then mop up). I am aware the new Moixa battery software has been upgraded to give a variable discharge rate and respond quicker to changing load but I wonder how many other batteries have similar issues.

I'm no expert on any of these, but basically the battery is monitoring import and trying to minimise it. Like a water heating diverter that's trying to minimise export, the batt does the same during the charging part. So, within the charge/discharge limits, the batt will try to minimise both import and export. So an entirely variable rate of discharge (within the batts limits).

I recall something from Tesla where they hope to get the accuracy down to a zero import/export, where the battery only discharges the exact amount needed (when it's charged of course).

As an example, let's say you are generating 2kW and consuming 1kW, then the batt will charge 1kW. But switch on a 3kW kettle (increasing total household demand to 4kW, and the batt will start discharging 2kW to avoid import (when added to the 2kW PV generation). Then generation rises to 2.5kW and the batt discharge drops to 1.5kW ..... etc etc. [Again, depending on the charge/discharge limits of the batt.]

zeupater

JKenH wrote: »

I thought it worthwhile to check with Moixa about the charge and discharge rates and have been in touch with their Smart Battery Product Manager. This was his response.

“The 750W and 850W are the correct figures for the new battery. We are not actively selling the new battery yet so we will make sure to update the website before we start doing that.”


Thanks Mart for your comments on the discharge rates. Are you aware whether the discharge rate on the likes of the Powerwall are variable? Can it discharge at, say, 1kW and how rapid is the response to changing load. My old battery had a flat discharge rate and in the evening when I went to bed and the load dropped it would start to discharge the excess to the grid ( which incidentally the IBoost would then mop up). I am aware the new Moixa battery software has been upgraded to give a variable discharge rate and respond quicker to changing load but I wonder how many other batteries have similar issues.

Hi

The model I'm using is based on there being a decent energy management ability which can react to changes in demand as quickly as a PV inverter reacts to energy power levels on the DC side, I wouldn't touch a system that had no EMS ... additionally the model is charge & discharge threshold based (as a battery system should be!) and demand following is assumed to be accurate ... to achieve this the system needs accurate load monitoring, which obviously excludes simple clamp based current monitors without associated continuous voltage monitoring!

Anyway, regarding the newer Moixa battery having larger charge(750W)/supply(850W) capacity, they're so small that they make little difference with your type of setup ... the +battery figure reducing from £687 to £670, almost insignificant.

As a direct comparison, if the 4800W system could charge at 3680W and discharge at 5000W (similar to PW2) then the bill reduces to ~£590 ... adding in the PW2 capacity (13.5kWh useable) reduces this to ~£465 and using the PW2 claimed 90% round trip efficiency further reduces this to ~£455 (a £215pa saving over the improved Moixa) ... adding a second PW2 battery pack (27kWh useable) only has the impact of reducing this to ~£429 a meagre £26/year additional saving, so going on a 200 year payback for the additional unit, proving that there's a sweet-spot for capacity beyond which you simply throw money down the drain!

With your high usage it's not the capacity of the batteries that make a difference, it's the ability to charge & discharge ... 6900kWh/ year basically resolves to a continuous power load of ~800W(6900/365/24) and that simply isn't how you'll be using the power, suggesting that you need both a serious ability to feed energy into the batteries during the day & a system capable of meeting your demand ... unfortunately even the 'improved' Moixa looks to fall short of what you need ....

However, having raised this the figures simply come back to what kind of payback you're looking at ... A PW2 may save you £400/year with your large PV system and high usage, but ~£6500 represents a ~16years return, whilst assuming a ~£4500 price tag for the 4800W Moixa the return would be closer to 25years (4500/180) .... all confirming that fully supplied & installed prices need to move significantly before storage makes any sense!


Food for thoughts ... ?

HTH
Z

zeupater

Martyn1981 wrote: »

... I recall something from Tesla where they hope to get the accuracy down to a zero import/export, where the battery only discharges the exact amount needed (when it's charged of course) ...

Hi

The model assumes this level of accuracy and additionally has a variable threshold before charge/discharge commences before honing in on zero differential. In absence of this level of detail being available in any specifications, the threshold is currently set to a reasonable/practical 10W at this stage as this would likely be a logical switching level used by manufacturers/installers.

HTH
Z

JKenH

zeupater wrote: »

Hi

The model I'm using is based on there being a decent energy management ability which can react to changes in demand as quickly as a PV inverter reacts to energy power levels on the DC side, I wouldn't touch a system that had no EMS ... additionally the model is charge & discharge threshold based (as a battery system should be!) and demand following is assumed to be accurate ... to achieve this the system needs accurate load monitoring, which obviously excludes simple clamp based current monitors without associated continuous voltage monitoring!

Anyway, regarding the newer Moixa battery having larger charge(750W)/supply(850W) capacity, they're so small that they make little difference with your type of setup ... the +battery figure reducing from £687 to £670, almost insignificant.

As a direct comparison, if the 4800W system could charge at 3680W and discharge at 5000W (similar to PW2) then the bill reduces to ~£590 ... adding in the PW2 capacity (13.5kWh useable) reduces this to ~£465 and using the PW2 claimed 90% round trip efficiency further reduces this to ~£455 (a £215pa saving over the improved Moixa) ... adding a second PW2 battery pack (27kWh useable) only has the impact of reducing this to ~£429 a meagre £26/year additional saving, so going on a 200 year payback for the additional unit, proving that there's a sweet-spot for capacity beyond which you simply throw money down the drain!

With your high usage it's not the capacity of the batteries that make a difference, it's the ability to charge & discharge ... 6900kWh/ year basically resolves to a continuous power load of ~800W(6900/365/24) and that simply isn't how you'll be using the power, suggesting that you need both a serious ability to feed energy into the batteries during the day & a system capable of meeting your demand ... unfortunately even the 'improved' Moixa looks to fall short of what you need ....

However, having raised this the figures simply come back to what kind of payback you're looking at ... A PW2 may save you £400/year with your large PV system and high usage, but ~£6500 represents a ~16years return, whilst assuming a ~£4500 price tag for the 4800W Moixa the return would be closer to 25years (4500/180) .... all confirming that fully supplied & installed prices need to move significantly before storage makes any sense!


Food for thoughts ... ?

HTH
Z

Thanks again.

I was a little surprised at the minimal difference the increased discharge rate makes but you may well be right. That is presumably because we have a high consumption and most of the year would use all the energy saved in the battery anyway. (Our baseload is 230w which is 5.5kwh/day of which 1.6 kWh (7/24) are covered by E7 leaving 3.9 kWh at the day rate which equates to the battery’s effective capacity.)

In the summer I thought the higher discharge rate might help. Solar PV would cover most of our non E7 base load leaving about 2-3 kwh in the battery for us to use for other things mainly cooking and boiling kettles or putting ASHP on in the evening. With a 850w discharge rate we would have 620 watts available after base load from the battery rather than 270 watts, i.e. 130% more which would really help us to use up that spare 2-3 kWh effectively while minimising what we take from the grid using our kettle, plasma tv and other appliances or even putting on ASHP on a cooler night.

I am not sure we would benefit tremendously from the very high discharge rate of the Powerwall.

A couple of the lifestyle changes we have made are to use a 1250w (actually draws about 1270w) insulated kettle and use our top oven which is 1400w as opposed to our main oven which is in the region of 3000w. While it still takes the same watt-hours to boil a kettle, using a lower wattage spreads the load over a longer period and allows a full boil on solar when we have about 1.5kw of PV (allowing for 0.23kw base load) rather than using a 3kw kettle and about 1.7kw being taken from the grid.

With a 500w battery discharge rate we would be drawing 1000w for 5mins from the grid but with an 850 watt battery only 650 watts for the same amount of time to boil our kettle. Similarly with the oven. We never put the cooker and kettle on at the same time unless there is spare PV and we would adopt a similar policy with the battery. As they are the main two heavy load items we use and rarely much after 6pm I would have hoped in summer we would use virtually no power from the grid other than E7.

While a Powerwall would meet all our needs in the summer we just wouldn’t be able to fill it for a large part of the year.

If we look at September, daily PVGIS estimate is 17.83 kWh. Some days we will make more, some days less. Our household consumption before we installed solar panels averaged 19 kWh of which baseload will be 5.5 kwh. In September half of that (2.75 kWh) will be covered by PV (daylight) and each day we typically put 6kwh into the IBoost. That leaves just over 9 kWh of solar to meet our other daytime usage boiling kettles, cooking, washing, watching daytime tv etc so I would be surprised if there was on average much more than 4-5 kWh available to go into the battery.

On average this September we used 2 kWh of E7 and 4 kWh at day rates). On a sunny day there might be more PV available to go into the battery but our demand from the battery would also be less and anything over 4kwh would just go towards displacing E7. On a rainy day our demand might be higher but there just wouldn’t be as much spare PV to go into the battery. The further we get into winter even less PV is available to go into the battery.

I think the Powerwall is just too big for most solar installations in the UK. In sunnier climes it might work. By comparison the Moixa is designed specifically for UK applications. It also has a lifetime warranty.

Neither, though, is suitable for my situation at the moment.

Bricroq

Hi,

I’m having a PW2 installed and want to move to an E7 tariff to enable battery charging overnight along with charging an Outlander PHEV. The problem I have is that my energy supplier (Utility Warehouse) say they only fit an E7 meter with a dedicated night time supply output (a fifth port on the meter). I want to be able to use any appliances I have overnight at the lower rate. Does anyone do this and if so with whom and/or how?

Thanks for any advice.
Bri

EricMears

Bricroq wrote: »

Hi,

I’m having a PW2 installed and want to move to an E7 tariff to enable battery charging overnight along with charging an Outlander PHEV. The problem I have is that my energy supplier (Utility Warehouse) say they only fit an E7 meter with a dedicated night time supply output (a fifth port on the meter). I want to be able to use any appliances I have overnight at the lower rate. Does anyone do this and if so with whom and/or how?

Thanks for any advice.
Bri

My present E7 meter was fitted in 1997 by what was then East Midlands Electricity Board and has always worked in such a way as to charge any electricity imported between 1am & 8am at the 'night time rate' and everything else at the 'normal' rate. For the ten years before that in my previous house the meter (also supplied by EMEB) worked in exactly the same way.

The practice of charging all electricity in the house at 'normal' rates all the time but only giving an off-peak discount for a particular circuit (usually storage heaters) was ISTR common in the 1960s but I thought it had disappeared many years ago.

But Utility Warehouse surely wouldn't be supplying the proposed E7 meter anyway - wouldn't that be the responsibility of the distribution company for your area. I strongly suspect that the UW minion who answered the phone was making it up as he went along ! Ring again and see if you get a brighter spark

JKenH

My E7 meter experience is similar to the last post. We have only one cable going through the meter to the distribution board which feeds all our circuits although the E7 period runs from 00.35 to 07.35. I understand that the switch over time is staggered and can be anytime between midnight and 01.00 to between 07.00 and 08.00.

When my meter was changed recently it was organised by my electricity supply company Scottish Power rather than the DNO.

thisreallyisme

Bricroq wrote: »

Hi,

I’m having a PW2 installed and want to move to an E7 tariff to enable battery charging overnight along with charging an Outlander PHEV. The problem I have is that my energy supplier (Utility Warehouse) say they only fit an E7 meter with a dedicated night time supply output (a fifth port on the meter). I want to be able to use any appliances I have overnight at the lower rate. Does anyone do this and if so with whom and/or how?

Thanks for any advice.
Bri

I'm having a PW2 installed, too, and have decided to go for a smart meter instead of E7 and that way I can choose any tariff. I gather E7 meters are being phased out so another reason to go smart I suppose.

I am going with Octopus and their Go tariff (designed really for EV users but not limited to them) which has a very low rate for 4 hours at night which I think will be long enough to fill my battery.They also offer a longer night rate tariff if I remember correctly.

Coastalwatch

Following on from Lightsource BP’s announcement of the signing of their PPA arrangement with Budweiser and Sonnen’s Virtual Power Plant supporting the grid in Germany has got me thinking about what would be the most useful(largest) size of battery that might accompany our system. As a minimum we should need 5 kWhs for daily overnight use in winter, then perhaps as a sensible maximum it could be as much as our system is capable of generating on any given day in the summer. This last year saw several days when output surpassed 60 kWhs. Given that we are made an allowance for the estimated 50% donated to the grid that we don’t use then it could be argued that a battery of around 30 kWh’s might not be a bad compromise.
At current storage costs, inconceivable, but in the fullness of time and with the benefit of grid smoothing payments(V2G etc) would it be out of the question?
I have to confess to hankering after larger rather than smaller as in winter time there is often two, three or more days without any measurable sunshine. So with a minimum 7 kWh’s daily required(not including hot water) then I can see 30 kWh’s disappearing quite quickly!


Can anyone make a case for going even larger?