Best battery set-up for large(ish) solar array but with low export limit set by DNO?

zeupater

Newbie_John said:

.... Even DIY for £2500 means that each your solar charged kWh costs 5p more..

Hi

..... and each imported low cost overnight kWh used during the day in autumn/winter/spring (including 17kW.t heat pump!) costs how much less than standard daytime tariffs ???? ....

The issue here is simply one of joined-up thinking .... we all know the difference between summer & winter solar generation, however many still fail to understand that solutions are based on the whole picture and that concentrating on component strengths & weaknesses is effectively illogical ....

On a similar thread of thought, the OP would logically be ill advised to match any potential battery size to cover mid-winter max rated operation of the HP (probably ~70kWh) whilst not allowing for overnight temperature set back & average temperatures etc ... as a pure guess (not knowing heat-loss etc!), the maximum battery size in order to cope with most conditions would be ~30kWh, with going down to ~15kWh (or below) requiring an acceptance that there would still need to be considerable reliance on mains daytime tariff import during the darker months. The issue then becomes an annual financial balance of what the rate of return is as the storage capacity incrementally increases above ~15kWh (/30kWh etc ... )  

HTH - Z

ed110220

Newbie_John said:

QrizB said:

Newbie_John said:

£5k battery with 10kWh makes every kWh 10p more expensive

I don't think batteries have been £500 a kWh any time this decade. Back in 2020, Pylontech 3kW batts were around £1200, if memory serves?

ed110220 said:

OK I realise it's not for everyone because it was a DIY kit, but I have a Seplos 15 kWh battery.

The Fogstar DIY kits are around £100/kWh, but even pre-built they're only £200/kWh. By the same logic, if a £500/kWh battery adds 10p to each kWh of electricity you cycle through it, those Fogstar ones add 2p and 4p respectively.

Sure DIY projects are cheaper but how many of us will do that? Less than 5%. The 95% majority will give a call to multiple installers and get quotes around £5k.
Meaning +10p cost of each charged kWh.
Even DIY for £2500 means that each your solar charged kWh costs 5p more..

I'd say your assumptions in the calculation are off. That type of battery is stated to be 8000 cycles, but that is to 70% state of health (ie after 8000 cycles it will hold 70% of the original capacity), not that it will be dead or essentially useless after 8000 cycles. But lets stick with 8000 cycles for simplicity and to be conservative, for a 14 kWh/cycle (15 kWh battery but not discharged right down to zero) that's 112 000 cycles. At £2500 retail for the battery, say £2500 for installation (I'm not up to date on installation costs) that gives <4.5p/kWh. I would also bear in mind that if you have a heat pump you'll probably want to store more than 15 kWh and the installation cost isn't going to increase proportionally with the battery size. 

I had a 6 kWh battery installed with my solar in 2022 for ~£3100. Late last year I sold it and replaced it with the 15 kW £1500 Seplos kit myself (I was confident in swapping one battery for another, but not installing a battery and inverter etc from scratch). Selling the original batteries and buying the new kit was a net loss of £300. So £3400/112 000 kWh gives a cycle cost of 3p/kWh. 

Martyn1981

zeupater said:

Newbie_John said:

.... Even DIY for £2500 means that each your solar charged kWh costs 5p more..

Hi

..... and each imported low cost overnight kWh used during the day in autumn/winter/spring (including 17kW.t heat pump!) costs how much less than standard daytime tariffs ???? ....

The issue here is simply one of joined-up thinking .... we all know the difference between summer & winter solar generation, however many still fail to understand that solutions are based on the whole picture and that concentrating on component strengths & weaknesses is effectively illogical ....

On a similar thread of thought, the OP would logically be ill advised to match any potential battery size to cover mid-winter max rated operation of the HP (probably ~70kWh) whilst not allowing for overnight temperature set back & average temperatures etc ... as a pure guess (not knowing heat-loss etc!), the maximum battery size in order to cope with most conditions would be ~30kWh, with going down to ~15kWh (or below) requiring an acceptance that there would still need to be considerable reliance on mains daytime tariff import during the darker months. The issue then becomes an annual financial balance of what the rate of return is as the storage capacity incrementally increases above ~15kWh (/30kWh etc ... )  

HTH - Z

Damn good guestimate that. Our 20kWh*** batt is covering almost all days. Only 2 days so far this winter it fell short. Both in Jan, one was 'washing day', with near zero PV gen* and batt ran out about 6.30pm. The other was just a very cold day, no PV gen, and ran out around 9.30pm.

We are not a perfect comparison for a full ASHP wet system, as the A2A units warm through from downstairs with some oil rad top ups upstairs. So the oil rads will only have a COP of 1.

Economically, I suspect 15kWh would be a better match, with batts falling short more often (but not every day), so perhaps an additional average dayrate import 2kWh per day in Dec and Jan. So a monthly bill increase of 31days x 2kWh x 17p** = £10.50.

*For comparison, yesterday was washing day, with decnet PV gen, and the batts hit 48% before starting to charge again at 11.30pm.

** dayrate of 25p, minus the avoided night rate cost of 7p with battery losses, perhaps 15% giving me 8p.



*** Getting back to the thread question, the 20kWh DC side battery, allows us to avoid clipping on the 6.7kWp system, that goes through a 3.68kW inverter, due to DNO limitation on our export.

nday

Martyn1981 said:

Hi nday and welcome.

That's a big system, do I assume you have something like a 7kW inverter, that's been capped by the installer to 4kW output?

Just wondering about the setup, as you may need a hybrid inverter and DC side batts to prevent clipping. That's what I have on one system, with 6.725kWp running through a 3.68kW inverter.

Edit - Batts* are still expensive, but the lost/clipped units could be worth ~15p/kWh export, plus you mentioned an ASHP, and the batts could be used to charge up on cheap rate leccy, for use during the day.

*Cheaper DIY products are now available, but I assume for this you will need DNO/MCS approved install, but I'm really not sure, and better battery advice than I can give, is needed.

So a bit more background, we got the ASHP and Solar installed as part of the governments ECO4 grant scheme, including a new wet system install, so consider ourselves very fortunate and had little input into what we were given.

The installer originally put in a G98 application for a 5kW SolaX inverter but I persuaded them to install an 8kW inverter instead given the size of the array. This meant they ended up putting in a G99 application for the SolaX X1 Smart 8kW String inverter late in the day and the approval didn't come through until after the array had been installed.

I was looking at the Intelligent Octopus Flux tariff which would currently give around 25p/kWh which seems like a pretty good rate for otherwise lost clipped energy, this rises to around 35p in the peak period. This tariff wouldn't let me charge up on cheap overnight rates though.

QrizB

nday said:

. So a bit more background, we got the ASHP and Solar installed as part of the governments ECO4 grant scheme, including a new wet system install, so consider ourselves very fortunate and had little input into what we were given.

The installer originally put in a G98 application for a 5kW SolaX inverter but I persuaded them to install an 8kW inverter instead given the size of the array. This meant they ended up putting in a G99 application for the SolaX X1 Smart 8kW String inverter late in the day and the approval didn't come through until after the array had been installed.

I was looking at the Intelligent Octopus Flux tariff which would currently give around 25p/kWh which seems like a pretty good rate for otherwise lost clipped energy, this rises to around 35p in the peak period. This tariff wouldn't let me charge up on cheap overnight rates though.

25p and 35p/kWh look like the Flux import rates? Export is more like 13p and 26p.

Flux import definitely has a cheaper overnight rate. In my region it's currently 14.99p/kWh.

Qyburn

QrizB said:

nday said:

I was looking at the Intelligent Octopus Flux tariff which would currently give around 25p/kWh which seems like a pretty good rate for otherwise lost clipped energy, this rises to around 35p in the peak period. This tariff wouldn't let me charge up on cheap overnight rates though.

25p and 35p/kWh look like the Flux import rates? .

"Intelligent Octopus Flux", import and export rates are the same. And no cheap overnight.

Qyburn

nday said:
The installer originally put in a G98 application for a 5kW SolaX inverter but I persuaded them to install an 8kW inverter instead given the size of the array. 

8kW is presumably within the DNO's total system size limit. And 4kW export limit applied via CT clamp or something monitoring the grid connection. What the rating of your heat pump, in terms of power draw? Just a reminder that assuming a DC coupled battery, no matter how much charge you won't be able to power more than 8kW, in fact the battery may have a lower limit.

Example we have a 6kW inverter but the battery can only deliver 5kW. So max we can power is 6kW if there's at least 1kW of solar, or 5kW if purely running off battery. Above that has to come from the grid.

I was looking at the Intelligent Octopus Flux tariff .

Be aware that only certain batteries are supported.

Martyn1981

nday said:

Martyn1981 said:

Hi nday and welcome.

That's a big system, do I assume you have something like a 7kW inverter, that's been capped by the installer to 4kW output?

Just wondering about the setup, as you may need a hybrid inverter and DC side batts to prevent clipping. That's what I have on one system, with 6.725kWp running through a 3.68kW inverter.

Edit - Batts* are still expensive, but the lost/clipped units could be worth ~15p/kWh export, plus you mentioned an ASHP, and the batts could be used to charge up on cheap rate leccy, for use during the day.

*Cheaper DIY products are now available, but I assume for this you will need DNO/MCS approved install, but I'm really not sure, and better battery advice than I can give, is needed.

So a bit more background, we got the ASHP and Solar installed as part of the governments ECO4 grant scheme, including a new wet system install, so consider ourselves very fortunate and had little input into what we were given.

The installer originally put in a G98 application for a 5kW SolaX inverter but I persuaded them to install an 8kW inverter instead given the size of the array. This meant they ended up putting in a G99 application for the SolaX X1 Smart 8kW String inverter late in the day and the approval didn't come through until after the array had been installed.

I was looking at the Intelligent Octopus Flux tariff which would currently give around 25p/kWh which seems like a pretty good rate for otherwise lost clipped energy, this rises to around 35p in the peak period. This tariff wouldn't let me charge up on cheap overnight rates though.

Hiya. So regarding my bold, sorry but does the last sentence mean that you got approval for 8kW, it just came in later? I'm assuming I'm misunderstanding, as that would solve the problem wouldn't it, allowing you 8kW output/export?

Or do you mean they installed a 5kW inverter because the approval for the 8KW was too late?

Just wondering, as this affects (I think) where the problem is. If it's the export limit, then you need to get the high PV gen past the DNO limited inverter, so in my case, the solution was a DC side batt, to effectively put it through over a longer time period, at the approval limit.

If high PV gen is allowed, then as others have posted, there are AC side battery solutions to 'catch' the export for later use by the household, to reduce/avoid high priced daytime units.

Or even, if it's an export from the house limit (a slightly different situation), then you can use some, or store some, to get the export down, thus allowing the PV inverter to not cap. But I've no idea if such a setup is now/yet available.

nday

QrizB said:

nday said:

. So a bit more background, we got the ASHP and Solar installed as part of the governments ECO4 grant scheme, including a new wet system install, so consider ourselves very fortunate and had little input into what we were given.

The installer originally put in a G98 application for a 5kW SolaX inverter but I persuaded them to install an 8kW inverter instead given the size of the array. This meant they ended up putting in a G99 application for the SolaX X1 Smart 8kW String inverter late in the day and the approval didn't come through until after the array had been installed.

I was looking at the Intelligent Octopus Flux tariff which would currently give around 25p/kWh which seems like a pretty good rate for otherwise lost clipped energy, this rises to around 35p in the peak period. This tariff wouldn't let me charge up on cheap overnight rates though.

25p and 35p/kWh look like the Flux import rates? Export is more like 13p and 26p.

Flux import definitely has a cheaper overnight rate. In my region it's currently 14.99p/kWh.

So I was referring to the Intelligent Octopus Flux Tariff, I just had a look at the Octopus site and seems I exaggerated the rates slightly for my area but Import and Export rates do track at the same price.




The issue is that only certain battery manufacturers are compatible with this tariff like GivEnergy and the Tesla Powerwall3 and it requires you to hand control of the battery to Octopus.

nday

Qyburn said:

nday said:
The installer originally put in a G98 application for a 5kW SolaX inverter but I persuaded them to install an 8kW inverter instead given the size of the array. 

8kW is presumably within the DNO's total system size limit. And 4kW export limit applied via CT clamp or something monitoring the grid connection. What the rating of your heat pump, in terms of power draw? Just a reminder that assuming a DC coupled battery, no matter how much charge you won't be able to power more than 8kW, in fact the battery may have a lower limit.

Example we have a 6kW inverter but the battery can only deliver 5kW. So max we can power is 6kW if there's at least 1kW of solar, or 5kW if purely running off battery. Above that has to come from the grid.

I was looking at the Intelligent Octopus Flux tariff .

Be aware that only certain batteries are supported.

Max running current for the Grant Aerona HPID17R32 is 25.3(A) according to literature.

Unfortunately 8kW is the total system size limit as I understand it, we have not yet made an application for the addition of a battery. The installers had already put in the 8kW inverter before approval had been received, they now have to return to limit the inverter and I have been requested not to turn the system on until this is done.