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

nday

Martyn1981 said:

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.

Sorry, could have done with a bit more of a joined up explanation on my part......

Original design was for a 5kW inverter limited to 3.68kW (G98). Late in the day the installer "upgraded" to an 8kW inverter and put in a G99 application for the full 8kW, the 8kW inverter was then installed before they had received G99 approval.

A couple of weeks back the installer received DNO approval for the SolaX 8kW inverter but on the basis that it was limited to 4kW so current system export limit is 4kW meaning I have only gained 0.32kW over the standard G98 limit of 3.68kW. I am now waiting for them to return to limit the inverter and I have been requested not to turn the system on until this is done.

Sounds like a DC side battery is the way to go which starts to limit battery options further if I also want to access the IOF tariff to maximise export payments...assuming I am going to be a net exporter given the size of the array.

nday

I have had a quote of £8k for a Tesla Powerwall 3, including install, which has an 11.5kW max import and can sit DC side, replacing the Solax 8kW inverter. The Powerwall3 would also allow me to access the IOF tariff with higher export rates but still feels like a big initial cost when I have limited usage data and a low export limit (relative to array size) to feed into any ROI calculation.

Newbie_John

£8000 it's a big NO.

If you put that into best 5% ISA - you'll get £33 every month (£400 a year) - which you can use against your bill all year round. Or invest in any of the "ripple" like schemes - no issues moving houses, no lost space.

What's your typical bill in July?

Martyn1981

nday said:

Martyn1981 said:

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.

Sorry, could have done with a bit more of a joined up explanation on my part......

Original design was for a 5kW inverter limited to 3.68kW (G98). Late in the day the installer "upgraded" to an 8kW inverter and put in a G99 application for the full 8kW, the 8kW inverter was then installed before they had received G99 approval.

A couple of weeks back the installer received DNO approval for the SolaX 8kW inverter but on the basis that it was limited to 4kW so current system export limit is 4kW meaning I have only gained 0.32kW over the standard G98 limit of 3.68kW. I am now waiting for them to return to limit the inverter and I have been requested not to turn the system on until this is done.

Sounds like a DC side battery is the way to go which starts to limit battery options further if I also want to access the IOF tariff to maximise export payments...assuming I am going to be a net exporter given the size of the array.

Many thanks. That all makes sense now, and fits with what I originally thought. So the real problem then, is how to get the leccy from the panels to your house/export, but via a DNO limit.

So yep, one solution is the DC side batts with a hybrid inverter, which is what I have. The Powerwall 3 (unlike the PW2) actually does that, as it has 3 MPPT's, and can take ~20kWp I think (might have that wrong, sounds a lot). Again, you'd need to cap the output to 4kW for the DNO, but I think that's OK. But with your big system, even at max discharge to the grid, it will fill up fast in the summer.

Also in the summer, you will need to force discharge the batts in the morning, to empty enough space for the new generation. Eg - say the battery has 10kWh in it, and your PV is generating 7kW, then at that point, even with 4kW of export, the batt will still be fill in 1hr from the 3kW DC going into the batt, and will effectively cap at 4kW generation. Does that make sense? Once the generation drops to 4kW, or in poorer weather/months, then even if the batt is full, it will still discharge/export the 4kW (or less) so no loss to you/your export income.

Another option maybe, but I'm making this up, no idea if it's correct, but could you ask the DNO for a zero export permission for the DC side battery, then keep 5.22kWp through the 8kW inverter (or even get a smaller inverter). Then the remaining PV, perhaps the 4.35kWp system goes to the DC battery. The battery is set to not discharge more than household demand not being covered by the 5.22kWp system. [Not sure I could have worded that worse.] You'll still need DNO permission as that battery (let's call it the PW3 just for ease) will be connected to the grid (via your house).

But don't underestimate even the smaller system, in the summer that could generate ~24kWh on a great day, which will need a big battery to avoid clipping, and then how do you use that up, before the next day's generation starts.

A variant on that last idea, would be to get some portable powerstations/add on batts, such as Ecoflow, Jackery etc. You connect them up to the 4.35kWp system, and won't need DNO permission. But the big problem then is how to use the leccy - they will have sockets to plug in devices, but that will be a lot of hassle. Doubt this is at all viable, as you'll be generating far more than you'll be able to consume ........ I can only think of a BEV as having enough demand. The ASHP will be hard wired, and winter generation will be lower anyway.

Back to your large system and batts in general, good news is that you'll have some generation in the winter for the ASHP, and you'll be able to charge the batt with cheap rate leccy to cover some of the daytime consumption.

Sorry, I'm just rambling now as I'm thinking out loud.

nday

Newbie_John said:

£8000 it's a big NO.

If you put that into best 5% ISA - you'll get £33 every month (£400 a year) - which you can use against your bill all year round. Or invest in any of the "ripple" like schemes - no issues moving houses, no lost space.

What's your typical bill in July?

We only moved in end of July 24 when the property had a gas converted Aga and one plug in electric radiator heating the downstairs and storage heaters in the bedrooms upstairs. We had the ASHP, wet central heating system with rad's and solar PV installed as part of the ECO4 Grant scheme mid November so no useful usage data for the summer months at the moment. Also only had a smart meter installed mid-November to allow me to track usage.

QrizB

nday said:

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.

Yes, you just happened to quote the (approximate) Flux import rates rather than the Intelligent Flux ones, which confused me rather - hence my asking in a equally unclear fashion!
I think we're all on the same page now.