Adding battery to existing solar PV system - any thoughts/advice please.

Screwdriva

JKenH said:

)If I scrap it and replace with new then I am chucking away around £7k worth of system on current values. It is cheaper just to bolt on some new panels if the DNO will allow it.

The DNO will not be involved unless you re-apply for higher export allowance, which may be worth the effort especially as you are now expanding your system. Your G99 export remains valid regardless of what panel is doing the generation. 

JKenH

Screwdriva said:

Qyburn said:

I've been looking at IOF but haven't been able to take it further as Octopus have only just fixed my meter. I don't think it would be worth switching just now when production is ramping down.

Taking your example customers it looks like they're exporting 2,000 kWh per month. That’s twice our best month but if you assume the same degree of variation that probably equates to a range or up to 90kWh per day, average 60. How do you manage to get so much of that into the peak three hours?

IOF will stop delivering for most households from late October and begin again late March. A good time to switch to Intelligent Go or Cosy with the standard 15p export tariff. 

The household in question has 29 X 435W bifacial panels (with reflective background roof paint) for a ~12.6kWP system size - it has generated between 75-95kWh on a consistent basis, with outliers on both ends of that range. 

They begin exporting 11kWh from roughly 10:45am to 7pm. 4-7pm pays them over 30p per kWh (33kWh X .32p = £10.5) and the remainder of the day pays them their off peak rate in the mid 20p range for the 45-55kWh they export during the rest of the day (assume 45kWh X .24p = £10). Your mileage may vary. 

(Shared with permission of the owner)

Those panels do seem close to the roof edges.

Screwdriva

JKenH said:

Those panels do seem close to the roof edges.

9cm from the sides and 16cm from the top/ bottom, as per the homeowner's measurements following the homeowner's guidance.

NedS

Screwdriva said:

JKenH said:

Those panels do seem close to the roof edges.

9cm from the sides and 16cm from the top/ bottom, as per the homeowner's measurements following the homeowner's guidance.

MCS guidance states:

5.9.7 Solar PV modules should not be mounted within 400mm from any edge of a domestic roof unless specific measures are taken to:

• Resist the increased wind uplift forces in the edge zone through additional fixings and, where necessary, additional roof timbers for those fixings

• Ensure ridge-tiles remain secure

• Ensure rainwater run-off patterns are not affected

• Ensure build-up and shedding of snow cannot cause injury or property damage

https://mcscertified.com/wp-content/uploads/2023/05/MIS-3002_Solar-PV-Systems-V5.0-Final-for-publication.pdf

I would not have wanted to install onto the roof overhang as that will be substantially weaker and more prone to wind uplift.

I'm sure they could have squeezed one more panel on in landscape, above the door if they'd tried.

NedS

Screwdriva said:

Qyburn said:

I've been looking at IOF but haven't been able to take it further as Octopus have only just fixed my meter. I don't think it would be worth switching just now when production is ramping down.

Taking your example customers it looks like they're exporting 2,000 kWh per month. That’s twice our best month but if you assume the same degree of variation that probably equates to a range or up to 90kWh per day, average 60. How do you manage to get so much of that into the peak three hours?

IOF will stop delivering for most households from late October and begin again late March. A good time to switch to Intelligent Go or Cosy with the standard 15p export tariff. 

Absolutely. My spreadsheet shows that our generation just about matches our consumption in October, so that will be the time to keep a very close eye on the forecasts and switch to Cosy. Likewise, March is the same so we are probably switching 1st April and 1st October, plus or minus a couple weeks (e.g, 2 weeks of warm sunny weather at the start of October would delay the switch).

The spreadsheet says September is still an IOF month for us, but the best of the summer is behind us now, The switch will ultimately be defined by when the wife wants the heating turned on.

Screwdriva

NedS said:

I'm sure they could have squeezed one more panel on in landscape, above the door if they'd tried.

Two more would have fit in landscape. But a single PW3 has its limits too..

NedS said:

MCS guidance states:

5.9.7 Solar PV modules should not be mounted within 400mm from any edge of a domestic roof unless specific measures are taken to:

• Resist the increased wind uplift forces in the edge zone through additional fixings and, where necessary, additional roof timbers for those fixings

• Ensure ridge-tiles remain secure

• Ensure rainwater run-off patterns are not affected

• Ensure build-up and shedding of snow cannot cause injury or property damage

Well aware - I have the MCS document as a Screensaver

The installers who fit panels < 300mm from the edge should always use reinforced mounting hardware on additional mounting points to compensate (this should be a rule for all installers). The installers I recommend do this as a default but sadly not all do. I believe this is why MCS refer to it as guidance. 

NedS

Screwdriva said:

NedS said:

MCS guidance states:

5.9.7 Solar PV modules should not be mounted within 400mm from any edge of a domestic roof unless specific measures are taken to:

• Resist the increased wind uplift forces in the edge zone through additional fixings and, where necessary, additional roof timbers for those fixings

• Ensure ridge-tiles remain secure

• Ensure rainwater run-off patterns are not affected

• Ensure build-up and shedding of snow cannot cause injury or property damage

Well aware - I have the MCS document as a Screensaver

The installers who fit panels < 300mm from the edge should always use reinforced mounting hardware on additional mounting points to compensate (this should be a rule for all installers). The installers I recommend do this as a default but sadly not all do. I believe this is why MCS refer to it as guidance. 

I know you know

For others, this reddit post interprets the MCS standards perfectly:

https://www.reddit.com/r/solar/comments/1htpk8l/placing_solar_panels_within_10cm_of_roof_edge/

This MCS Installation Standard makes use of the terms ‘must’, ‘shall’ and ‘should’ when

prescribing certain requirements and procedures. In the context of this document:

• the term ‘must’ identifies a requirement by law at the time of publication;

• the term ‘shall’ prescribes a requirement or procedure that is intended to be

complied with in full and without deviation;

• the term ‘should’ prescribes a requirement or procedure that is intended to be

complied with unless reasonable justification can be given.

and in this case 5.9.7 goes on to detail the prescribed mitigations to be taken if not following the should not be mounted within 400mm

Qyburn

Screwdriva said:

The household in question has 29 X 435W bifacial panels (with reflective background roof paint) for a ~12.6kWP system size - it has generated between 75-95kWh on a consistent basis, with outliers on both ends of that range. 

They begin exporting 11kWh from roughly 10:45am to 7pm. 4-7pm pays them over 30p per kWh (33kWh X .32p = £10.5) and the remainder of the day pays them their off peak rate in the mid 20p range for the 45-55kWh they export during the rest of the day (assume 45kWh X .24p = £10). Your mileage may vary. 

(Shared with permission of the owner)

That's a full roof.  Sadly although our panels occupy only about 30% of the SSW aspect, a DNO limit of 6kW means it wouldn't be worth filling it without a huge battery.

We face 191° and the Summer output is more or less centres and around 13:00, so starts sooner but also tails off sooner than yours. It could still be viable though given that 22.75p is already 50% more than 15p.

NedS

Qyburn said:

Screwdriva said:

The household in question has 29 X 435W bifacial panels (with reflective background roof paint) for a ~12.6kWP system size - it has generated between 75-95kWh on a consistent basis, with outliers on both ends of that range. 

They begin exporting 11kWh from roughly 10:45am to 7pm. 4-7pm pays them over 30p per kWh (33kWh X .32p = £10.5) and the remainder of the day pays them their off peak rate in the mid 20p range for the 45-55kWh they export during the rest of the day (assume 45kWh X .24p = £10). Your mileage may vary. 

(Shared with permission of the owner)

That's a full roof.  Sadly although our panels occupy only about 30% of the SSW aspect, a DNO limit of 6kW means it wouldn't be worth filling it without a huge battery.

We face 191° and the Summer output is more or less centres and around 13:00, so starts sooner but also tails off sooner than yours. It could still be viable though given that 22.75p is already 50% more than 15p.

Yes, for my region, IOF day rates (22.36p) are ~50% higher than SEG (15p) and almost 100% higher at peak times (29.82p). Last month we averaged 26.3p on our export, 1.75 times the SEG rate.

Having a battery may also allow you to manage/buffer your export. Octopus will commonly discharge the battery down to 20% at peak times, and not recharge until the following afternoon, meaning the battery has the capacity to soak up excess solar above your export limit during the peak part of the day when you would otherwise be clipping, and then export that electricity at peak rate, so it's a win-win as generation that may have been lost to clipping and/or export limitation is now exported at ~30p/kWh.

The battery doesn't have to be huge to buffer generation above your 6kW export limit plus any background use.

JKenH

NedS said:

Qyburn said:

Screwdriva said:

The household in question has 29 X 435W bifacial panels (with reflective background roof paint) for a ~12.6kWP system size - it has generated between 75-95kWh on a consistent basis, with outliers on both ends of that range. 

They begin exporting 11kWh from roughly 10:45am to 7pm. 4-7pm pays them over 30p per kWh (33kWh X .32p = £10.5) and the remainder of the day pays them their off peak rate in the mid 20p range for the 45-55kWh they export during the rest of the day (assume 45kWh X .24p = £10). Your mileage may vary. 

(Shared with permission of the owner)

That's a full roof.  Sadly although our panels occupy only about 30% of the SSW aspect, a DNO limit of 6kW means it wouldn't be worth filling it without a huge battery.

We face 191° and the Summer output is more or less centres and around 13:00, so starts sooner but also tails off sooner than yours. It could still be viable though given that 22.75p is already 50% more than 15p.

Yes, for my region, IOF day rates (22.36p) are ~50% higher than SEG (15p) and almost 100% higher at peak times (29.82p). Last month we averaged 26.3p on our export, 1.75 times the SEG rate.

Having a battery may also allow you to manage/buffer your export. Octopus will commonly discharge the battery down to 20% at peak times, and not recharge until the following afternoon, meaning the battery has the capacity to soak up excess solar above your export limit during the peak part of the day when you would otherwise be clipping, and then export that electricity at peak rate, so it's a win-win as generation that may have been lost to clipping and/or export limitation is now exported at ~30p/kWh.

The battery doesn't have to be huge to buffer generation above your 6kW export limit.

Thats interesting to note. I have now placed an order for a PW3 plus a further 5kW of panels. It remains to be seen what the DNO has to say. If necessary I may have to scale back the additional solar. We only generate a maximum of around 4.5kW from our existing ENE/WSW panels so we have almost 3kW of spare capacity on our existing system.  

If the PW3 does leave charging until the afternoon that would be great for me as on a sunny day it could mop up any generation in excess of our export limit which is likely to peak around lunchtime.

The new panels are going on our SSE facing garage roof which this time of year starts going into shade at 4pm and is fully shaded by 5.15pm. The PW3 apparently isn’t compatible with optimisers so the 10 panels will be wired to the three DC inputs on the PW3 on a 3/3/4 split to deal with the shading.

Does Octopus ever cycle the battery more than once - say to meet the breakfast time surge as well as the 4-7pm slot? And does it cycle fully everyday? How much control over charging and discharging do you have?