Solar export limit

patchyX2

I'm looking to get solar installed. The general consensus seems to be to install as many panels as will fit; the cost of panels are fairly small, so while you've got the installer out and the scaffolding up you might as well get it done.

Is there a limit to this though? By my calculations, I think I could get somewhere between 30 and 40 panels (E/W split) on my roof. My electricity usage isn't particularly high, so I imagine a lot of the time I'd be generating more energy that I need, and therefore looking to export it to make a few ££.

However, I've just read that there can be limits placed on how much you can export (SEG export limit?), and this amount varies depending on your location. Is there a way to find out how much this limit is before getting too far down the installation path? Or is is this something that only tends to affect commercial or huge installations (50+ panels)?

QrizB

Almost anyone can have 8 or 9 panels (3.68kW). Much bigger than that and you need permission from your DNO. Your installer will usually deal with the negotiation for you.

40 panels is potentially 16kW which is definitely on the larger side!

Qyburn

The DNO will almost certainly have some limit on the power you can export. That will depend on the characteristics of your supply, and the characteristics and loading of the network where you're connected. For example we're limited to 7kW total system size and 6kW, the limit in our case is due to our connection.

In all cases you're permitted 3.68kW export. They used to express that as 16A per phase but more recent use power assuming 230V, hence the fraction. That's the "G.98" limit and assuming type approved equipment installed by MCS installer, you just notify them once it's commissioned.

For higher power you need to apply, or the installer does so on your behalf. My experience was that we applied for what we wanted, and SSEN responded with a (steep) price for "reinforcement" work that would be needed before they'd permit this. However they also indicated what they'd accept without reinforcement, which is what we ended up going for.

You can over provision panels, the DNO is only concerned with the inverter. So we have 6kW inverter but 8.5kW of panels.  You may be able to go further since your E and W aspects won't be generating flat out at the same times.

patchyX2

Thanks very much for the detailed breakdown Qyburn. Couple more questions:

1) Are you able to find out your limit by querying the DNO (SSEN in my case) directly before engaging with installers? Reason for asking is that I'll be having building work done, so am leaning towards in roof panels. But if my export limit is tiny then I might want to reduce the number of panels and ensure that my builder's quote includes enough tiles.

2) What happens if you were to get say 20kW panels, but only a 6kW inverter? Presumably in the summer you'd just have a load of wasted energy, but would this damage the setup in some way? Just thinking that even if you can't utilise them all in the summer, they might be beneficial in the winter. 

nologo

If you have more panels than you can export, nothing will happen, but you could use the excess to charge batteries to use later, or heat water or do something else with, like cooking/dishwasher

Qyburn

patchyX2 said:

1) Are you able to find out your limit by querying the DNO (SSEN in my case) directly before engaging with installers?

I'm not sure. I did correspond directly with SSEN but that was after my installer had made the G.99 application. That was to clarify what was involved in their suggested reinforcement.

2) What happens if you were to get say 20kW panels, but only a 6kW inverter? 

Any given inverter is likely to have a maximum over subscription. I have Solaredge and their installers have access to quite a sophisticated tool for the purpose, taking into account orientation of the different sets of panels, so for example would support more if they were split E/W than if they were all facing S.

Allowing for that limit then you're correct, even though you wouldn't fully benefit then it would still do better in Winter. For example today I don't think we hit 2kW so would actually have benefitted from 60 panels.

If you go for a DC coupled battery most systems can charge the battery at the same time as the inverter is whacking out it's full power.

QrizB

Qyburn said:

2) What happens if you were to get say 20kW panels, but only a 6kW inverter? 

Any given inverter is likely to have a maximum over subscription.

You can also get slightly fancier inverters that implement export limitation, so you might have an inverter that's capable of 8kW but will throttle generation to stop you exporting more than 4kW.

The advantage there is that, on a sunny day when you're exporting your max, if you turn the kettle on the inverter will raise it's output to supply the load and still export to the limit.

tamste

Just to point out, the DNO G98 (do and tell) application to the DNO is up to 3.68 kW per phase.
If you have a 3 phase supply, this is 11.04kW (3.68kW per phase).
To export more your installer would have to apply for permission from the DNO for the higher export before installing.

I am on 3-phase and have 26 panels, limited to 10kW export by the inverter. Also on E7 as storage heaters used, so pretty expensive this time of year, but hoping the solar export will claw back quite a chunk in the summer. Did consider putting batteries in but couldn't get the sums to work.

Qyburn

QrizB said:

You can also get slightly fancier inverters that implement export limitation, so you might have an inverter that's capable of 8kW but will throttle generation to stop you exporting more than 4kW.

Just a slight qualification if we're going into this detail. Going by the way SSEN handled our application, they may set a total system size limit as well as export. The reason being that export limiting is imperfect so they assess the effect on their network if full system power was exported.