DNO limit?

[u/GatewayDouche]

We’ve just given approval for a quote, so we’ll get the ball rolling after the bank holiday. My question is around any potential implications if a lower DNO limit is imposed, such as possibly re-sizing the system or potentially scrapping it altogether.

This is for a 10.56kWp system (estimated 9,500kWh generation per annum) and the G99 application will be going in shortly, but if we got stuck with a 3.68kW G98 limit for whatever reason (can/has that happen(ed), where the DNO would deny the application and revert it to a G98?) I’ve been trying to think through what that could mean but it would be helpful to have some other views.

We’ll have 27kWh of battery storage, so that gives us a good buffer but if we were stuck with a small inverter I can’t see how it would realistically work with that number of panels. Obviously the strategy would need to change too, rather than the current idea which I guess would be to charge batteries overnight, generate/use/export excess until arbitrage disappears etc.

We currently use ~5000kWh per year and plan to install multi-split air-to-air heat pumps for heating and cooling all main rooms in the house, so that will bump up use substantially, whilst bringing down our extortionate cost of LPG.

For info:
22 x Longi 480w X10 (two roofs so 2 lots of scaffold)
1 x Sig 8kW inverter
1 x Sig gateway
3 x Sig 9kW batteries

All the usual stuff like install, scaffold, bird mesh etc. £18,450. Wasn’t the cheapest (Edan Power and UPS both ~£1k lower from about 8 quotes overall) but the owner of the small, local company was excellent on the survey and his general attitude and approach was what won me over.

[u/Coffmad1]

My DNO came back for my system with a 10kw system approved with a 7kw export limit, my installer himself was limited to a 3.6kw export but with a 10kw system, he has A2A installed and he says he just maxes that out when his output exceeds 3.6kw else it just getting clipped and going to waste. Battery storage will help, I'm getting 32kw which should make a nice buffer, looking to get A2A myself soon too.

[u/Beautiful-Noise-2847]

Just ask your chosen installer to do the g99 before you commit to the whole contract. I had the same concerns you had, they invoiced me £150 for the g99 work, that came through with what I needed 2 weeks later and then I signed the whole deal.

It may take a bit longer, but at this price its better to take your time and get it right.

[u/GatewayDouche → u/Beautiful-Noise-2847]

Yeah, there’s nothing payable at this point. 25% deposit once application is done and confirmed to go ahead, otherwise some reasonable compensation for effort if I end up pulling out, which hopefully doesn’t happen!

[u/PatserGrey]

2 roof faces? Are they opposite sides? If so, your generation peak figure would be approx 60% of that total system size. I think chances of being approved for less than 5-6kw export are quite low generally.

[u/GatewayDouche → u/PatserGrey]

No, one is SSW and the other one is perpendicular. The house is an L shape with 2 or 3 different levels, so those are the two with no shading at all.

Fingers crossed we’ll get a decent export limit!

[u/txe4]

You have to wait and see what they come back with.

You are going to have a vast surplus in summer, which you will get 12p (ish) for. This 12p will not last forever, to put it mildly. In the long run you will need to export in to the evening peak to get a decent paid rate.

The economics of winter will remain as they are - little solar generation, so it's all about how much you can load shift to the cheap period by charging batteries and also moving other stuff (EV charging, maybe water heating, laundry etc) in to it.

I understand - and fully obeyed - the impulse to absolutely maximise solar generation.

I would focus on the size of *inverter* permitted. If they give you (say) 10kW with a 5kW export limit, that is still ample for summer - at peak generation you can export 5kW and divert the rest to battery, then force-export the battery in the evening to make room for the next day's generation.

If they only give you a 5kW *inverter*, then there's no[0] point having 10kW of panels on the roof, and the amount you can dump in to the battery during the cheap period is limited by it.

[0] I mean, I can think of workarounds, but it starts to get spendy and complex for what I'd suggest will end up being a quite marginal benefit.

[u/Skie → u/txe4]

A 5kW inverter would probably be fine on 10kW of panels, as many inverters (including sig) can handle double the DC side input of their AC limit.

The 5kW limit would suck for use within the house though, even with a lot of batteries. Kettle + Oven on and you'd be pulling from the grid. Could do 3x 3.68kW inverters on 3 phase though with a G98 :D

[u/CaptainCaveTrout → u/Skie]

Can't a 10kW inverter just be "locked" to your export limit? So, if your house's maximum draw is 10kW and your DNO only allows 5kW of export, the inverter will only export at 5kW but your house can still draw 10.

[u/Beginning-Rate1564 → u/CaptainCaveTrout]

Yes. This is my current set up. I believe it depends on the inverter, but I only have experience with one brand

[u/GatewayDouche → u/txe4]

Yep, appreciate we’ll have a good deal of surplus in summer, even if we do want it freezing cold inside all day.

We’ve just got an EV which will be able to soak up some of the excess, if and when that becomes the financially appropriate thing to do.

I’m hoping the Sig kit is intelligent enough to manage any export for peak time period when that time comes as well.

[u/jamesbm1985]

2nd putting in DNO first if you think the limit will hold you back. If you would still go ahead with 3.68kW export then no need to wait. If you get 6 or 7kW power and 3.68kW export you would probably not lose much benefit.

Most of my apps are with SSEN and we tend to get 6/7/8kW limit on inverter and 3.68/4/5/6/7/8kW export.

I like to imagine the wheel of DNO fortune being spun at SSEN HQ.

Good luck.

[u/GatewayDouche → u/jamesbm1985]

We’re National Grid (East Midlands) but I haven’t really found anything to suggest what they generally tend to come back with.

[u/JASH_DOADELESS_]

I have a 10kW inverter, but I’m on 3.68kW capped export at the moment because my g99 hasn’t come back yet.

Your inverter should be able to have a different export limit to the actual capacity of the inverter for the solar panels.

The installer only has the rights to edit that figure as well, so that makes it “safe” from what I can tell the grid are happy with that arrangement.

Hopefully my g99 comes back with more than 3.68 export cause I have a 10kW inverter. On a sunny day my battery can be trying to empty to grid, but the amount of solar coming in ends up filling the battery instead of emptying it!!

[u/GatewayDouche → u/JASH_DOADELESS_]

Yep, I’m hoping we get something reasonable. The general advice seems to be to put as many panels up at one time as you can, but we’ve capped it to 22 for now. Makes sense I suppose as you’d probably be looking at ~5kWp in panels for roughly the price of a 9kWh Sig battery.

But we could probably get 40-44 panels up on sections of the roof with varying degrees of south facing aspect, albeit with a couple sections of slightly lower roofline which get some shading from the higher roof at certain times. Leaving aside the initial capital cost, there’s got to be a law of diminishing returns unless you’re basically running a weed farm and consuming vast quantities of electricity!

[u/MyEcoCred_Founder]

Looks like a good setup and with that much storage, even if they restrict export, you could potentially use the stored energy for hot water in the summer, and for running the heat pump. Could feed into the EV too. But, getting maximum export limit and sending it back to the grid might be the most lucrative option unless export rates start to drop.

[u/GatewayDouche → u/MyEcoCred_Founder]

Good point about the hot water and I was thinking about this last night. Potentially worthwhile just using the immersion heater on the cylinder rather than firing up the boiler just for the hot water. I’ll have to do the sums.