KWP vs. Inverter capacity

chamelion

Martyn1981 wrote: »

This can all get a bit misleading. First consideration is an appropriately sized and wired inverter for the job. For instance, most normal 4kW inverters are designed to have two strings of panels. So let's say two strings of 8 panels.

What that does is double the amperage, but half the voltage v's a single string of 16 panels. So you could have a situation where the inverter is happy with 16 panels on 2 strings, but 1 string of 12 panels would exceed it's voltage max causing it to shutdown - but that would happen almost immediately it was turned on, and it's unlikely a decent installer would make such a mistake.

In the case of Solaredge, they run on a single string so the inverter has to be correctly sized for the whole string. You'll see from my earlier link that their latest inverters are able to cope with large amounts of oversizing, which makes sense if you think about it, as Solaredge is a specialist in 'odd' installs, where you have multiple orientations, shading, pitches etc, so you might have a lot of PV, but less output than say a nearby system that faces south and has no shading.

Looking again at the SE spec sheet, the 3.68kW inverter can cope with 5.7kW DC, and the 4kW model can cope with 6.2kW DC. I don't know what model the installers would recommend, they may even go bigger, such as the 5kW model, but their Solaredge modeling will tell them what can, can't be done, and suggest the best model too.

When they install it, they'll then need to set max output at 3.68kW during the configuration stage, and then they'll report the install to your DNO stating a max output of 3.68kW from your system.

Obviously most of this is theory, the important part is that the correct inverter will be chosen to cope with the number of panels and the sum of their voltage, and after that it will be set to cap as per the DNO rules.

I can't thank you enough for your inputs - they've helped more than you can imagine!

Any idea what an application costs to exceed 3.68kW in the London area? I've found the uk power networks G59 simple application process at https://www.ukpowernetworks.co.uk/internet/en/our-services/list-of-services/electricity-generation/distributed-generation-connection/ but there's no indication of cost.


Also - what's the maximum potential kWh you can generate in a month if your inverter is capped at 3.68kW? I thought it was a simple calculation but am getting ridiculous figures

Martyn1981

chamelion wrote: »

I can't thank you enough for your inputs - they've helped more than you can imagine!

My pleasure. I found it tricky when I was in your shoes, and received lots of help, so just paying it forward. Also I'm an early adopter on the higher FiT, so also trying to pay that forward too.

chamelion wrote: »

Any idea what an application costs to exceed 3.68kW in the London area? I've found the uk power networks G59 simple application process at https://www.ukpowernetworks.co.uk/internet/en/our-services/list-of-services/electricity-generation/distributed-generation-connection/ but there's no indication of cost.

Fraid I don't know. Mine was in mid 2012, so snuck through as a G83/2 at the time which was free, but has since changed. I seem to recall someone saying what the cost was on here and I don't think it was huge. You could ring and ask, they should be able to chat with you informally, but have to deal with the electrician.

chamelion wrote: »

Also - what's the maximum potential kWh you can generate in a month if your inverter is capped at 3.68kW? I thought it was a simple calculation but am getting ridiculous figures

Not sure it's possible to work out, so a guess might be the best you can make. Going back to my earlier waffling, I was suggesting around 10% loss on a great day, and perhaps we could assume half of June or July are 'great', then the rest good, so maybe an average loss of 7.5%, but call it 10% anyway for the month.

So take a look at PVGIS (see section 5 of the FAQs), just put 6kWp east or west (+/-90) it'll be good enough. Then look at the June and July figures and knock off 10% .... but this is all a bit of a guess.

There will be losses in other months, but less and less as the sun won't see both rooves as much, at the same time.

BTW, what pitch are your rooves at, mine are 30d? If yours are steeper, then losses might be a bit less as the sun won't have as good an angle of 'attack' at midday.


[Edit - Just to confuse you even more, there is a plus side to undersizing inverters too, as this makes them more efficient at low generation levels. It's not recommended for high sun areas like Spain, but in the UK, 10%-20% might actually be better, so 4kWp running through a 3.6kW inverter as opposed to a 4kW inverter capped at 3.68kW, might generate more across the year. This doesn't impact you directly, but just goes to show that predicting generation is very hard, you really have to wait a year and see what actually happens, as there are so many potential variables. M.]

chamelion

Martyn1981 wrote: »

My pleasure. I found it tricky when I was in your shoes, and received lots of help, so just paying it forward. Also I'm an early adopter on the higher FiT, so also trying to pay that forward too.




Fraid I don't know. Mine was in mid 2012, so snuck through as a G83/2 at the time which was free, but has since changed. I seem to recall someone saying what the cost was on here and I don't think it was huge. You could ring and ask, they should be able to chat with you informally, but have to deal with the electrician.




Not sure it's possible to work out, so a guess might be the best you can make. Going back to my earlier waffling, I was suggesting around 10% loss on a great day, and perhaps we could assume half of June or July are 'great', then the rest good, so maybe an average loss of 7.5%, but call it 10% anyway for the month.

So take a look at PVGIS (see section 5 of the FAQs), just put 6kWp east or west (+/-90) it'll be good enough. Then look at the June and July figures and knock off 10% .... but this is all a bit of a guess.

There will be losses in other months, but less and less as the sun won't see both rooves as much, at the same time.

BTW, what pitch are your rooves at, mine are 30d? If yours are steeper, then losses might be a bit less as the sun won't have as good an angle of 'attack' at midday.


[Edit - Just to confuse you even more, there is a plus side to undersizing inverters too, as this makes them more efficient at low generation levels. It's not recommended for high sun areas like Spain, but in the UK, 10%-20% might actually be better, so 4kWp running through a 3.6kW inverter as opposed to a 4kW inverter capped at 3.68kW, might generate more across the year. This doesn't impact you directly, but just goes to show that predicting generation is very hard, you really have to wait a year and see what actually happens, as there are so many potential variables. M.]

Haha - I've quickly come to see how complicated modelling up the math behind this can be - and it's terrible if you're a numbers guy like me I'm going to end up putting worst-case scenario for everything and hopefully overdeliver at the end.

My roof pitch is 45 degrees....

Martyn1981

chamelion wrote: »

Haha - I've quickly come to see how complicated modelling up the math behind this can be - and it's terrible if you're a numbers guy like me I'm going to end up putting worst-case scenario for everything and hopefully overdeliver at the end.

My roof pitch is 45 degrees....

Do you want the good news or the bad?

Bad news is that whilst a steep roof is fine for south facing, it does knock generation for E/W facing. I've just popped a pin in Oxford (on PVGIS) and a 1kWp system west facing at 30d pitch is 786kWh pa, whilst a 45d pitch is 739kWhs, so about 6% less.

Good news, the steeper the pitch the less you'll generate when the sun is high in the summer and in the south, as the 'angle of attack' on the panels will be a bit worse, so at least that means that the losses from capping will be reduced ..... swings and roundabouts perhaps, some of what you lose in annual generation you'd have lost in capping anyway?

Probably worth mentioning that whilst the capping will reduce income from FiT and export, it probably won't reduce your leccy savings since any time you are being capped, you'll have 3.68kW of generation, which is more than enough to run the big items like a dishwasher, washing machine, kettle etc, just don't run them all at the same time.

Also, anytime your generation is 3,679W's or less, then you're generating more than you would have been, had you installed a smaller system, so at low generation times when you are probably using all the generation, then the extra bit can be valued at full import prices.

...... I'm trying to find the positives.

chamelion

Martyn1981 wrote: »

Do you want the good news or the bad?

Bad news is that whilst a steep roof is fine for south facing, it does knock generation for E/W facing. I've just popped a pin in Oxford (on PVGIS) and a 1kWp system west facing at 30d pitch is 786kWh pa, whilst a 45d pitch is 739kWhs, so about 6% less.

Good news, the steeper the pitch the less you'll generate when the sun is high in the summer and in the south, as the 'angle of attack' on the panels will be a bit worse, so at least that means that the losses from capping will be reduced ..... swings and roundabouts perhaps, some of what you lose in annual generation you'd have lost in capping anyway?

Probably worth mentioning that whilst the capping will reduce income from FiT and export, it probably won't reduce your leccy savings since any time you are being capped, you'll have 3.68kW of generation, which is more than enough to run the big items like a dishwasher, washing machine, kettle etc, just don't run them all at the same time.

Also, anytime your generation is 3,679W's or less, then you're generating more than you would have been, had you installed a smaller system, so at low generation times when you are probably using all the generation, then the extra bit can be valued at full import prices.

...... I'm trying to find the positives.

I like the positives!

I've just jumped on an offer - install in mid december Probably worst time of year to get solar, but hey ho!

I'm of course already building up a spreadsheet to track estimate vs. reality. Is there any way to see how much of the generated energy is actually being used? That'll give me an estimate on what i've saved in my leccy bill too!

Martyn1981

chamelion wrote: »

I like the positives!

I've just jumped on an offer - install in mid december Probably worst time of year to get solar, but hey ho!

I'm of course already building up a spreadsheet to track estimate vs. reality. Is there any way to see how much of the generated energy is actually being used? That'll give me an estimate on what i've saved in my leccy bill too!

Unless there's been a rule change, then installing in December should mean your FiT gets an inflationary uplift in April, whereas an install in January wouldn't get an uplift till 2019 as it starts in the year after the install.

Without any kind of monitor to measure export, you can't really work out how much you use, other than comparing bills and seeing how much import goes down.

But, the Solaredge system does now monitor export, import and from that info and the generation puts all sorts of tables together to show you percentage used, exported etc.

Whilst mine seems to accurately record import, so therefore export is probably correct, it doesn't 'know' that there is the WNW system, so that confuses all of the other tables on my monitoring site, especially when export appears to be greater than generation (well, generation it knows about).

EricMears

Not sure if it's still being sold but the Immersun monitoring package gives daily figures for import, export, generation and diversion. It also gives instantaneous figures for those parameters. It isn't however very helpful on (say) hourly averages for results and I have some reservations about how accurate the results are (thought they're still a lot better than trying to check your meter and comparing with a similar day last year !).

glennevis

"Is there any way to see how much of the generated energy is actually being used".


A simple energy monitor which displays both instantaneous power consumed and a daily total would do, for about £30. As the clamp is attached to the household meter feed this measures net energy use.


If however you want to export the data to create pretty graphs then you're looking at a more sophisticated device and higher cost.

chamelion

glennevis wrote: »

"Is there any way to see how much of the generated energy is actually being used".


A simple energy monitor which displays both instantaneous power consumed and a daily total would do, for about £30. As the clamp is attached to the household meter feed this measures net energy use.


If however you want to export the data to create pretty graphs then you're looking at a more sophisticated device and higher cost.

I have a loop energy monitor I'll likely just work out a way to make use of that. I'd probably become too obsessive if I had a full day profile - just day-by-day or even monthly would suffice to keep my analysis OCD needs in check!