Questions from newbies! PV capture 'capping' ? Different PV voltages?

nitsuanomis

Hi all,

We are new to the solar generation world and are trying to understand the wealth of information we are getting from our monitoring application.

We have a 6.84kWh solar panel array (18x JA Solar 380W Mono panels) on a south facing roof in Northumberland, with a Luxpower LXP 3600 Hybrid inverter and 2x Aoboet Uhome-LFP 2400 batteries.

In looking at our solar PV generation for the last few days, it seems like it is 'capped' at just under 4kWh - we understand that it depends on available sunlight, etc., but it has plateaued at this point on the sunniest days of the year and seems so consistently maxed out at that level; is this to be expected?

We believe the output to the grid is not allowed to exceed 3.68kWh (which, combined with our base level consumption, is about this number), but we don't see it climb above that even when we have higher consumption in the property; should it?

Here are some graphs from the past few days to help give an overview of what we are seeing:



Is anyone able to kindly shed some light and help us understand our new system?

Thanks in advance,

Simon

70sbudgie

From what I can tell from a quick look at the specs for your inverter and the batteries is the following:
The inverter is 3.6kW AC. The max input is 6.6kW DC. So it looks like your solar panels are oversized for the invertor and are charging your batteries first. (As would be expected). Your batteries look to be 2x 4.8kWhr (I think) so are charging pretty quickly with the recent sun.

From the graphs above, I can't see a huge amount of battery discharge (where the blue line goes above zero), but then again you are not importing much - only a couple peaks where the battery doesn't appear to be able to respond quickly enough.

I think I read that the batteries have a good controller, so you might want to consider delaying their charging to the middle of the day to offset some of the peak capping. 

May also be worth trying to smooth your consumption a bit more to remove the occaisional peak, though I don't think you have a huge scope for that.

Edit to add: the batteries and the PV appear to be DC coupled, so the batteries charge "before" going through the invertor. Once your batteries are fully charged, the inverter will limit the output to 3.6kW AC.

Spies

Your inverter is woefully undersized, send this information to the installer and ask them to resolve it with a larger inverter.

Martyn1981

Spies said:

Your inverter is woefully undersized, send this information to the installer and ask them to resolve it with a larger inverter.

Yes, but! They need to ask the DNO for permission to go bigger, something they really should have explained and done, before installing such a large system (v's the 3.68kW limit).

Fingers crossed the DNO says, otherwise perhaps bigger batts, and as Budgie suggests, delayed battery charging in the summer till after 10am to gain a bit more.

QrizB

Spies said:

Your inverter is woefully undersized, send this information to the installer and ask them to resolve it with a larger inverter.

A third vote for your inverter being far smaller than your array really deserves. You've got a large solar array in a good location and the inverter is definitely restricting performance.

Someone  - I think @Reed_Richards - has an inverter that's technically more powerful than his export permission, but the inverter monitors the net export from the property and can increase power in response to domestic loads.

For example, if the export is limited to 3.68kW but someone turns on a 2kW kettle, the inverter output will rise to 5.68kW in response.

From the graphs you've posted, it seems your invertwer is either unable to do this, or has not been configured in this way. It's a question you need to ask your installer.

Alnat1

Those batteries are only 2.4 I think.

I know (from energy-stats-uk) that 2 Pylontech 2.4 batteries on a Lux 3.6 inverter can only discharge a max of 2.4kWh so assuming these batteries could work the same. Would explain the morning peaks (when the 3kWh kettle goes on?) at 5.30 and 8.00.

3 batteries gives the expected 3.6kWh discharge.

Mikeyboy01443

I like the idea of delaying charge til later in the day, can someone explain the logic behind this? At the moment I'm only on 1 battery still waiting on second so sometimes by the morning the battery is nearly gone by morning...

70sbudgie

Whether you can do it will depend on the battery controller and the inverter logic.

The common logic is that the battery charges from the PV panels before exporting to the grid. So you use as much generated electricity locally as possible.

But in the case above, the inverter appears to be designed to charge the batteries (DC) and export maximum to the grid (AC) at the same time. So the solar panels have been sized for maximum export plus maximum battery charging rate.

The batteries seem quite small capacity, though and therefore charge very quickly (if they are charging at about 3.6kW and are only 4.8kWhrs, they will charge from empty to full in less than 2 hours full power). 

Once the batteries are fully charged, any electricity generated above 3.6kW is wasted and that is the capping. To maximise total output you want to balance your load with your generation (the age old story! ). Though in this case load includes export. So the logic is to initially prioritise export over battery charging.

Ideally the batteries should start charging when the panel generation exceeds 3.6kW. If the panels never reach peak generation, then the batteries will just charge more slowly and there's no waste if generation minus battery charge stays below the inverter AC maximum. But you also want your batteries to be fully charged when generation meets demand, so it may take some tweaking to get it right.

On 3 of the 4 days posted above, the panels seem to reach 3.6kW about 10am, which seems a reasonable time to start charging the batteries. It is also highly possible that the last two weeks have been slightly anomalous and that with normal weather you wouldn't see capping. The 17th looks like a more reasonable amount of capping and is perhaps what you might normally expect.

To be honest, your installer should have done this for you and provided you with a report with the reasons for the recommended sizes. But it also takes time to understand how your particular install works and if you don't yet have the full compliment of batteries, combined with unusual weather, you may be getting slightly misleading data.

Magnitio

The following graph shows an example of battery charging as a way of using the output from solar panels when it exceeds the AC limit of the inverter.

In this case, the AC limit on the inverter is 5.5kW. By setting the battery to charge at 1.25kW, output from the panels up to 6.75kW can be handled.

And here is an example on the same system where the battery was fully charged too early in the day:

As you can see, the output was limited to 5.5kW for several hours.

If you have Agile Outgoing Octopus and the ability to control the battery, then you can export some of the energy at high rates in the early evening or early morning and then let the battery fill up during peak solar hours. This does mean checking the forecast and electricity rates and fiddling with the setup, but it is quite interesting.

NoobSolar

Your system needs an inverter that is more powerful in order to handle the full generation of the panels. About 6 or 6.5 kW.

You then need to apply for something called g99 certification from the DNO. Call them to arrange this. They will need your mcs certificate and other info from your installer.

G99 will give you the right to export to the grid more than 3.68kw, which is the 'permitted development' maximum you are currently allowed. Do this now as theoretically in the future when more people have self- generation the amount they allow you to export could reduce.

The DNO g99 certification will only relate to the setup applied for.

It may also be worth getting an iboost once you have that much power.

Mikeyboy01443

Magnitio said:

The following graph shows an example of battery charging as a way of using the output from solar panels when it exceeds the AC limit of the inverter.

In this case, the AC limit on the inverter is 5.5kW. By setting the battery to charge at 1.25kW, output from the panels up to 6.75kW can be handled.

And here is an example on the same system where the battery was fully charged too early in the day:

As you can see, the output was limited to 5.5kW for several hours.

If you have Agile Outgoing Octopus and the ability to control the battery, then you can export some of the energy at high rates in the early evening or early morning and then let the battery fill up during peak solar hours. This does mean checking the forecast and electricity rates and fiddling with the setup, but it is quite interesting.

I might experiment with this, as you say though its not a set and forget thing as on a rubbish day like today, it isn't charging the battery much so if the sun suddenly comes out i would want it to try and fill the battery as quick as it can and not be limited.. but i may try next time it's sunny! 

Just had a look and i can set charge / discharge to a certain Amps, not kw, so i havent got a clue. 
Don't understand all these different units!