Total output power from two inverters

[Serge]

Hi everyone,

I'm not very technical and completely new to solar so this will likely sound as a stupid question.

I have two 6kW inverters (H1-6 and F6000). The system went live mid July and with the weather we've had, I couldn't really see what two inverters could actually do just after midday when the sky is clear and the sun is shining on both arrays (ESE, 110 degrees and WNW, 290 degrees). Arrays are 6.56kWp each. This is an example of a random day in July with constant changes of sunny/cloudy spells:

These last two days the weather has been good with little clouding and I've realised that for the peak time of 11am to 3pm, two inverters won't output more than ~6.6kW combined. I first thought that it could be down to ESE array peaking around 10.30am and WNW array peaking at around 4pm but then after the occasional cloudy spell, as soon as the sun is out, both inverters would jump 4kW+ each to then quickly, a few seconds or so later, settle at ~3.2kW each as though something is limiting the total output from the two. Here's today's graph with green circles highlighting just before or after short cloudy spells:

I haven't tested it yet but I think if I was to power down one of the inverters at around 1pm, the other one would start generating at a higher kW.

Single phase, 100A, DNO approval for 12kW.

What am I missing? Am I being naive assuming that the system could generate more at around midday? I wasn't expecting full 12kW but more than 6.6kW with a clear a sky.

[calum]

How are the panels distributed across the two strings? I'm wondering if each array is all on a single string for each inverter...

[Serge]

How are the panels distributed across the two strings? I'm wondering if each array is all on a single string for each inverter...

Hi Calum,

Thank you very much for your reply. This is where I'm not up too speed on a terminology yet. Do you mean PV1, PV2 etc? If so, this is how my installer said PV1 and PV2 are split and the same is on the other side of the house:

This is PV1 and PV2 Power from the cloud for both inverters today:

[calum]

Inverters frequently have two inputs for panels to be connected to, certainly the H1 does (I have an H1-3.7, basically the same as yours except with less capacity).

Anyway - the overall ability of the inverter to take solar PV (which is DC current) and convert it into AC for your home is divided across the two inputs. If you connected all the panels to one input then you would max out that input and you wouldn't get the full capability of the array. There's a thread somewhere on here where that had happened and they weren't getting the full output they should have been.

Fortunately it's easy to check how it's been set up. You should have cables going into both PV1 and PV2 inputs of the inverter, and if you look at the front panel and go into the menu, choose System -> Solar, you should see volts amps and power readings for both U1 and U2 (which are the designations the menu gives to PV1 and PV2).

Assuming you do have the expected connections and can see readings for both strings, the problem will be elsewhere, but it's good to validate the basic topology before moving on to other things.

[Serge]

Thank you for your patience and taking time to explain, Calum.

Both inverters have 4 cables going into them.

H1 (WNW array):

And its readings:

F6000 (ESE array):

And its readings:

Also my thinking that something is throttling the output got even stronger today. Yesterday when the sky was clear, F6000 (ESE) was already pulling 5kW+ by 9am. H1 (WNW) didn't reach 1kW until 11.50am. Today was mostly cloudy, especially in the morning so F6000 was fluctuating between 2kW and 4kW until about 10.30am while H1 was already pulling 1kW+ by 9.15am, fluctuating between 1kW and 1.30kW. After about 10.40am as the sky become a little clearer, F6000 started pulling above 5kW and H1 output went down to 0.5-0.8kW until almost midday (once F6000's output started declining).

[calum]

You do see bigger peaks and troughs on days with sunshine and cloudy spells, as the panels cool off and are temporarily more efficient when they are suddenly illuminated.

I do wonder though if this may be partly down to the orientation of the panels. As you'll be aware you get the most power from south facing panels, but yours are all either east or west. As such you're missing out on having direct sun on the panels during the strongest part of the solar day. Of course when the sun is high both arrays will get some sun, but neither will be operating at full capacity. I'm not sufficiently familiar with the dynamics of this to say how much loss you ought to be expecting, but it will be having an effect. Overall though, an east west split gives you nearly as much energy as all south facing, but it spreads it out over a longer period of time, so in some ways it's actually more useful.

Looking again at the trace you posted from 26th July, you do seem to be sustaining around 8kW in the middle of the day, given the panel orientation I'm wondering if that might actually be what peak(ish) performance might look like? (Actual peak performance tends to be in June where the longer hours and somewhat cooler temps help out).

All that said, given that the panels are wired up in the right way, it's possible there's some mis-configuration somewhere. Have you asked your installer?

[Serge]

Hi Calum

I did consider drops in temperature of the panels after the cloudy spells as the reason for the output spikes. It's just those spikes don't last for more than a few seconds.

With regard to the 26th of July graph, I'm less reliant on the data as the weather kept frequently changing and where I'm still relying on the cloud, the refresh for each inverter/side doesn't take place at the same time. Hence I was waiting for the clear sky day to see the actual potential at midday. I often physically watch both inverters (I know, I need to sort out the modbus) and that's how I first thought that some kind of balancing act is going on as the values kept going up and down on both but the total wasn't as high as I thought it would be.

I'm completely with you regarding the sun's declining height as we move towards December and don't expect full 12kW. However, yesterday I've noticed something that also doesn't make sense. At 2.30pm, clear sky and with the sun was shining more on the western side as expected, the east facing inverter was pulling just over 4kW, the western 2.2kW. It's like the east facing inverter is allowed to output at its max capacity (available solar permitting) and west facing whatever is left to make up to 6. something. I should've really powered down east facing inverter to see if the west facing inverter ramps up but for whatever reason I didn't. Today's weather didn't allow me to run this test but I will definitely try it in the coming days.

I haven't asked the installer yet but will do so next week.

[calum]

Hi Serge

My point about your 26/07 trace was that it fairly clearly shows several hours of output that's at or close to 8kW, with a peak of 10kW - which suggests there isn't a "hard" limit on the system output. That said, I do take your point about what you were seeing yesterday. Which inverter is attached to which array?

What is usually done is that the hybrid inverter (ie the one attached to the batteries - I'm assuming you have batteries because I can see +ve and -ve HV DC cables connected in one of your photos) has a CT attached to the output of the "solar only" inverter so it can take into account the output of that inverter when "deciding" how much power to draw from the batteries, send to the grid, etc.

Assuming this is how things are set up (ie with the H1 as the "Master") it would be worth looking at the settings in the menu under Menu > Setting > On Grid to see if there's any sort of limits been set.

[Serge]

Hi Serge

My point about your 26/07 trace was that it fairly clearly shows several hours of output that's at or close to 8kW, with a peak of 10kW - which suggests there isn't a "hard" limit on the system output. That said, I do take your point about what you were seeing yesterday. Which inverter is attached to which array?

What is usually done is that the hybrid inverter (ie the one attached to the batteries - I'm assuming you have batteries because I can see +ve and -ve HV DC cables connected in one of your photos) has a CT attached to the output of the "solar only" inverter so it can take into account the output of that inverter when "deciding" how much power to draw from the batteries, send to the grid, etc.

Assuming this is how things are set up (ie with the H1 as the "Master") it would be worth looking at the settings in the menu under Menu > Setting > On Grid to see if there's any sort of limits been set.

Hi Calum

I understood what you meant about the 26th of July trace but I don't think I have explained properly what I meant by data not reliable.

As I've admitted before, I often stand in front of both inverters and watch the numbers. I first become suspicious when I noticed that the numbers fluctuate in opposite directions. Literally within a few seconds one inverter would go from 4kW+ down to 2kW while the other one, at the same very time, goes from 2kW to 4kW+. They both go down in sync when it's cloudy so that's understandable but other times it doesn't make sense. I must admit that I can't tell you if that was exactly what I've seen on the 26th though as I simply cannot remember when I first noticed fluctuations. Also that screenshot of the 26th of July was of 'Site' view so both outputs shown. Let me show you this in a different way.

I've got two screenshots here of individual inverters on the same day.
At the top is H1 (West) with a cursor on the sample at 13:56:07
At the bottom F6000 (East) with a cursor on the sample at 13:56:53

The total does come to 9.05kW and this is exactly what's shown on 'Site' level at 13:50:00 yet at the next sample at 14:00:00 it is showing 5.54kW. Going back to inverters and the time of 13:56, the difference was only 46 seconds but from what I've seen by standing in front of both inverters, a lot could've changed in that time.

I've also left a discharge line on H1 so you can see it was kicking in quite often making those numbers 'artificially' higher.

Thankfully to you I have browsed H1's menu today, checking all features/settings etc. In terms of 'On Grid' settings, 'Power Limit' was set at 100% so I've left it there. The 'Export Control' though was set at 6000W which is logical considering this is a 6kW version of H1 but I did wonder if having "solar only" inverter as well the number should've been higher. I did change it to 12000W but it was only after 1pm and of course, it was mostly cloudy since then. The only bright period was around 2.30pm so when I went to look at the inverters, the H1(West) was showing 4.3kW (no battery discharge as far as I could see) while F6000 (East) 2.8kW. This was the highest total I have consciously witnessed, live with my eyes. It only lasted a minute or so (it got cloudy again) but the numbers were stable throughout that time.

So I'm hopeful that that might be the reason for the fluctuations but only time will tell. Or it could've been a coincidence, I don't know. I also considered that incorrect 'feed in' reporting on the cloud may play the role in this too. I have previously posted about F6000 showing always feeding in: viewtopic.php?t=390. It is still like that and the installer hasn't come up with a solution yet.

As always, I'm very grateful for your input and I'll let you know if I see a higher total output numbers in the coming days.

[calum]

What I'm not completely clear on is how dual inverter systems like yours are configured, in that you have one inverter that only does solar, and one that does solar AND batteries.

The H1 inverter should have a CT clamp on your mains connection so it can see what power is flowing (and in what direction). It should also have a second clamp on the output of your other inverter, so that it knows the total aggregate generation across the two systems and can "decide" whether to send power to the battery, take it from the battery, send it to the grid, etc.

Now, your total DNO export approval is 12kW, what I don't know is whether the H1 being set to 6kW caps the total system output at 6kW (which would seem like a terrible idea!) or whether it only applies that to its own output? It would seem from your last post that it does seem to be applying some sort of overall cap but we are getting into a level of "how does it work" that is beyond my knowledge (which is purely that of an interested and engaged homeowner who owns a Fox inverter, I'm not involved in the trade nor do I have any formal electrical qualifications).

I would say your installer should have a good grasp of all this and ought to be able to explain to you how the system is meant to work - you can then see if it's behaving as they expect and go from there.