Correct Inverter Choice?

I hope someone on here can help with an opinion on my install.


The system was installed in December to beat the original deadline. I was quoted for 14 Bosch or similar panels and an SMA or similar inverter. Basically they said they would install what they could obtain as supplies were short.


The panels supplied ended up being 245watt Bosch panels giving a total system of 3.43Kw and a Theia Valera 4.4 HE-t inverter.


The panels are installed on a south facing 50 degree slope roof. There is a large chimney that shades some of the panels early on in the day. First of all it covers three panels then two then one. By mid-day the panels are completely shade free for the rest of the day. They are split into two strings of 7 panels with all the shaded panels being on one string. The DC run is approximately 24M.


There is also an additional shading issue in winter caused by nearby trees. These were a problem this winter but the sun is now high enough and so do not present a problem. I have a chainsaw and these will be dealt with before the Autumn :devilish:


The system hasn’t lived up to my expectations or the prediction provided by the installer although it is early days yet. So far the prediction to date would have been for 385Kwh’s and the system has actually produced 220Kwh’s.


The question is do I have the correct inverter for the system installed. The Theia only has 1 MPP tracker unlike others which appear to have two, does this make a big difference? The inverter also appears to be a larger size than needed, does this give any problems? Should a setup like this have used two smaller inverters?


Today it has been sunny all day so I have attached a graph of todays generation and you can see that up until mid-day the production is poor compared to the rest of the day. On a fully sunny day in March I think I could expect 15 (is that realistic for South facing sunny South Wales?) or more KWh but in fact got 10, is that reasonable?


I see on another thread that I should have been given a SAP calc. I was told a rough payback time, approximately 3000Kwh per year and quoted a return on investment but was never given a Sap calc. Does that matter?


Should I have received a Part P certificate for the work?


I should say that I am happy with the inverter. I connected it up to my router and it gives me all the stats I could ask for. Is that facility available with other inverters?


Sorry for all the questions, any opinions you may have on the above would be hugely appreciated as I can then have an educated chat with the installer if required.

Inverters oversized... yes you should get a part p..saps calc you shoud get but its not that important most are flawed by about 20% on a decent system from what I see...BUT you agreed to any system so I cant see you have any come back...

Your predictions would only be based on using a Sap calculation and would most likely assume no shade whatsoever. The shade will impact on your generation but if you are shade free by noon in the winter you will still get a healthy amount of sun come summertime.

I don't think you would necessarily need a dual inverter as all your panels are on the same roof.

I got 8kw today from a 2.7kw system (although it is not wired up correctly yet, so would hope to improve on that)

babba said:

BUT you agreed to any system so I cant see you have any come back...

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Thanks for your input babba. The agreement was for a correctly specified and correctly installed system with whatever equipment was available. Just because the equipment was in short supply should not mean that I end up with a deficient system. I get on well with the installer so don't think this should be a problem.

The inverter is due to be replaced anyway as it often has a fault code "low PV isolation resistance" so I am trying to find out if there is something more suitable that I can try and specify at this stage.

Anyone know what causes the above fault?

: The inverter LCD screen display “PV Isolation Low”, how can I deal with it? : PV Isolation Low is insulation fault. Every time before inverter trying to connect to grid, inverter will check the whole system’s insulation condition, if the insulation resistance is too low, inverter will show “PV Isolation Low” on its LCD screen and won’t connect to gird. Here are some suggestions to solve this problem:

1) Please first check whether the AC output wiring were connected properly or not. The PE wire in AC output must be connected to the earth. It is better connect to PE line from grid than to your resident earthing.
2) Please check casing of panels were grounded properly. Meanwhile, do not connect PV+ or PV- to ground.
3) Check whether DC switch or AC switch was moistened. There might be a little water inside switch, check and clean.
4) Use a multi-meter to measure the insulation resistance between PV+ (or PV-) and ground, it must be greater than 8 M ohm. Please measure at night.
Note: The PV isolation Low detection is very accuracy, it rarely have problems, so please check earth conditions carefully. The final suggestion is you take an additional inverter as a replacement to the site, replace the inverter if the problems still persist in. If the 2 inverters all have PV Isolation Low issue, it indicates the problem is not from the inverter but from the ground. The function of replacement is just for you to focus on attention to ground conditions.

If you are getting 'low pv isolation resistance' fault this is usually due to a wiring problem not the inverter. When in this condition the inverter will not connect to grid. Sounds like a rushed and poor system. I would suggest getting the company back to re-install it properly!

PS this is a copy n paste thank the internet..

I think a dual MPPT would be better as when the three panels are shaded then all 14 are working to the three shaded panels !!!

if a dual MPPT was there the 7 on the unshaed string would work to full.

MEP ELEC LTD said:

I think a dual MPPT would be better as when the three panels are shaded then all 14 are working to the three shaded panels !!!

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doesn't necessarily work like that, and doesn't seem to be from the graph. It looks like it's pulling the power at the peak MPPT of the unshaded string, so that one's unaffected, it just means that the 2nd string isn't operating at it's peak point.

So not ideal, but not as bad as if it was all run in one string.

Swings and roundabouts

Dave-geo said:

So far the prediction to date would have been for 385Kwh’s and the system has actually produced 220Kwh’s.

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If they only supplied you with a rough 3000kWh a year estimate where does this 385kWh to date prediction come from?

220kw seems about ok to me for the time of year and the lmitations you have described

Hi dave, I think there is severe problem with the system.I have check the pramater of the inverter as well as the panel.
Inverter, MPPt 230~500v, input power 4600w, output power 4400w.
Panel, 245w, open voltage37.7V, operating voltage 30.1V.

Are you sure there are in two separate strings? If so, the system's open voltage is 264V, operating voltage is 211V(which is out of the inverter's MPPt voltage range), which is too low for the inverter. It means the inverter can't working properly...

Meanwhile, I'd like to say some about the 'shade' and the 'dual mppt' issue.
As some of the panels are in shade sometimes, a dual-mppt is needed. I agree with MEP ELEC LTD, who said----

"I think a dual MPPT would be better as when the three panels are shaded then all 14 are working to the three shaded panels !!!if a dual MPPT was there the 7 on the unshaed string would work to full."

The voltage of the shaded panels will drop. If the two strings are not equal in voltage, the higher will charge the lower_this will reduce the lifetime of the panels!!!

babba said:

: Inverter will show “PV Isolation Low” on its LCD screen and won’t connect to grid.

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My inverter still connects to the grid when this fault is showing.

Gavin A said:

If they only supplied you with a rough 3000kWh a year estimate where does this 385kWh to date prediction come from?

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I have a friend who had a 4Kw system installed by the same company before me. He lives about 15 miles from me as the crow flies and he was given a prediction. I have kept the meter readings for the two systems and increased my readings by 16.65% to account for my system being smaller. As you can see from the attached graph he is slightly behind prediction but I am considerably behind what I would have hoped to acheive.