Some advice needed as getting conflicting info

[Gavin A]

Recently did a 12 (east) +4 (South) using a Power-One PVI 3.6 OUTD (single inverter 2 MPP trackers) remember to lower the start up voltage on the second string (easily done) system works absolutely fine. Some installers don't know as much as others. (supplied by Ken of course )
We looked at Power-one Micro inverters for that job and couldn't justify the extra expense over yield. Also installed an ImmerSUN at the same time, customer is over them moon with 'free' hot water by 10:30 - 11;00 am most days (he is still in the daily habit of looking at it )
For an 8+4 the power-one PVI 3.0 OUTD should be just fine though the calcs should still be done, and remember to set the start up voltage on string 2.

4 panels is very borderline. Obviously it can be made to work as you've proven, but I'd expect that 4 panel system to struggle to hit even the lowest switch on voltage in lower light conditions, so there'll probably be entire days in winter when it does nothing at all IMO.

over a year though I guess it probably is going to be less than 1% generation impacted, so you're probably right that it makes more financial sense than the micro inverters.... though the inverter on that tracker will also be right at the bottom of its efficiency range as well, so the micro inverters would probably have a 3-5% overall performance advantage as well.

probably a swings and roundabouts situation really.

 

[wersolaruk]

Definitely would use solar edge far more advanced than any other system, for a start panel optimisation, online panel by panel monitoring, fixed string voltage adjusting panel voltage to create optimal output, dc cut off switch at the optimiser when grid voltage lost create a safe dc system( which is a regulation to come soon, external dc cut off switch )can be re programmed via Internet , monitored and maintained from solar edge in Israel who can re programme to adjust grid settings and parrameters,we install a lot of this equipment with no fault yet (1000 optimizers+)As for the company I find nadav the uk service agent very helpfull and has taught me a lot about these products, I have had more problems with power ones being replaced and they didn't pay us for replacement, there customer service is terrible also. Just my opinion but the edge is leagues ahead of power one. But every installer has there product, mines not power one

 

[wersolaruk]

And not on that installation 4 panels is to low

 

[Worcester]

That was the other challenge - the micro inverters went below their start up voltage - that's actually something that too many installers miss. Kick in one bypass diode and the voltage will be too low on a very wide range of panels for the micro-inverter to kick in, whereas in a string environment there is still sufficient to kick start the inverter. Micro-inverters aren't always the panacea that some people think they are ...

@wersolaruk, we've tested it, we've configured it, and we've have done a significant number of installations.. It works fine with the particular panels that we installed on that project, I'm not sating it will work for all panels if you read my post I did put that caveat in.

 

[ExtremePV]

4 panels - if the correct panels works - Fact - not borderline. Its been used repeatably and reliably for several years now since the PowerOne PVI's were introduced - thats one of its biggest selling features.
The start voltage can be reduced to 120vdc on the short 4 panel MPPT input. 4 panels will hit this voltage, and power will be generated down to about 80vdc.

As to efficiency - Remember this is only one of the inputs, the other has 8 panels on it. The only datasheet I can find with Efficiency / Rated output I can find is the PowerOne Micro inverter - Enphase seem very shy to this...?
At 10% rated output, (25W per panel during winter say) the micro efficiency is 90%
The 12 panels on PVI 3.0 (12 x 25W = 300) 10% rated output efficiency is 92-92.5%

Yes micros potentially can provide slightly higher yield, but I do question some of the marketing claims, as I do with optimisers.

On this job, if money was no object I would probably go with micros, but money always comes into it. The original question was advice on a suitable solution for a 8 + 4 panel system. Micros are expensive, and a single dual MPPT inverter is far more preferable than two separate inverters.
Adding optimizers is possible - you could add 4 optimizers to the short 4 panel string with the PVI 3, but again this is adding additional cost and isn't necessary. It comes down to money. It concerns me when statements like '4 panels is too low' are made, when we don't even know the panels! It could be Sanyo / Panasonic HITs for all we know.

Ken

 

[Gavin A]

That was the other challenge - the micro inverters went below their start up voltage - that's actually something that too many installers miss. Kick in one bypass diode and the voltage will be too low on a very wide range of panels for the micro-inverter to kick in, whereas in a string environment there is still sufficient to kick start the inverter. Micro-inverters aren't always the panacea that some people think they are ...

interesting point - we had that discussion recently with Enphase technical rep, they reckon they will work with 1 string bypassed, but not 2.

I was a bit surprised by that, it seems a bit odd, but then I suppose there's diminishing returns when it comes to the difference between 33% of unshaded panel output with just the one string functioning vs maybe 15% for the full panel running at the output of the shaded cells.

We still spec power-one or sma with the shading functions enabled on the vast majority of shaded systems because I reckon they should perform around as well as the optimisers when the system is designed properly, particularly if the optimisers can't really use the bypass diodes fully.

enphase had to accept that their published comparison data wasn't fair as it was based on a comparison with an old SB3800 inverter IIRC.

microinverters do offer a good option for those who can't be arsed / don't know how to properly design a string inverter based system to mitigate the shading impact though.

 

[Gavin A]

4 panels - if the correct panels works - Fact - not borderline.

as I said, it obviously works, so yes I'd agree that's a FACT.

Borderline might not be quite the right word, but it's right at the extremes of the range of the power-one inverter, and as you yourself indicate, is a situation where the microinverters might be preferable. What I was trying to get at was that it's really a judgement call, there's unlikely to be a definitive right answer in performance / payback terms between the PVI 3.0 and enphase / power-one micro inverters.

fwiw, if possible I'd opt to use 5 x 36V 200Wp 800mm panels in place of the 4 x 30V 250Wp panels to bring the system voltage up to a more efficient point, but would probably still recommend the power-one 3.0TL for the 8 + 4 panel system if this wasn't an option.

Where we're struggling now with string inverters is on single 4 panels systems now that the SB1200 seems to have been withdrawn. That's where we've ended up switching to micro inverters for the sorts of 4 panel new build installs we seem to be getting a few of.

 

[FB.]

Also, have you considered using the north east facing roof? I know it sounds ridiculous, but you may fit a 4kWp system if you do. I would say that your ROI would be much better if you did.

Agreed - North-ish-facing can sometimes be worth considering because of the relatively fixed costs of inverter, cables, generation meter, add-on energy monitors, paying the wages and paying for all the "back office" accreditation, certification, qualification and so on for the company doing the work.
My signature shows the performance of two arrays, the second of which has half the panels facing exactly North and is delivering a good return on investment with 21p/kWh FiT payments on track to pay for itself after about ten years, plus electricity bill savings likely to knock another couple of years off the payback time.

 

[wersolaruk]

Solar edge optimisers still allow by pass diodes to work as they should and would not reduce output, as for micro inverters the would have an effect as voltage would be reduced, that's why in my opinion optimisers are a better system as the inverter dictates what optimal voltage the string needs at different irradiance levels

 

[Worcester]

@wersolaruk, I would agree in principle, though, if, as in this case it is simply two strings (albeit one small) with a well designed system, they may not offer any extra bang for their bucks over a twin tracker inverter.

Secondly specifically referring to solar edge (rather than tigo or others) there is a signifcant cost risk to the installer or client with regard to their current track record on reliability and dealing with warranty claims. - Don't get me wrong, one of my own arrays is a solar edge one - the ONLY options for that array were micro's or optimsers and tigo coudn't cope and micros were not cost effecitve, so I know what it is like to have a solar edge system fail from first hand experience.

 

[SolarCity]

Does anyone know of a micro-inverter/optimiser manufacturer that does offer a warranty that covers replacement costs? (scaffold and labour)?

 

[wersolaruk]

I can only speak as I find guys, hand on heart never had a problem. the edge is spot on for us. If companies do go bust the workmanship warranty will cover it? Is this right? If the system is not in correct operation?

Heard of tigo and enphase but the idea of micro inverters does not convince me, plus the idea of 230v floating around on a roof scares the s@@t out of me! Plus all these connectors and branches you need to use with enphase, over 20 years I think they will fail.

If you take the optimisers apart there really isn't much in them to go wrong but I am no electronic engineer.

Anyway I am new to this forum so would like to say hi !!! It's good to be able to throw ideas about between installers.