Some advice needed as getting conflicting info

[GOJO]

OK, here goes.......
I want to have a 3KW solar PV system onto my roof. Due to the roof size etc, I can have a 2KW system (8 panels on one roof - facing sw) and a 1KW system (4 panels facing se)
I have had 3 companies out to quote for the job, all giving different ideas for inverters - one company has said that it would need 1 inverter on each set of panels? (An inverter set up for the 1kW system and another for the 2kw system both linking into the generator meter). The others are saying only 1 inverter needed as both sets of panels would link to one inverter (2 strings). I relayed this info to the first company who said that there wouldn't be enough energy provided by the 1KW system to register anything going through 2 strings (1 inverter)? hence I would need an additional inverter. My head is turned and I just feel like throwing my hands up in the air and not bothering having panels installed.
Please could l ask what in your experience as PV electricians would you recommend for this type of installation?
Your feedback will be appreciated.
Cheers, Jo

 

[SpitfireW'sale]

I'm guessing there is a bit lost in translation here, but from what I can gather the first installer is correct.

As the 2 roofs face different directions they need different MPP trackers that (in laymans terms!) make sure the panels perform as well as they can. This can either be done by having 2 inverters with one MPP each, or by having one inverter with 2 X MPP trackers. The second option will not work in this case, as 4 panels will probably not be sufficient power for an inverter with a second MPP tracker to start. I think this is what the first install company is saying, therefore they would seem to have more credibility.

If the other companies are just saying they need 2 strings, and not 2 strings going into different MPP trackers, then I would steer clear as that isn't correct.

Either get them to exaplin clearly what they mean, or post your location on here and a local company that posts on the forum will no doubt quickly set you straight.

 

[Ian-LS]

You'd prob need micro inverters for the 4 panel system unless the voltage of the panels is more than the standard 31 V. I believe the SMA Sunny Boy 1200 works with 4 panels but I don't believe they are manufactured anymore.

 

[GOJO]

I'm guessing there is a bit lost in translation here, but from what I can gather the first installer is correct.

As the 2 roofs face different directions they need different MPP trackers that (in laymans terms!) make sure the panels perform as well as they can. This can either be done by having 2 inverters with one MPP each, or by having one inverter with 2 X MPP trackers. The second option will not work in this case, as 4 panels will probably not be sufficient power for an inverter with a second MPP tracker to start. I think this is what the first install company is saying, therefore they would seem to have more credibility.

If the other companies are just saying they need 2 strings, and not 2 strings going into different MPP trackers, then I would steer clear as that isn't correct.

Either get them to exaplin clearly what they mean, or post your location on here and a local company that posts on the forum will no doubt quickly set you straight.

SpitfireW'sale thanks for your advise, not sure if I am confusing the issue by saying 2 strings, I think I mean (1 wire from the 8 panels and 1 wire going from the 4 panels) going into one inverter which I can only assume that the other companies are relating to an inverter with 2 MMP trackers (Power One was one of the names I remember hearing)
So you mention that the 4 panels would properly not have enough power for the second MPP tracker which ties in to what the first company said to me when I questioned them about needing 2 inverters (which they quoted) - this kinda makes sense now. So really because my 1KW is so small I do need to have two inverters installed as anybody who says that the 1kw can be put into an inverter that has a second MPP tracker would be incorrect as it wouldn't have enough juice to get it going, right?

 

[SolarRoofSols]

Agree. We're doing one in Rotherham this week. 8 panels on south roof, 4 panels on west. Wouldn't consider anything but microinverter or micro optimiser (Solar Edge) now they are on the market. Did a few way back using 2 small inverters which have been performing well, but i like the panel monitoring you get with the micro versions - worth paying the extra for.

Go with the first company, it sounds like they know what they are doing!

 

[ExtremePV]

You can get 4 panel systems to work if the panel OC voltage is high enough.

The Power One 3.0 Inverter has twin trackers, so can easily cope with 8 + 4 configuration.

Depends on your choice of panels.
Micro inverters are an option, but it depends on budget. They are more costly than standard string inverters, but useful in situations like this.

If you or your installer is unsure - give us a call.

Ken

 

[Worcester]

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.

 

[Ian-LS]

Ah youve just reminded me that we've done this in the past but not with standard 31v panels, 36 v panels. Can the voltage on one mppt be lowered to say 80-100 v then?

 

[whinmoor]

I seem to recall HJ Solar have pretty high voltages. Might be worth considering.

 

[ExtremePV]

Minimum PV start voltage can be set down to 120VDC on the Power One PVI 3.0.

Its not ideal with such short strings, but far more efficient to use one inverter, rather than 2 - and significantly cheaper than micro inverters. In these small systems 2 inverters really should be the last resort.

If your unsure, let me know what panels and I'll check.

Ken

 

[GOJO]

Minimum PV start voltage can be set down to 120VDC on the Power One PVI 3.0.

Its not ideal with such short strings, but far more efficient to use one inverter, rather than 2 - and significantly cheaper than micro inverters. In these small systems 2 inverters really should be the last resort.

If your unsure, let me know what panels and I'll check.

Ken

Ken
The panels would be the SolarWorld Sunmodule SW 250
I have been given the option of 11 panels (8/3) with 8 on a Samil and the 3 on a Soladin
Cheers, Jo

 

[SolarCity]

I'd definitely go for the Power One 3.0 and lower the start up voltage to 120v.

 

[SolarCity]

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.

 

[wersolaruk]

Solar edge, easier and far more efficient, plus I would use your north facing roof to get 4kw done loads of north facers with the solar edge. Plus the monitoring is great to

 

[SolarCity]

Definitely wouldn't use Solar Edge. It's a good product but the warranty issues are too problematic. We've already changed one optimiser for an install where the original contractor ceased trading. Cost the customer a days work and scaffold.

If Solar Edge can't back their product with a warranty which covers the labour and scaffold to replace the unit then I don't see why anyone else should either.

 

[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