Advice invited re: 4kW system with string or micro inverters

[Joe Public]

Hello all

I'd appreciate advice / views on a system we're considering having installed. We have a large roof with room for up to a 4kW system (faces 8 degrees east of south; 45 degree pitch; single expanse from first floor to ground floor level; rectangular lower section c10m wide, upper triangular section, 8m total from apex to eaves).

I've been looking at:

16 x Sanyo 250 HIT with string inverter or micro inverters
16 x Suntech 250 mono with string inverter or micro inverters
Other options including Yingli (poly), Phono (mono) and Sharp (poly)

There's potential for some shading at either end due to trees (outside of summer). Can't quantify this or even be sure it's an issue. Also potential for some shading to bottom of lower section from neighbouring roofs - again, not sure.

I like the idea of micro inverters, but don't want to pay an unreasonable premium to have them. Attractions for me:
- ability to monitor panels individually (including identifying any faults)
- benefits of increased power generation
- flexibility to swap out one or more panels with something not identical if required in future

Concerns regarding microinverters:
- if they're behind each panel, they'll be costly to access if needed
- 16 x microinverters = 16 things that could break / catch fire, etc
- they seem to be less efficient than transformerless string inverters

Other specific questions:
- how much should I be paying for each of the above options? I don't have money to burn
- could micro inverters be loft mounted or would that create a weatherproofing nightmare? (could overcome ease of access issue)
- do Sanyos really outperform Suntechs (or others) to such an extent that they're worth the extra money?

I'm trying hard to make the right decision, but appreciate that a little knowledge is dangerous. Am tying myself up in countless knots over this and would really appreciate some advice from this great community. Thank you!

 

[SolarCity]

It depends on what you mean by potential for shading. Very few sites are shade free in the winter months but this doesn't mean that they are a huge problem.

My opinions on micro inverters is that there are very few occassions where they are worth it. Dual MPPT inverters cope well enough with shaded areas and they will generally offer you better efficiency anyway. Your concerns over micro-inverters are mine also. The DNO doesn't appear to like them either.

Micro inverters CAN be loft mounted but doing so and maintaining the weatherproofing of the felt is virtually impossible in my opinion.

I would personally advise going for Suntech panels if you have the space. However, if by using Sanyo panels you can avoid some shading issues then it may well be worth going down this route. A good survey from a good firm should clear up some of these issues.

 

[series530]

I guess that time will tell: we are in the process of having 14 Sanyo HIT250's and Enecsys inverters installed at the moment.

I looked and read , and the read some more. Our roof is roughly SSW and suffers from some (but as the analysis shows, not that much) shading in the morning. Interestingly, as I noticed this evening while standing 6M up on the scaffolding, it also suffers from some later evening shading due to our chimney. Our roof also is pretty large and has a 52 degree pitch. The panels, for reasons of dormer shading are arranged in a line just below the ridge.

We had various candidates out for survey and I decided upon the micro inverter route for a number of reasons: obviously, shading is one of them and the others include the fact a string will only work as well as the worst link in the chain. Most robust string inverters will offer up to 2 MPPT channels and that means that your system will be more efficient but you still have only 2 working points of transfer. Fine for many, but shading, if not isolated to a single string, will still drag the whole system down. Look at an Enecsys system running on the web and it is quite amazing how a set of panels on a roof are varying in output as you watch over a few minutes or a few hours. the mppt is tracking the panel and attempting to harvest at a maximum. A string cannot do that. Also, with the variation in harvest efficiency offered by any given panel manufacturer, a string wont be able to operate at the optimum while a micro inverter can.

The other consideration is that micro inverters come with a longer warranty. While you can extend the warranty on a string it comes with a price premium (admittedly, overall, still less than the price hike for micro inverters).

We deliberately chose to put our inverters inside the roof space. Enecsys have told us that they are warranted for use in that way. In the event of a failure it is a simple job to replace the offending item. Furthermore, in the event of a failure or reduction in harvest, it is much easier to fault find with micro inverters.

The issue of weather protection is a valid one. That said, I am confident that a decent installer will make the installation water tight. I have confidence in our installer.

Micro inversion will cost more, of that there is no doubt. However, I much prefer to take a longer term view. Lets say that the aberations in shading caused through a string make it 10% less efficient than an micro inverter but that a string inverter is 5% more efficient than a micro inverter. Let's say that your average yield is, say, £1500 per year. A 5% upside on that will cover the extra outlay over the FIT. Factor in the additional cost to extend the warranty on a string and you will get better value with MI.

Our installer quotes a much higher upside than I have stated above. He bases his numbers upon the SMA prediction vs. his actual installations. Only time will tell if his predictions are valid. I am pinning my hopes on this. At the same time though I am trying to embrace new technology and trying to be that little bit greener.

 

[SolarCity]

He bases his numbers upon the SMA prediction vs. his actual installations.

By SMA prediction, do you mean Sunny Design? Sunny Design isn't intended to give accurate yield data and it isn't really good science to compare vs. actual installations. I've yet to see the install that you could accurately compare with another. There are just far too many variables - pitch, shading, geographical location, panel, inverter, orientation, length of cable run, inverter location.

 

[genawin]

How important is inverter location - I'm concerned that during the summer i know my roof space gets very hot and it has been the only location suggested by several companies and i can't find an easy alternative ?

 

[series530]

By SMA prediction, do you mean Sunny Design? Sunny Design isn't intended to give accurate yield data and it isn't really good science to compare vs. actual installations. I've yet to see the install that you could accurately compare with another. There are just far too many variables - pitch, shading, geographical location, panel, inverter, orientation, length of cable run, inverter location.

Fair point, the supplied data from the installer came from Sunny Design (as well as the SAP data obviously). The SAP, unsurprisingly, was low. The SMA data provided by the installer came within a few KW of a data set that I also ran through Sunny design. I also ran the same data through PVGIS and this came out pretty close to SMA as well.

I know that the installer has err'd on the side of caution by de-rating the efficiencies for the purpose of calculation. He claims that the real world data that his customers are achieving, on average, has a significant harvest uplift compared to the numbers that he is quoting. Assuming that this is true, I put it down to a cautious estimate and a thoughtfully designed and sited system (he did, unlike most, run a shading analysis prior to quote). Obviously, as with any installation, the quotes are just a guide and there is no redress in the event of a harvest not meeting prediction.

If the work done so far is anything to go by, these people are really trying to do the job properly. I'm impressed by the fact that the roofer has decided use a different set of roof anchors because the roof waves more than was first though (it's only a matter of an inch or two). The anchors specified don't have enough adjustment on them. They didn't have enough of a type with more adjustment and so will come back on Tuesday. There were enough to do the lower rail and the 13 or so metre run of this rail is mm perfect even though the roof waves in and out. The wiring looks neat, tidy and fit for purpose (none of that 2.5mm twin and earth stuff that some suppliers quoted!).

Their quote was the highest, of that there is no doubt. I am taking a gamble with micro inversion. For me though, the shading is something of an issue. Even with a dual mppt, I can see half of the system being heavily compromised until about noon because of some shading due to the neighbours chimney. mppt per panel, I am hoping, will pay for itself over the longer term. Who knows... ? !!

 

[series530]

How important is inverter location - I'm concerned that during the summer i know my roof space gets very hot and it has been the only location suggested by several companies and i can't find an easy alternative ?

Personally, I wouldn't want to have a string inverter in the roof. The Sunnyboy is rated to operate from -25 to +60. As mentioned previously, electronics is affected by heat and dust. In the latter case, dust collecting upon a hot surface reduces the opportunity for the component to dissipate the heat and, over time, it will cook itself. A micro inverter such as the Enecsys is rated from -40 to +85. The operating range, being wider, gives more margin. In real world terms, most roof spaces wont go beyond the +60 centigrade limit. The Enecsys modules are sealed. In effect, the whole case of the unit is acting as a heat sink. This should, in theory, allow for a more even heat dissipation over the longer term and less of a localised over heat. It could also be argued that with a given efficiency figure (albeit, micro inversion being slightly less efficient) dissipating that efficiency loss in a fully operating system will cause a larger localised heating effect in a string inverter than a distributed set of micro inverters. That said, I suspect that the overall temperature in the roof would be far more of an issue than the heat generated by any inverter.
The other consideration of an inverter in the roof is the fact that any work, checking, resetting, etc requires a trip into the roof space. Not an issue for many, but something of a pain for some.

The final thing, which favours a roof location, is the length of the DC feed from the cables. Minimise those cable runs and you move toward a more optimised efficiency. It's false economy going for a super efficient inverter and then feeding it with cheap high resistance cable because you will dissipate power in the cable. This will add heat and cause a harvest loss. The problem can be mitigated almost entirely by using decent quality low loss cable.

Do you have a location in mind for your inverter other than the loft? If so, how would you anticipate getting the cable to it?

 

[SolarCity]

Fair point, the supplied data from the installer came from Sunny Design (as well as the SAP data obviously). The SAP, unsurprisingly, was low. The SMA data provided by the installer came within a few KW of a data set that I also ran through Sunny design. I also ran the same data through PVGIS and this came out pretty close to SMA as well.

I know that the installer has err'd on the side of caution by de-rating the efficiencies for the purpose of calculation. He claims that the real world data that his customers are achieving, on average, has a significant harvest uplift compared to the numbers that he is quoting. Assuming that this is true, I put it down to a cautious estimate and a thoughtfully designed and sited system (he did, unlike most, run a shading analysis prior to quote). Obviously, as with any installation, the quotes are just a guide and there is no redress in the event of a harvest not meeting prediction.

If the work done so far is anything to go by, these people are really trying to do the job properly. I'm impressed by the fact that the roofer has decided use a different set of roof anchors because the roof waves more than was first though (it's only a matter of an inch or two). The anchors specified don't have enough adjustment on them. They didn't have enough of a type with more adjustment and so will come back on Tuesday. There were enough to do the lower rail and the 13 or so metre run of this rail is mm perfect even though the roof waves in and out. The wiring looks neat, tidy and fit for purpose (none of that 2.5mm twin and earth stuff that some suppliers quoted!).

Their quote was the highest, of that there is no doubt. I am taking a gamble with micro inversion. For me though, the shading is something of an issue. Even with a dual mppt, I can see half of the system being heavily compromised until about noon because of some shading due to the neighbours chimney. mppt per panel, I am hoping, will pay for itself over the longer term. Who knows... ? !!

It sounds like they're doing a great install and they've got a great team together.

I'm going to cautiously avoid micro inverters for a good few months yet until we have some solid data on exactly how they're performing. It'll be interesting to see how many malfunctioned inverters are reported.

 

[SolarCity]

How important is inverter location - I'm concerned that during the summer i know my roof space gets very hot and it has been the only location suggested by several companies and i can't find an easy alternative ?

I'd say inverter location is critical. If you can, get it in the garage. If you can't then get an outdoor rated one and put it outside - preferably in an area where it doesn't receive too much sunlight

 

[genawin]

I'd say inverter location is critical. If you can, get it in the garage. If you can't then get an outdoor rated one and put it outside - preferably in an area where it doesn't receive too much sunlight

That's exactly what i thought you might say - i expect efficeincy drops proportionally as the temperature rises , but i don't have an alternative - should i consider biulding an encloser on an outside wall ?
(Sorry for going off subject please feel free to move my post)

 

[SolarCity]

A lot of inverters are rated for outdoor use so get one of those and only put it in an enclosure if it is likely to get a lot of sunlight - and make sure you give it plenty of ventiliation.

 

[Joe Public]

Choices, choices, choices!

Thanks for all your feedback so far, folks - all very helpful.

series530: could you please elaborate on how your installer is ensuring weatherproofing re your loft mounted microinverters? This configuration could give good peace of mind re servicing access, but I'd welcome knowing what I should be hearing from a good installer if I ask them this question.

BiggsSolar: Thanks also for your advice. We have all options open for string inverter location: a utility room on the south facing wall, the loft or on that outside wall (electricity meter is there). Point of interest: we've had a couple of quotes and one installer said they wouldn't recommend external mounting, as inverters get nicked - rubbish or valid concern?

 

[series530]

There are three things in our install that I think help with water tightness. Firstly, the pitch of our roof is 52 degrees and water has a pretty hard job running up hill even with driving wind. Secondly, the tiles are all pretty small and light and wont pinch on the cables too much. Thirdly, the water proof membrane, rather than being a large sheet pinned to the rafters consists of several long but narrower sheets overlaid upon one another.

The plan, as I understand it, is to thread the cables under the tiles and up between the overlaid membrane sheets. As the diameter of the cable is relatively small compared to what is needed on a long string, they can thread up under the tiles without dis-lodging them too much. The installer has also said that they will double check the integrity of each connection and add membrane if necessary.

The feed from the isolator to the energy meter is done inside the house. We have pretty easy access in this regard. I think that feeding a heavy gauge cable to the outside and then down to the garage under the roof tiles is asking for more trouble. This feed has already been done. The connections from the panels to the roof void is being done this coming week and then I have to wait for Enecsys.

 

[SolarCity]

Point of interest: we've had a couple of quotes and one installer said they wouldn't recommend external mounting, as inverters get nicked - rubbish or valid concern?

I'm not sure. I always mention the possibility - at the end of the day, people will nick anything. It does have its plus points though - with a bit of luck you might find that a pikey will end up killing himself trying to take it.

I don't see the resale market of inverters. They're only of any use to MCS installers and I can't see an MCS installer buying a second hand one. The scrap metal value of them isn't huge either.

 

[SolarCity]

The plan, as I understand it, is to thread the cables under the tiles and up between the overlaid membrane sheets. As the diameter of the cable is relatively small compared to what is needed on a long string, they can thread up under the tiles without dis-lodging them too much. The installer has also said that they will double check the integrity of each connection and add membrane if necessary.

How wide are the felt sheets? I can't see how the cables will be long enough to come in between the folds without extending them.