Micro invertor on a solar PV installation

RightRevBod

Sorry in advance for a long post...

We are currently in the process of trying to purchase a solar PV system. We are in a bungalow with a fairly large, almost exactly south facing roof, with a fairly normal slope (looks about 45 degrees). The only complication is an area of flat roof dormer which projects out from the middle section of the roof. This means that in the early part of the day, this would be likely to shadow some of the PV panels on one side of it and in the evening shadow some of the panels on the other side.

There should be space for a 4kWp system.

Last night we had the “marketing” person from Solatricity come to visit. After about 45 minutes I showed him the door. Even with my very limited knowledge of the field I could tell he was talking rubbish and being apparently deliberately misleading. We are getting quotes from some local suppliers. We are in the fortunate position of having 2 within a mile.

With the issue of the partial shading for some of the day, I was thinking that a 2 string solution might be best, with one string placed each side of the central flat root. That way at least 1 string should be totally unshaded for the whole day. These would presumably be connected to a SB4000TL inverter.

From looking around the forums, another option would seem to be using a micro inverter on each panel. If I understand correctly, this would mean that only the individual panels that were shaded would be impacted.

From what I have read, it would seem that the micro inverter route is not very common. I assume that there must be a good reason for this?

I will of course listen to the advise of the installers and take this into consideration, but I would like to get as much information in advance as possible.

Are there any good reason to either follow or not follow the micro inverter route?

Would the same monitoring options be available to record the amount of power being generated?


On a possibly related note, I assume that if we do go for micro inverters then all of the panels do not need to be identical?

We were looking at monocrystalline cell panels, but I understand that hybrid panels are more efficient in shaded areas. Is there any reason why we should not use hybrid panels in the areas most likely to be shaded and monocrystalline for the rest?


Sorry for all the questions, but it seems that there is a real font of knowledge here and it would be a real share not to make use of it.

zeupater

RightRevBod wrote: »

Sorry in advance for a long post...

We are currently in the process of trying to purchase a solar PV system. We are in a bungalow with a fairly large, almost exactly south facing roof, with a fairly normal slope (looks about 45 degrees). The only complication is an area of flat roof dormer which projects out from the middle section of the roof.......

.....Sorry for all the questions, but it seems that there is a real font of knowledge here and it would be a real share not to make use of it.

Hi

Another possible solution would be to use some DC/DC optimizers, effectively small individual MPPTs which could be used for the panels which have shade potential leaving the main DC/AC work to the central inverter which is normally more efficient than micro-inverters, you would also then benefit from being able to use a remote monitor with your inverter. I believe that optimizers are around £60 for a 250W unit and a few more pounds for a 500W unit which could support a pair of panels, so they're possibly a much cheaper alternative than DC/AC microinverters at around a couple of hundred pounds each.

HTH
Z

General question ..... Does anyone have experience with, or have optimizers ?

RightRevBod

zeupater wrote: »

Hi

Another possible solution would be to use some DC/DC optimizers, effectively small individual MPPTs which could be used for the panels which have shade potential leaving the main DC/AC work to the central inverter which is normally more efficient than micro-inverters, you would also then benefit from being able to use a remote monitor with your inverter. I believe that optimizers are around £60 for a 250W unit and a few more pounds for a 500W unit which could support a pair of panels, so they're possibly a much cheaper alternative than DC/AC microinverters at around a couple of hundred pounds each.

Thanks. Another variable to throw into the mix

s2art

Check out Enecsys's duo range of micro-inverters. There would appear to be little price premium over the old string inverters, and should be more reliable (20 year warranty). The question is now; Why wouldnt you use micro-inverters?

rogerblack

Microinverters (and it's not very micro if you're planning on 2) are more expensive than one larder inverter.
If the panels are setup in two series strings, in parallel, then in equal light, power will be drawn equally from both.
In unequal light, depending on the setup, the other panel may contribute little or nothing.

However, this may not be much more than it'd be providing anyway.
If you can arrange them into >2 strings - then put those in parallel - that may be the best config for a shaded area without additional hardware.

s2art

rogerblack wrote: »

Microinverters (and it's not very micro if you're planning on 2) are more expensive than one larder inverter.

No longer true. As I mentioned above. Micros may be cheaper if you compared string inverter + 20 year warranty to micro-inverter with standard 20 year warranty.

zeupater

s2art wrote: »

No longer true. As I mentioned above. Micros may be cheaper if you compared string inverter + 20 year warranty to micro-inverter with standard 20 year warranty.

Hi

Welcome to the forum ...

Does the warranty include the cost of scaffolding/access and labour to test/replace units fitted under panels on the roof ??

The issue which seems to be missed by many is the basis for calculating the system MTBF including multiple microinverters against system inverters. If a system consisting of 1 unit with a MTBF of say 30000Hrs was to be compared to another consisting of 16 units with a more reliable MTBF of 60000Hrs you will find that you would expect to average 1 failure in 30000Hrs v 8 failures in 30000Hrs ....

What must be remembered is that specialised labour is expensive. A consumer unit which you could purchase for around £60 will probably cost around five times it's value for a suitably qualified installer's labour and margin ...... that's why the scaffold/access and labour warranty cover becomes important and probably why it's better to have a single system inverter unless there are particular shading or orientation issues ....

HTH
Z

s2art

zeupater wrote: »

Hi

Welcome to the forum ...

Does the warranty include the cost of scaffolding/access and labour to test/replace units fitted under panels on the roof ??

The issue which seems to be missed by many is the basis for calculating the system MTBF including multiple microinverters against system inverters. If a system consisting of 1 unit with a MTBF of say 30000Hrs was to be compared to another consisting of 16 units with a more reliable MTBF of 60000Hrs you will find that you would expect to average 1 failure in 30000Hrs v 8 failures in 30000Hrs ....

What must be remembered is that specialised labour is expensive. A consumer unit which you could purchase for around £60 will probably cost around five times it's value for a suitably qualified installer's labour and margin ...... that's why the scaffold/access and labour warranty cover becomes important and probably why it's better to have a single system inverter unless there are particular shading or orientation issues ....

HTH
Z

Current MTBF estimates for micro-inverters are in excess of 300 years. All solid state, no electrolytic condensers etc etc. If thats even close to being true then your point is moot. With the Enecsys duo thats only 8 micro-inverters, not 16.
In addition if one fails you only lose a panels output (maybe two if the enecsys duo is used), if a string converter fails you lose the lot.

Gizmosmum_2

Shame Enecys have currently stopped production because of faults ......

In 2004/5 I was involved in the installation of the first ashp's in England, courtesy of innovation funding - the benefit of which was when all the ashp broke down over Christmas I could go to the local diy stores and buy oil filled radiators so our customers would be warm. The problem was fairly easily resolved a few weeks later but it would have been costly if we didn't have the funding behind us to iron out the problems.

The moral of the tale - if you're happy to be a guinea pig and pay more for micro inverters they may be for you. If you're not simply ask for a Fronius with a 20 year warranty. If it's installed before the end of October it'll only cost you £200 more than the normal cost of a Fronius inverter. No issues with roof access if anything goes wrong and much cheaper and lower risk than micro inverters which are currently unavailable anyway :-)

zeupater

Gizmosmum wrote: »

Shame Enecys have currently stopped production because of faults ......

In 2004/5 I was involved in the installation of the first ashp's in England, courtesy of innovation funding - the benefit of which was when all the ashp broke down over Christmas I could go to the local diy stores and buy oil filled radiators so our customers would be warm. The problem was fairly easily resolved a few weeks later but it would have been costly if we didn't have the funding behind us to iron out the problems.

The moral of the tale - if you're happy to be a guinea pig and pay more for micro inverters they may be for you. If you're not simply ask for a Fronius with a 20 year warranty. If it's installed before the end of October it'll only cost you £200 more than the normal cost of a Fronius inverter. No issues with roof access if anything goes wrong and much cheaper and lower risk than micro inverters which are currently unavailable anyway :-)

Hi

Are they the units which were manufactured in the reign of Queen Anne ? ...... 300years, just about time to see a few failures then ... :D

I can see a case for MTBF of 300years on a couple of the components, but for a module ?? ..... do screws even last that long ? ... anyway, why would anyone waste their time reading my thoughts on a 300year MTBF when they could read Enecsys's own viewpoint ..... http://www.enecsys.com/downloads/UnderstandingMTBF.pdf ... :cool:

300Years .... :rotfl:

HTH
Z

zeupater

s2art wrote: »

Current MTBF estimates for micro-inverters are in excess of 300 years. All solid state, no electrolytic condensers etc etc. If thats even close to being true then your point is moot. With the Enecsys duo thats only 8 micro-inverters, not 16.
In addition if one fails you only lose a panels output (maybe two if the enecsys duo is used), if a string converter fails you lose the lot.

Hi

Then the average failure rate becomes 1 in 30000Hrs v 4 in 30000Hrs ..... the issue of labour and access hasn't been addressed. If the warranty includes all costs, ie labour & access/scaffolding, and is independent insurance backed then I would agree that the option could be competitive with a system inverter made by an established manufacturer ... if not I'd personally prefer to wait and see and I doubt that I'll be waiting 300years either ....

Just looking at comparing costs, a system inverter is likely to cost around £0.35/Wp against duo microinverters at £0.55/Wp and single panel units at £0.80/Wp, so a ratio of 1:1.6:2.3, or alternatively, looking at a 4kWp system, a premium of £800 for duo units or one of £1800 for single panel units .... a cost which is really only justifiable under shade conditions due to improved performance as it would either cover the cost of a repair or a long term warranty, or even pay/part pay for a new inverter at some time ....

HTH
Z