inverter on every panel or not ?

[SG-1]

Planning to install a 15KWDC ground mount grid tied PV system.

Not sure if I want a single inverter or have the inverters on each panel.  Seems to be an upfront cost vs monitoring & troubleshooting convenience.   

Appreciate your thoughts on the matter.

[NiHaoMike]

I would go with the single inverter so you can have the option of backup power without batteries. Some even support adding batteries later on.

[fourtytwo42]

IMOP panel inverters are a waste of money and simply serve to extend the payback time of the system.
As for monitoring that is something exciting to play with on a new system but the novelty soon wears off.
Anybody involved with electronics understands the more components the poorer the reliability.
If you have a severe shading problem arrange the shaded panels in there own substring and attach them to a multi-mpp inverter on there own input, still far fewer components and lower cost than per panel modules.

[tszaboo]

Quick search and napkin calculation:
1 ) Enphase Micro-inverter IQ7+: 120 EUR.
Three phase cabling: 20 EUR each
You will have some 42 Panels. Total inverter cost: 5900 EUR, probably an extra 500 for accessories.

2) SMA Sunny Tripower 15000TL-30: 2380 EUR

3) Solaredge P370 : 45 EUR
42 power optimizers (1900 EUR) + 1x SolarEdge SE16K SetApp, 1329 EUR,
Total: 3200 EUR
You get per panel tracking, monitoring


You can translate this to your local currency. IMHO, micro inverters are good, if you get them for free.
You have to decide yourself, if that extra 800 EUR or so is worth to get the max out of each panel. I did invest into them.
Ask quotations before deciding for any technology, your local pricing will vary.

[HB9EVI]

by coincidence, there just was an article about that topic in the recent swiss amateur radio league magazine, which seems to prove, that the efficiency gain is minimal and the interference level is going through the roof compared to a single inverter solution.

[SG-1]

Appreciate the insight into this.  I have one quote from my Electric CoOp using micro-inverters on each panel & one quote from a private company using either one or two inverters. There is 15,000 US dollars difference & the micro-inverter system is smaller.

[tszaboo]

Appreciate the insight into this.  I have one quote from my Electric CoOp using micro-inverters on each panel & one quote from a private company using either one or two inverters. There is 15,000 US dollars difference & the micro-inverter system is smaller.

Smaller in price or in power output?
Keep asking quotes, the ones I got had about 25% variance in price for the same nominal power.

[bdunham7]

I have an Enphase system that is now eight years old, has completely paid for itself, possibly twice.  Almost no issues and almost 100% uptime, the only exception being when I replaced the communication module with a more advanced system due to spotty communication.  No interference issues at all.  However, it was only nominally more expensive at the time, and it is on a roof.  With that sort of price difference, I'd keep shopping and on a ground-mount system, I really wouldn't want to pay a lot extra for microinverters, certainly not that much.  How much are the quotes?  $15K is almost what I paid for my entire system after tax credits and prices have supposedly gone down.

[nctnico]

by coincidence, there just was an article about that topic in the recent swiss amateur radio league magazine, which seems to prove, that the efficiency gain is minimal and the interference level is going through the roof compared to a single inverter solution.

Not just that, you are also adding a lot more moving parts. Recently I visited someone who now has 2 blown Enphase separator relays (to disconnect the solar array from the grid) for no appearant reason. I opened one up and the internals didn't instill much confidence. A single inverter looks like a much more reliable solution to me (at the cost of needing extra wiring).

[bdunham7]

Not just that, you are also adding a lot more moving parts. Recently I visited someone who now has 2 blown Enphase separator relays (to disconnect the solar array from the grid) for no appearant reason. I opened one up and the internals didn't instill much confidence. A single inverter looks like a much more reliable solution to me (at the cost of needing extra wiring).

Those relays aren't required or used here, AFAIK.  On an Enphase system like mine, there are 30 separate systems and no common points of failure other than the actual generic wiring going to it.  If they start monkeying with that formula, I'll switch my opinion pretty quickly as to which is better.

[PaulAm]

Microinverters certainly make the system easier to work on; they're dead unless line power is supplied.  You don't have to worry about 300V DC from energized panels, but the trunk cables can be expensive.  I have a 10kw Enphase microinverter array and they've been working fine for a few years.

They do complicate things if you want to use them for backup or go offgrid.  There are microgrid solutions for that, but it starts getting pricey.  I fell into a deal for another 25KW of panels with the goal of going offgrid and I'm going to have to figure out how to incorporate them.

[fourtytwo42]

Microinverters certainly make the system easier to work on; they're dead unless line power is supplied. 

I thought they were permanently connected to a panel! not exactly dead IMOP.

[bdunham7]

I thought they were permanently connected to a panel! not exactly dead IMOP.

They have no output unless the grid voltage is present.  How dead do you want them to be?  Obviously the DC side is still energized by the panel if there's light.

[Monkeh]

I thought they were permanently connected to a panel! not exactly dead IMOP.

They have no output unless the grid voltage is present.

Remind me how that's different to any other grid-tie inverter.

[nctnico]

I thought they were permanently connected to a panel! not exactly dead IMOP.

They have no output unless the grid voltage is present.  How dead do you want them to be?  Obviously the DC side is still energized by the panel if there's light.

One of the potential problems is other phases to keep receiving power which could lead to a big inbalance between phases if one phase gets interrupted. Over here a decoupling relay is mandatory for both single and multi-phase micro-inverter systems.

[bdunham7]

Remind me how that's different to any other grid-tie inverter.

The DC voltage present on a single microinverter, which at maximum is the OCV of the panel,  is low enough to not present a significant electrocution or arcing hazard and is confined to the area immediately under the panel with only two connections, even less if it is an integrated module.

The HVDC voltage present at the inputs of a central inverter in a string system is highly lethal and represents a significant arcing and fire hazard and it has to make its way down to that inverter via a long chain of wiring, connectors and often other devices.  And even if there is a roof-mounted HVDC disconnect, HVDC is still present in a significant part of the system.  HVDC is responsible for the vast majority of solar installation fires and the shock hazard is the motivation behind most of the cumbersome NFPA rules regarding solar installations in the US.

But I suspect you knew all that already? 

[bdunham7]

One of the potential problems is other phases to keep receiving power which could lead to a big inbalance between phases if one phase gets interrupted. Over here a decoupling relay is mandatory for both single and multi-phase micro-inverter systems.

The Enphase microinverters only see the phase they are on, so if you lose one phase those inverters will stop but the other ones will continue.  That is, unless the control system intervenes and stops them, which may well happen on newer systems.  Either way, if you lose a phase in a 3-phase system, the inverters wouldn't change the situation much.  The decoupling relays are clearly to satisfy bureaucrats that either don't understand or don't trust the Enphase system.  In large, complex systems a robust secondary disconnect system might make sense.  On a house, not so much.

[Monkeh]

Remind me how that's different to any other grid-tie inverter.

The DC voltage present on a single microinverter, which at maximum is the OCV of the panel,  is low enough to not present a significant electrocution or arcing hazard and is confined to the area immediately under the panel with only two connections, even less if it is an integrated module.

The HVDC voltage present at the inputs of a central inverter in a string system is highly lethal and represents a significant arcing and fire hazard and it has to make its way down to that inverter via a long chain of wiring, connectors and often other devices.  And even if there is a roof-mounted HVDC disconnect, HVDC is still present in a significant part of the system.  HVDC is responsible for the vast majority of solar installation fires and the shock hazard is the motivation behind most of the cumbersome NFPA rules regarding solar installations in the US.

But I suspect you knew all that already? 

I'm well aware of the not-HV DC side of matters, yes. And yet, the AC side behaves no differently.

[bdunham7]

I'm well aware of the not-HV DC side of matters, yes. And yet, the AC side behaves no differently.

My reply was to fourtytwo42's comment, which was a retort of sorts to PaulAm.  So outside of that context there's not much point to a debate.  On a microinverter system with the grid off you can take the system apart with your bare sweaty hands and are unlikely to suffer any injury, whereas with the string system you can easily electrocute yourself or burn  your house down.  I think that qualifies as 'easier to work on'.

[Monkeh]

I'm well aware of the not-HV DC side of matters, yes. And yet, the AC side behaves no differently.

My reply was to fourtytwo42's comment, which was a retort of sorts to PaulAm.  So outside of that context there's not much point to a debate.  On a microinverter system with the grid off you can take the system apart with your bare sweaty hands and are unlikely to suffer any injury, whereas with the string system you can easily electrocute yourself or burn  your house down.  I think that qualifies as 'easier to work on'.

In the context of working on the DC side of the system, yes. I was considering it from the AC side. Not that I would consider it 'easy' to electrocute myself or burn my house down if I had to touch the DC side of the inverter - it has a DC isolator, access to the panels is not 'easy', and it's a floating system. MC4 connectors have deeply recessed contacts, so it's quite easy to safely disassemble the string if I did gain access to the panels..

[fourtytwo42]

Using a tool to deliberately obtain contact with an otherwise touchproof MC4 connector requires about the same level of stupidity as using a tool to open a panel inverter to obtain contact with the high voltage inside. So the use of this argument in favor of panel inverters is completely spurious as is the case with much so called advise regarding solar power.

[Jeroen3]

Ease of installation is also a big plus of enphase systems. You only need to bring the TX8 drill with you for the railing. No cable assembly required on the roof!
If you're going for a DIY installation and you never crimp stuff, definitely get the micro's.

The number of places where you can make a mistake is reduced, and the severity of mistakes you can make is small.

[tszaboo]

Microinverters certainly make the system easier to work on; they're dead unless line power is supplied.  You don't have to worry about 300V DC from energized panels, but the trunk cables can be expensive.  I have a 10kw Enphase microinverter array and they've been working fine for a few years.

They do complicate things if you want to use them for backup or go offgrid.  There are microgrid solutions for that, but it starts getting pricey.  I fell into a deal for another 25KW of panels with the goal of going offgrid and I'm going to have to figure out how to incorporate them.

With the Solaredge DC optimizers, if you disconnect them, the open circuit voltage drops to 1V per panel. Until you connect them back to the inverter. They communicate continuously. They even passed the regulation, so you dont need the DC contractor for the system anymore.
And it is cheaper than those microinverters.
I was skeptical at first, probably you can even find my thread about them here. But then the more I read about them, the more technically convinced I got, and that's what I ended up installing.

[paleogizmo]

I believe that microinverters can be more cost-effective if it means you can hire an electrician who is not certified for working with high-voltage dc wiring (and therefore cheaper). Panel costs have gone down so much that your loss in efficiency from using a central inverter doesn't matter - put your cost savings into more panels. This is ignoring some pathological case such as your neighbor putting up a 3-story teardown that shades half your roof.

[nctnico]

Ease of installation is also a big plus of enphase systems. You only need to bring the TX8 drill with you for the railing. No cable assembly required on the roof!

Sounds great! I didn't know Enphase has wireless power modules these days...

All things considered I'm not sure what qualifies as safer. Either way you need cabling on the roof. The mains grid has an awfull lot of power behind it. Sure current can be limited by fuses but still it can push 3kW+ into something to cause a fire. Not to mention a large current surge ability. A string of solar panels can push a lot less power.

[bdunham7]

Sounds great! I didn't know Enphase has wireless power modules these days...

All things considered I'm not sure what qualifies as safer. Either way you need cabling on the roof. The mains grid has an awfull lot of power behind it. Sure current can be limited by fuses but still it can push 3kW+ into something to cause a fire. Not to mention a large current surge ability. A string of solar panels can push a lot less power.

You may not be familiar with the system, but what he is getting at is that there are no splices, combiner boxes or any other wiring assembly done on the roof.  The cabling comes preassembled, you plug in the modules and the other end goes back through a feed-through so that all of the splicing and connection is done inside.

As for 'what qualifies as safer', instead of speculating whether a 240VAC (think about the AC part) limited by a 20A breaker is less of a fire hazard than 800VDC limited to ~8-10A typical with no off switch, we can just look at the incident rate for installed systems over the past decade or so.  I think there are three reasonable conclusions:

1) Most if not all of the fires have been in HVDC systems.  That includes all the burnt Wal-Marts as well as small burnt-up things like Dave Jones' isolator box.

2) The overall reliability rate of each type is not readily ascertainable, at least not to me.  Neither is perfect.  Enphase had a whole batch of unreliable inverters early on and apparently you've seen some issues with the contactor-thingies.  OTOH, when I installed my system installers were routinely replacing the HVDC inverters which failed fairly often.  Most systems that I see or hear of nowadays that aren't working are plain string systems, but that's a small, localized sample.

3) Either type of system works pretty well if you manage to avoid the defective components and the installer does a good job.

[nctnico]

I agree. Without proper statistics you can't tell what is safer or not. It is very likely that there are more DC systems out there versus micro-inverter so in absolute numbers you'll likely see more failure from DC systems. But that doesn't mean they are relatively more toublesome. Also add to the equation that a lot of (older) solar systems have been installed by non-certified engineers and there is a lack of regulations / certifications where it comes to installing solar systems.

A plug & play system like the micro-inverters offer does take away some of the complexity of installing a solar system properly but OTOH you can do the same using pre-made wires for DC systems.

[kaz911]

My setup is now Enphase as well.

I would say - IF you have a "complex" setup - I prefer Micro Inverter. So if you have panels scattered around for both SE/SW or S/W etc. it is by far the best solution. But if you have an easy to access ground based installation and you don't plan on changing it a lot - then DC Inverter is probably the "best". But I would still go MicroInverter.

DC Inverters are most efficient at close to their rated input range. So if you have "split" systems you are rarely running fully optimised unless you run multiple inverters. But with Micro Inverters you are. Then each inverter outputs to the best of its abilities and the panels abilities.

Right now I'm testing different panel setups for our "re-design" of our house. So I'm "pitching" a few panels against each other before I make the decision. Right now I have a Q-CELL up against a Panasonic HIT and plan to add 2-3 more to get some good data. The Q-CELL is about 1/2 the price of the Panasonic HIT - and the price difference more than pays for the micro inverter. I wanted to test the Panasonic as it is said to be "much better" in low light. It is better - but enough to warrant the price difference (so far - lets see what winter brings)

Our plan is to "over dimension" the setup enough to have surplus to charge the car and house battery.

My plan is for energy independence - not necessarily "<10y capital return" - we are building the house to be cheap to live in "post pension age" so we can afford to stay and operate the house 30 years from now.

And Enphase can run off-grid with the correct setup both in US and UK. Don't know the regulation elsewhere.