single inverters vs micro-inverters

[Brewhound]

I have had 8 estimates on pv systems for my home and I am definitely going to get a system. I still have one large question, however, and I hope someone might share some wisdom with me to answer it:
Typically most solar installers connect the panels in series to a single inverter. However, a few installers (including Suntricity out of Delaware) will use a micro-inverter for each panel. The claim is that the micro-inverters are 5 to 20% more efficient than using a single inverter. Also, if there is shading from a tree or a cloud, using a single inverter will not be as effective as micro-inverters - all of the panels will only produce as much electricity as the shaded ones, where with micro-inverters each panel produces electricity independently. I've asked other installers if they would consider using micro-inverters in their systems - most say no, but a few said they would at a greater cost. The companies are telling me that Enphase (the micro-inverter company) has only been producing the micro-inverters for about 4 years now and since the technology (as well as the company) are relatively new, they are not positive that they will be successful. Thus, the companies charge more in case they have to come out and replace failing micro-inverters. The company warranties the micro-inverters for 15 years, versus most single inverters warrantied at 10 years.
I want to make sure I get the most for my money and I will not be happy if I pass up the micro-inverter technology if it becomes the norm in the next few years. At the same time, I don't want to take any unnecessary risks with unproven technology. Does anyone know anything about the micro-inverter technology? Can you make a recommendation? Any help is greatly appreciated!

[Jason]

Hi Brewhound,

Welcome and thank you for registering to ask your question.

Here are a couple links that may be of interest to you until you get some replies in this thread:

http://solarpaneltalk.com/showthread.php?t=256&highlight=enphase

http://solarpaneltalk.com/showthread.php?t=1127&highlight=enphase

[Sunking]

Brewhound sounds like you are getting the jest of it or the pros and cons.

As stated the micro inverter route is a bit more efficient, and with each panel having its own inverter gets around some shading issues.

On the flip side though is they are more expensive, more complexity, and one thing you might not have thought of yet, monitoring fees and software.

[TekArt]

I installed my own Enphase system last year and have nothing but good things to say about the product and the company. They are growing so fast that they can't keep up with orders. I'm sure they will be around for a LONG time.

The installation is MUCH quicker and simpler than central inverter systems so don't let installers charge extra, they are saving labor big time. The equipment is totally reliable and they are very responsive to small production issues. Their tech support is excellent and very friendly.

You can see my detailed blog about my Enphase installation here:

http://www.arttec.net/SolarPower/index.htm

It includes a web cam view of the panels and real-time energy stats.

I have just finished the final draft of an article for the next (Mar/Apr) issue of Home Power magazine that is based on my blog. I highly recommend subscribing to Home Power:

http://homepower.com/home/

You can learn a lot about sustainable energy systems from them and it's all written by hands-on folks like me.

[Brewhound]

Regarding the monitoring of the Enphase micro-inverters: one thing I failed to mention was that the installer, Suntricity out of Delaware, offers free 15 year monitoring of your PV system - and the customer has free access during this time to see the performance as well. I'm thinking I'm going to go this route!

[Mike90250]

Regarding the monitoring of the Enphase micro-inverters: one thing I failed to mention was that the installer, Suntricity out of Delaware, offers free 15 year monitoring of your PV system - and the customer has free access during this time to see the performance as well. I'm thinking I'm going to go this route!

Well, about time. Usually, your data is held hostage, and you have no idea if it's working or not.

[Mike90250]

.....The installation is MUCH quicker and simpler than central inverter systems so don't let installers charge extra, they are saving labor big time. ...

Roof racks - needed for both, and rack grounding system.

Still have to connect all the panels together. DC has MC connectors that are weather proof, snap together. About as easy as it gets. How are AC cables easier?

DC combiner box.
scheme to combine all the AC cords into a single conduit. (AC or DC, both need it)

Conduit from rooftop to ground level (AC or DC, both need it)

DC disconnect Are the micro-inverters to code for areas that require a DC disconnect?

Maybe you don't have to lift a 50lb inverter onto a wall bracket. micro wins
What's involved if a inverter need a replacement? DC wins here.

AC disconnect Same for both systems.

I guess I don't see the "big advantage" you state for roof top micro inverters.

Central systems can be monitored with ethernet, modbus or RS232

roof top micro inverters monitored with Central Service, Local interface box (converts the carrier current powerline signal to encrypted internet connection. Monitor fees vary. DC wins over this scheme.

What am I missing?

[TekArt]

WELL Suntricity is offering a deal on the monitoring - but they are just adding the cost to your bill. Enphase charges $2.00/inverter/year, it doesn't matter who you pay - you'll be paying the same price unless Suntricity is marking up that fee.

But the Enphase monitoring system is WAY better than central inverters because it monitors EACH PANEL individually. If anything goes wrong on a panel you know right away. It's actually really cool to watch the real-time data per panel as the sun comes up or sets - or shading issues that crop up. If you have snow you will see every clump of snow that has not been cleared. Cool!

Now try putting a few leaves the panels and watch your DC system shut down while the Enphase inverters just lose the panel that is shaded. You'll know exactly which panel by looking at the web page - from your iPhone in another state!

[Mike90250]

What is the Big Advantage for the low install costs?

re: scattered leaves or bird "artwork" yes, that's about the only advantage, is only the panels that have leafs, loose power. put a leaf on each panel, and it's even.

Each has some advantages, and dis-advantages.

[Sunking]

Enphase inverters cost just over $1.00 per watt compared to $0.50 per watt.

[fromport]

Mike, i really appreciate your lengthy/detailed answers on this forum.
You really seem to know a lot (more than me) about what you writing about.

Just wanted to give my feedback on some of your comments:

Still have to connect all the panels together. DC has MC connectors that are weather proof, snap together. About as easy as it gets. How are AC cables easier?

AC connecters are twist & turn and are also weather proof.
Not much difference here it seems.

DC combiner box.
scheme to combine all the AC cords into a single conduit. (AC or DC, both need it)

Not needed with microinverters. Although there is a maximal amount of panels in a string.
In a system that i helped install with 40 panels, we needed 3 strings.

Conduit from rooftop to ground level (AC or DC, both need it)

DC disconnect Are the micro-inverters to code for areas that require a DC disconnect?

Yes, absolutely.
Fireinspectors like microinverters even better. When they disconnect AC, the maximum voltate on the roof they can encounter is the voltage of one panel vs string voltage on a DC system, up to 450 volt of DC while you are spraying water...
Even when the DC disconnect is engaged near the big inverter inverter downstairs.

Maybe you don't have to lift a 50lb inverter onto a wall bracket. micro wins
What's involved if a inverter need a replacement? DC wins here.

AC disconnect Same for both systems.

I guess I don't see the "big advantage" you state for roof top micro inverters.

Central systems can be monitored with ethernet, modbus or RS232

roof top micro inverters monitored with Central Service, Local interface box (converts the carrier current powerline signal to encrypted internet connection. Monitor fees vary. DC wins over this scheme.

What am I missing?

Let me explain my setup: I have 2 systems:
2 x 3strings of 165Watt panels = 990Watt DC on a conventional DC system with batteries & grid tie
and
5 enphase inverters (4 x 175 watt & 1 x 165 watt = 865 Watt DC )
I meter both systems.
Last week on a nice and sunny day the DC setup fed 4 kWh back into the grid.
The 5 enphase produced 4.3 kWh into the grid.
More power with less (panel) power.
The conventional unit draws 20 Watt during the night from the mains, the enphase inverters (when in sleep) consume less than 30 mW each ( 0.15 watt in my setup)
In my setup i have a lot of shading in the winter months, so this is a clear advantage for the micro inverters compared to the string setup. Maybe it equal out in a few months from now.

I am fairly new to the solar (install) business (2 years). Having installed and also owning both systems, _i_ prefer the enphase over the conventional setup.
Installation takes a bit more time/material and price/watt is bit more expensive but over a 10+ year time i think that will easily compensate due to higher efficiency (in my setup)
I love the monitoring but $2 per inverter per year is a about 15 kwh per panel/inverter to compensate for the monitoring. That's half a month in summer. That's (imho) way to much. But that's why one of my hobby projects is to get to all the info myself.
Things are looking good so far.
Of course if you intend to have battery backup system there is no replacement for a DC setup. But for a lot of people with grid tie only system the micro inverter could be a pretty good alternative imho.
But it's also matter of taste, and that can be different for each individual.

[Mike90250]

there is a new report out

http://www.renewableenergyworld.com/rea/blog/post/2010/04/micro-inverter-misconceptions?cmpid=rss

about central / micro inverters

[dave]

Report by RenewableEnergyWorld

I read the report with the Truth vs Myth - First of all I am not an expert BUT
The comments he made on the two technologies do not appear to reference any particular devices - He appears to be more familiar with the HVDC devices then the micro inverters. He assumes that everybody is going to monitor there solar panels like their weather stations = too much rain over here - not enough there. He assumes all or nothing criteria - If one fails they will all fail and the manufacturer will MAKE SURE that they cannot be repaired.
I would not buy anything wrapped in epoxy either - but without stating which ones and what percentage most would be at a loss to comment = I have not seen one yet.

[coffeepodman]

Interesting discussion

I have been comparing different inverter systems lately and have made a few discoveries ... As mentioned throughout these blogs there is a statement regarding the higher costs of Micro Inverters .. this does not appear to be true in the real world ... do your pricing people ... I also don't want to discuss the MTBF issue ... if it's going to break it's going to break ! I just don't want to have to pay for half a 10kw inverter system when I can have a micro inverter on a shelf in the shed ... Warranty for MI is longer than central inverters and it's not because you need it .. it's because you don't ..why do you think someone puts a longer warranty on something .. it's not because they think it's going to crap out .. it's because they have faith in the fact it won't !

[russ]

İ didn't realize 'faith' was an engineering term?

They have done their best analysis that says (either or both):
1) Proper engineering analysis has been done - now we hope and pray
2) This is what we have to have to get into the market and by the time the failures start to come we will have come up with a defense.

Warranties only count from companies that still exist at (after) the time of failure. That excludes probably 90% of the companies in the solar field today.