HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Solarseeker · October 2010

I am hoping a few of you experts are in a gracious mood to help this ignorant poster!

I am looking to install an 8.1 Kw system using 36 Sunpower 225 Watt panels.

One of my challenges is that I have some shading (The roof peak of the house next door) that gradually crosses over the bottom quarter of my roof from approximately December thru March.

Neither company offers microinverters.

Here are the proposed solutions:

1) Company A wants me to use one 7000 watt Inverter (basically an SMA badged as their own product) as they said this is the best option to maximize harvest according to Sunpower's proprietary software.

They said that they could break it down to four (4) strings so that the shading will only affect one of the four panel arrays which would effectively reduce only 25% of the harvest.

2) Company B says that it's better to use two 4000 Watt Inverters because they could use even more strings than the 7000 watt inverter which would minimize the impact of shading further.

My question is, all things being equal, which solution would you use?
I humbly thank you in advance.

BB. · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Solar Guppy is probably the gentleman that can give you exact numbers.

From what I understand, if you get one string substantially blocked (more than 10% of the panels are shaded), you would have to assume that they will simply no longer participate.

I am not sure that two 4kW inverter vs one 8kW inverter will make much of a difference... SMA are very good inverters/company--So that should not be an issue.

Solar Guppy is the only person here that has actually tested configurations like this to know for sure.

So, this reply is also bumping the thread so he can answer (if he chooses).

-Bill

Solar Guppy · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Partial shading = Use Multiple Inverters

Solarseeker · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

BB- Thank you for response and the courtesy of bumping this up for the board expert in this area to respond!

Solarseeker · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Hi Solar Guppy. I appreciate your taking the time to respond, but I am confused...I think? When you say "multipliable inverters", was that a spelling error meant to say "multiple inverters" or is there some brand/type of inverter called a "multipliable inverter?"

Thanks for the clarification!

jcgee88 · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Solarseeker wrote: »

I am looking to install an 8.1 Kw system using 36 Sunpower 225 Watt panels.

One of my challenges is that I have some shading (The roof peak of the house next door) that gradually crosses over the bottom quarter of my roof from approximately December thru March.

Neither company offers microinverters.

Here are the proposed solutions:

1) Company A wants me to use one 7000 watt Inverter (basically an SMA badged as their own product) as they said this is the best option to maximize harvest according to Sunpower's proprietary software.

They said that they could break it down to four (4) strings so that the shading will only affect one of the four panel arrays which would effectively reduce only 25% of the harvest.

2) Company B says that it's better to use two 4000 Watt Inverters because they could use even more strings than the 7000 watt inverter which would minimize the impact of shading further.

My question is, all things being equal, which solution would you use?

I would not claim to be an expert, but I went through a
similar thought process when I contracted and installed my
PV installation earlier this summer. My site's roof layout
forced me to multiple strings, has shading considerations,
and because of space constraints I looked very closely at
SunPower.

Microinverter architecture shines when you have shading
issues. With such architecture you get "n" strings, where
"n" is the number of PV panels you install. Thus, one or
more shaded panels have no effect on the other unshaded
panels.

Both your companies' solutions are in my view an attempt
to grossly replicate a microinverter approach using a central
inverter. Their solutions give you more than one string,
thus reducing the loss to "only" 25-50%.

You mention that your contractors don't do microinverters.
Some contractors will refuse to take a certain approach
that you might like, simply because it is out of their
comfort zone, as opposed to the solution not being a
good one. If that is true with your prospective contractors,
I would reach out to them and try to push their envelope
a bit. "Comfort zone" is not a valid justification for not
considering a potentially superior solution.

In this case, Enphase with SunPower is feasible (I have
a friend putting that combination in right now). However,
the relationship between those two companies cannot
be characterized as optimal, especially since SunPower
competes with Enphase in the inverter business (with
re-branded).

Here are some questions for you:

1. Why are you considering SunPower? They have an
advantage in being more efficient, thus smaller size,
at reportedly about $1/watt more cost. Are you space
constrained? If you are not space constrained, you
could divert the premium you pay for SP to do multi-
inverters.

2. How much leaway do you have on your budget? If you
go with two central inverters, that is typically more than
the cost for one inverter. If you were to consider
microinverters, 36 of them would probably be the most
costly solution (36 * $200/microinverter + $450 for
gateway) of the three alternatives.

--

For myself, I liked the Enphase approach not just because
it helped maximize harvest, but also because I can see
my array's performance down to the individual panel.
For example, I see how my furnace flue cap is shading one
panel up to around 9:30 am (and this tells me that I could
reduce the loss by installing a smaller flue cap).

John

BB. · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

And that was a typo--"Multiple Inverters"...

-Bill

Solarseeker · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

John, I may not know much about solar systems, but I know class when I experience it. You and the others that have responded are so considerate with your time and vast knowledge/experience. I can't thank you enough for your long and thoughtful response!

First of all, I do have a space issue, thus the decision to focus on Sunpower (and Sanyo). A couple of the competitors (one of which suggested microinvertors) suggested Solarworld as they said that they actually produce more harvest due to the architecture of the panel (Angled edges that reflect more of the light to capture in the panel) and showed me Photon magazine as the third party company that conducted the test to validate their claim. However, when I saw that Sunpower and Sanyo were not included in the competition, that made me leery about them in general, so I didnt go any further with them.

The other issue I had was that they could only put 6.6 KW on the South roof (with some shading) and place the other 2.8 KW on the "west side" of the house (with no shading). They said that even though I would lose some production by having panels on the west side of the house, the fact that I was installing a 9.2 Watt Solarworld system (Cost is $43,000 including microinvertors) it would generate power equal to or greater than the 8.2 Watt Sunpower system (Cost is $48,000 including two 4000 watt inverters) placed on the South side of the roof (with some shading).

But when I asked them to prove it, they could not give me numbers to substantiate their claim. They said their "experience" is what they are going by...

Now as for the shading issue, I asked the company that was proposing the 7000 Inverter to give me an actual copy of the solmetric projection for the six shots they took on six different points on my roof.

On this report, it only showed shading issues for December and January in one location out of the six. It showed a 65% for December and a 78% for February.
The rest of the year showed a 99% number.

When I asked the other company to show me their solmetric results, they said that they did not have them as they got "lost". So know I am inclined to believe that my shading issue is not as severe as I originally was led to believe.

Therefore, there proposal for one 7000 inverter does not seem as unreasonable. But, clearly, if we are talking about two inverters (Two 4000 inverters) for the same price as one 7000 inverter, it would seem that is the better option having read Guppy's response.

Having said that, would microinverters still be more preferable over the two 4000 inverters in this scenario? On one hand the Enphase was rated at 95.5% efficiency while the SMA inverters were rated at 96.8%. On the other hand I keep reading about the "extra boost" in power production (5-20% roughly) by using enphase microinverters over regular inverters regardless of shading. Did I understand this correctly?

I know you are spot on about "comfort zones". I look at it this way. If SMA, the giant in the inverter business not only goes on record as saying microinverters can be appropriate for the residential market, but also buys a microinverter company, that tells me that microinverters have the ability to be a sound solution. More companies will go this route if the microinverters are as good as advertized.

But again, from the solmetric numbers provided by the one company, do still think microinverters make a compelling choice in my scenario?

Solar Guppy · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Sorry about the typo ...

Enphase has yet to be proven long term, in general, engineers don't like the idea of having 40 little inverters to a single large inverter, the basics of probabilities would indicate you will have more failures that requires expensive labor to address on the 40 micro inverter configuration long before you would have to replace a central inverter that is simple to access. There is also the issue of adverse conditions, under a solar panel is simply the worst possible choice for an electronic device, regardless what spam the inverter manufactures post as proven research

In your specific case, I would recommend two central inverters as rather than a single larger one as you going to have two issues, different orientations and the partial shading of a strings.

With good design practices, this have minimal impact, if you know what will be shaded, Try and balance between the strings so that both strings get shaded at the same time ( panels in pairs if two strings ). It's also wise to put the west array on its own inverter.

My personal opinion is if you have shading bad enough to warrant an inverter per panel then it makes no economic sense to install PV in that location to begin with.

People drinking the kool-aide would appear to believe everything printed is fact from the micro-inverter manufactures ( harvest gains , expected life times , cost savings ) when in fact none of it is proven in the context of actual field results or even neutral testing sites, such as Sandia National Labs.

Sunnyboy is the number one manufacture in the world with 15+years of experience, a good, safe and proven solution. The only thing i would review is the string placement and wiring for best results for your location

SG

Solarseeker · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Hey, no problem on the spelling thing. I just wanted to make sure that I wasn't savvy to some new type of inverter LOL!

I want to thank you again for your unselfish contribution of your highly valued expertise. THANK YOU!

System2 · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

I have 32 SunPowers 215 panels with a slight morning and late after noon shade problem. The SunPower dealer installed two 4kw inverters.

When monitoring the two inverters you can see the production difference between the two inverters. The shade inverter will produce half what the full sun inverter does.

Look at the 8,9 and 4 o'clock hours in the avatar. The west inverter is yellow and the east inverter is green. Power consumption is blue.

mike95490 · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

So, what's shadeing the Panels ? building? your tree? city tree? neighbors tree? your garage ? It won't work right till you get rid of the shade.

niel · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

mike90045 wrote: »

So, what's shadeing the inverters ? building? your tree? city tree? neighbors tree? your garage ? It won't work right till you get rid of the shade.

i assume you meant to ask what's shading the pvs and not the inverters.

shade may keep inverters cooler and let them last longer, but pvs run on the sun.:D

dwh · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Heh.

I think he means that the PV has shading on one part in the morning, and another part in the afternoon so they split the array onto two inverters so in the morning, one is putting out full and in the afternoon the other.

EDIT: Oh, full sun PVs on one inverter and partial sun PV on other inverter. Now I get it.

BB. · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Are both GT inverters behind the same configuration/wattage of solar panels?

If so, the east string looks like it is under producing (assuming no sun) in the middle of the day.

-Bill

stephendv · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

One more option is to use a single SunnyBoy that has multiple MPPT trackers internally. I can't find the model number on the SMA america site, but on the european one the TL (transformerless) inverters bigger than 5000W support 2 internal MPPT trackers. This would give you the same effect as having 2 different inverters but at a better price.

stephendv · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

Just another thought here, for those that like the micro-inverter approach because of shading, why does it have to be an all or nothing decision? Why not put micro's on the few panels that will be shaded, and stick a central inverter on the rest?

Seems like the best of both worlds to me.

System2 · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

mike90045 wrote: »

So, what's shadeing the Panels ? building? your tree? city tree? neighbors tree? your garage ? It won't work right till you get rid of the shade.

The AM shade is caused by an existing palm tree. I just need to get it cut down. The west side shade is caused by a neighbors trees. He has two very large Liquid Amber trees that he has agreed to have the top 25% to 30% cut off. Of course at my expense.

I also need to remove a TV antenna that will start to cast a shadow on some panels mid day as the sun gets lower in the winter sky. The install is only 60 days old so we still have a to do list.

System2 · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

BB. wrote: »

Are both GT inverters behind the same configuration/wattage of solar panels?

If so, the east string looks like it is under producing (assuming no sun) in the middle of the day.

-Bill

when you roll over the bar graph, with the mouse, it display the watts produced. Mid day they are very close to being the same 2650watts and 2640 watts.

techntrek · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

stephendv wrote: »

Just another thought here, for those that like the micro-inverter approach because of shading, why does it have to be an all or nothing decision? Why not put micro's on the few panels that will be shaded, and stick a central inverter on the rest?

Interesting idea. Would also be useful if your central inverter was maxed out but had room for a few more panels on the roof and the service panel had some room for a few more amps.

solarix · October 2010

Re: HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

First, two 4000W inverters will put you over the backfeed rule for a 200A service.

8000 / 240 x 125% = 41.6A which is more than 20% of the 200A main breaker.

There may be ways around this but generally is a problem. On the other hand putting a 8kW array on a 7kW SMA is really not a problem either as you will rarely see more than 7kW actually produced. But if shading, or multiple orientations is a problem at all I would recommend the SolarEdge distributed inverter system which has the best system solution IMHO for grid-tie.

HELP!: One 7000 Watt SMA Inverter or two 4000 Watt SMA Inverters?

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