Hi,
A question from a total novice.
We live in CA and are about to get solar. We will basically be selling into the grid. We plan on 4 arrays of 8 panels each. 2 arrays will be on a south facing roof and 2 arrays on a west facing roof. We have received a couple bids and one company is telling us that we need 2 inverters while the other company is saying that one inverter is enough. This will be about a 5000KWH system.

The company that says we need one inverter is basically saying that the system will have 4 strings of 8 panels. If any one panel is in shade, the output of the whole string will be compromised (in some complicated, non-linear way). This part I believe. They also said that if one string is compromised (because it is in shade) , that where the 4 strings come together is a parallel circuit, and it is additive, so all 4 strings will not be compromised.

The other company says that when some of the panels are in max sun (say the south facing panels at 10 in the morning), the west panels will be at lower voltage (because they are getting different sun access), therefore the inverter will be less efficient due to the unbalanced power from the separate arrays. This company claims we need two inverters...one for the west facing panels, one for the east facing panels.

The confusing part of all of this, is that both companies are using solar panels and inverters from the same manufacturer...

Any thoughts? Do we need to get one inverter or two?

 

this might not be very helpful, but i would recommend calling the manufacturer and asking them. without knowing what features are built into the inverter, i wouldn't know which contractor is right.

 

Sign up and post your question over at the Northern Arizona Wind & Sun forums. Lots of solar/inverter experts over there.

http://www.wind-sun.com/ForumVB/index.php

creakndale

 

This will be about a 5000KWH system.

5000KWh over what period of time? A year?

The company that says we need one inverter is basically saying that the system will have 4 strings of 8 panels. If any one panel is in shade, the output of the whole string will be compromised (in some complicated, non-linear way). This part I believe. They also said that if one string is compromised (because it is in shade) , that where the 4 strings come together is a parallel circuit, and it is additive, so all 4 strings will not be compromised.

This is all correct. There are some passive components in the combiner box called "steering diodes" which prevent non-shaded strings from being loaded down.

The other company says that when some of the panels are in max sun (say the south facing panels at 10 in the morning), the west panels will be at lower voltage (because they are getting different sun access),

Any module exposed to more than 200 Watts of light will operate at rated voltage - no less and no more. It is current which increases as light increases. If the Western-facing panels (or Southern ones near sunset) should be under-voltage due to less than 200W of light on them, those nifty steering diodes will still do their job! "Shade" is nothing but the absence of light, after all.

Now some vital questions for your second bidder: Are they planning to charge more than a dollar a watt for the two inverters? Are they planning to sell you two inverters of the same size the first contractor would sell? Or are they each "half-sized?"

 

"This will be about a 5000KWH system."

So it's a 5 megawatt/hr home solar system? I think that is the breakthough that we have been waiting for.

Lefty