Two string inverters vs one?

[ocdave]

Hi all. I'm pretty much set to take the plunge into solar for my home. This forum has been a great resource! I have a question for which I couldn't find any existing threads on...

One of the vendors quoted me a 8.3kW DC system (Kyocera 245W panels) with two inverters while another quoted me a system with just one inverter (a Power One 8000TL-1 inverter). I was wondering what the pros and cons are for these two configurations. Any thoughts would be appreciated.

The system is 34 245W Kyocera panels in three "clusters". Two are oriented in the same direction (SE) and one is oriented towards SW if that is relevant.

Thanks in advance for your input.

Dave

[bando]

we just signed our contract last week and are getting 2 SMA string inverters because we have two arrays, one with panels on the West (SW) the other with panels on the South (SE).

due to the different orientations, they use separate inverters so that the different arrays aren't affected by one another. as you may know, in a string array the loss of output of one panel affects the output of the entire array so you don't want that to affect the second array on another part of your roof if at all possible.

i guess the con is that you have to replace two inverters rather than one.. but then again, another pro of having two is that if you have one inverter failure, the whole system won't be down either.

[ocdave]

we just signed our contract last week and are getting 2 SMA string inverters because we have two arrays, one with panels on the West (SW) the other with panels on the South (SE).

due to the different orientations, they use separate inverters so that the different arrays aren't affected by one another. as you may know, in a string array the loss of output of one panel affects the output of the entire array so you don't want that to affect the second array on another part of your roof if at all possible.

i guess the con is that you have to replace two inverters rather than one.. but then again, another pro of having two is that if you have one inverter failure, the whole system won't be down either.

Thanks for the input. But I was under the impression that some inverters (the Power One TL ones for example) can support two or more orientations on a single inverter. I guess not having a single point of failure is a pro but I'm wondering if the cost difference (guessing around $2k) worth it?

[Naptown]

Hi all. I'm pretty much set to take the plunge into solar for my home. This forum has been a great resource! I have a question for which I couldn't find any existing threads on...

One of the vendors quoted me a 8.3kW DC system (Kyocera 245W panels) with two inverters while another quoted me a system with just one inverter (a Power One 8000TL-1 inverter). I was wondering what the pros and cons are for these two configurations. Any thoughts would be appreciated.

The system is 34 245W Kyocera panels in three "clusters". Two are oriented in the same direction (SE) and one is oriented towards SW if that is relevant.

Thanks in advance for your input.

Dave

The biggest Power one i could find is a 6KW
SMA makes an 8000TL but it is single mppt tracking only. so all strings would need to be exactly equal which wont happen with 34 panels

[ocdave]

The biggest Power one i could find is a 6KW

Hmm. The quote says one Power One 8000TL-1 inverter, but when I google the model #, I can only find an SMA inverter with that model #. Maybe it was a mistake on the quote and it is really an SMA inverter.

[Volusiano]

SMA makes an 8000TL but it is single mppt tracking only. so all strings would need to be exactly equal which wont happen with 34 panels

Does any other SMA inverter model support multiple mppt tracking? Maybe some models do and some don't? I just vaguely remember that when I was considering splitting my configuration into 2 arrays on different sides of the roof (ended up deciding to put them on the same side after all), my installer said that some SMA models have this capability.

[Naptown]

Yes
the 3000,4000,5000 TL-US-22 have two mppt inputs.They also have the advantage of the auxillary outlet for limited power if the sun is shining enough.

[bando]

Thanks for the input. But I was under the impression that some inverters (the Power One TL ones for example) can support two or more orientations on a single inverter. I guess not having a single point of failure is a pro but I'm wondering if the cost difference (guessing around $2k) worth it?

you could be right; i'm not sure about Power One specifics regarding orientation.

we had various bids and the proposal that used Power One we still needed two inverters (PVI-5.0). I didn't ask them why though because I assumed it was the same reason as the SMA proposal (different array orientations), but our system is also 11.7 kW so i think we needed 2 inverters no matter what ...

[bando]

Yes
the 3000,4000,5000 TL-US-22 have two mppt inputs.They also have the advantage of the auxillary outlet for limited power if the sun is shining enough.

ah, good to know.

[Volusiano]

I guess not having a single point of failure is a pro but I'm wondering if the cost difference (guessing around $2k) worth it?

You can look at it either way. Not having a single point of failure can be a pro (if one is down, at least not the whole thing will be down), but if you look at it the other way, it can be a con, too (2 points of failure, having to do 2 replacements at 2 different times instead of 1). That would be the same pro and con argument in the micro-inverter case, too (but on the extreme). One micro is down, the rest still work. But now you have many many points of failure with potentially many service calls.

In terms of down time, it's the same either way, right? Assuming that each inverter will be down once in their warranty lifetime. If it takes 3 days to replace an inverter, your down time for 1 inverter configuration is 3 days for all panels, vs a 2-inverter configuration which is 3-days for half the panels, and at another time, 3 days for the other half -> same total down time in either cases. At least with 1 inverter configuration, you'd only have to schedule 1 service call for replacement.

Regarding the cost difference of $2K for 2 different inverters, my installer originally proposed an SMA 8000 for a 10.5KW STC configuration. I told them I didn't like this because it'll most likely clip, so they said that's fine, they can give me 2 smaller SMA inverters for no extra cost instead. I ended up with 1 SMA 4000TL and 1 SMA 7000 for an 11KW STC configuration and they only charged me the extra cost for the extra 2 250W panels only. I asked them wouldn't 2 smaller inverters cost more than a bigger one, they said the difference is small enough so that they won't bother charging me the difference. At the time I hadn't signed with them yet, of course, so they were also trying to win my bid so they probably didn't want to appear to nickel and dime me either.

[Naptown]

Both the power 1 and SMA TL-US support 2 mppt inputs
these can be used for different string sizes different orientations etc.

[inetdog]

You can look at it either way. Not having a single point of failure can be a pro (if one is down, at least not the whole thing will be down), but if you look at it the other way, it can be a con, too (2 points of failure, having to do 2 replacements at 2 different times instead of 1). That would be the same pro and con argument in the micro-inverter case, too (but on the extreme). One micro is down, the rest still work. But now you have many many points of failure with potentially many service calls.

In terms of down time, it's the same either way, right? Assuming that each inverter will be down once in their warranty lifetime. If it takes 3 days to replace an inverter, your down time for 1 inverter configuration is 3 days for all panels, vs a 2-inverter configuration which is 3-days for half the panels, and at another time, 3 days for the other half -> same total down time in either cases. At least with 1 inverter configuration, you'd only have to schedule 1 service call for replacement.

For a grid tied inverter, the only loss caused by downtime will be the production of the unit for the time it takes to get it working or replaced.
Redundancy has a much greater value for an off-grid installation where you will either go without power or have to spend a lot more money running a generator.

[ocdave]

For a grid tied inverter, the only loss caused by downtime will be the production of the unit for the time it takes to get it working or replaced.
Redundancy has a much greater value for an off-grid installation where you will either go without power or have to spend a lot more money running a generator.

So here is the answer I got back from the vendor on why 2 inverters are required...

Due to my system's size (8.3kW DC) and the fact that there are two orientations, two string inverters are required. The SMA 8000TL-1 only supports a single MPPT so it can't be used for my installation with 2 orientations.

[Naptown]

That and the 34 panels if I haven't confused threads are a problem

[inetdog]

So here is the answer I got back from the vendor on why 2 inverters are required...

Due to my system's size (8.3kW DC) and the fact that there are two orientations, two string inverters are required. The SMA 8000TL-1 only supports a single MPPT so it can't be used for my installation with 2 orientations.

If your two orientations have subarrays of equal size, without any partial shading of either of them during those hours when they would both be producing, then they could be put in parallel on one MPPT input. Xantrex and, I believe, SMA have published white papers on this subject based on both theory and experimental data.
I would look more closely at the geometry of your proposed installation to see whether you fit this scenario.