How many panels in a string?

[wpns]

Designing an add-on to my existing solar system so there are a lot of moving parts and options, but I'm having a lot of confusion over some of the available 'kit' systems I'm seeing online. [I'd like to get a complete kit of parts, so there's one supplier, but of course I can roll my own, plus or minus supply chain issues and even more moving parts.]

My strong preference is a string inverter, so as to avoid RFI issues with cheap, poorly engineered inverter-per-panel systems. I'm not sure how SolarEdge Power Optimizers work, so I don't know if they are going to have the same RFI issues, but that's a question for another post.

I'm seeing systems advertised which produce "watts per hour" and "KW per day", and my person favorite, systems rated in "WK", so those are easy to weed out. However, a few systems look OK until I add up the OCV on the strings and compare to the maximum input voltage ratings of the inverter, and discover they are 35% over the limit! Are they cheating and using the MPP of the panels? Are they just not engineering it right?

My understanding is that OCV of the panels times the number of panels in a string at the lowest operating temperature should be less than the maximum DC input of the inverter, no?

Thanks!

 

[sunshine_eggo]

My understanding is that OCV of the panels times the number of panels in a string at the lowest operating temperature should be less than the maximum DC input of the inverter, no?

Correct.

 

[zanydroid]

All power optimizers have DC DC converters in them, which are switching mode devices. They are series connected so they need to match the current along a string to avoid stranding generation.

SolarEdge optimizers I believe have a wide range of current matching capability, while TIGO is more limited. SE also has an MPPT in it while Tigo does not. There’s probably a bypass mode if the current matching needed is out of range.

Why do you need optimizers?

 

[wpns]

Why do you need optimizers?

I’m not sure if I do, they come with one of the kits I was looking at.

 

[zanydroid]

I’m not sure if I do, they come with one of the kits I was looking at.

Do you have complex shading or multiple roof planes?

If you have multiple roof planes each plane needs a separate MPPT, you can also just get more MPPT channels in the SCC or string inverter. Unless you have a lot of tiny roof planes, then optimizers or microinverters make more sense.

SolarEdge optimizers are proprietary and locks you into using their inverters. If the inverter supply dries up you have problems. In practice I think places where eBay is good, DIYers just buy some old inverters ones to keep around.

 

[wpns]

Do you have complex shading or multiple roof planes?

I have a 'garage' with east and west facing roofs, so two planes. I'm guessing that two separate 5-6KW systems are as good as one 10-12KW system with separate MPPT strings?

 

[zanydroid]

Depends. I suspect you will have higher standby losses with two separate entry level AIO from the fixed amount of power they burn. You also have higher system complexity from sync'ing two AC outputs, but the AIO will say if they support it. Supporting it and you being able to follow the instructions successfully / dodge or debug the extra failure modes that can happen are separate questions. You can see some issues on this forum of people with stacked inverters that stop talking to each other. At that point they need to rely on good tech support or their own understanding of how to reset the inverters and split the stack to get it working again.

OTOH if you have two separate systems you have more redundancy if one explodes. And if you have one system that blows up, you also need some combination of tech support and your wits to swap it out.

 

[wpns]

I have an SMA WR11TU20 grid tie inverter with a Sunny WebBox Ethernet to RS-485 interface, which works fine. I'm looking at adding a couple of SMA Sunny Boy 7.7-US-41-V2 inverters (one on each roof side). Looks like the 7.7 have built-in Ethernet interfaces, so they'll talk to the SMA portal separately. Will I need to link (stack?) the three inverters together, or can they each work independently and feed into the grid depending on the DC power from either of their arrays?
Thanks!