General guidelines for rooftop string design?

[dgeist]

Hi, all. I've been a "backyard" solar user for a few years (with a very happy off-grid tree house to show from it). I'm planning a system for the main house and will likely start out with a whole-house battery/inverter setup but just a few PV panels so I can qualify for some solar tax credits.

I'm looking at hybrid controllers/inverters which can do either AC or DC coupled and am perfectly comfortable doing the work in either domain. I'm thinking DC coupled since the primary purpose is really to charge the batteries and the size I'm thinking won't justify any crazy wire guages, etc.

Is there a recommended minimum number of panels for certain charge controller capabilities, perhaps a primer on string design (i.e. reasons for arranging stings in your setup certain ways to maximize wiring efficiency, avoid partial shading losses, etc?). I expect to have to re-wire things at some point as the system grows, but I'd like to get it decently configured initially so we get as much production as we can.

Also, I know current (and recent) NEC requires disconnect capabilities at microinverters. In the DC realm, does this equate to a disconnect at a roof-mounted PV box? Simple breakers/fuses? An active string optimizer with disconnect? Just trying to weed through the myriad of products out there to figure out what I actually NEED for a fairly small grid-tied setup.

 

[Hedges]

Rooftop probably requires AFCI and RSD.
For a DC coupled system, Midnight has SCC that support arc-fault. For AC, most GT PV inverters support that; don't know about the various off-brand hybrids.

RSD (and AFCI) are avoided by microinverters due to voltage and wattage of panel. For string inverters, RSD box per panel is needed. If AFCI causes RSD to disconnect it is probably more effective. DC coupled you may be able to avoid RSD with a lower voltage system (< 80V)

DC coupled SCC is more efficient at charging batteries. Any battery system likely wastes excess PV production part of the time, so maybe efficiency not so important (except off-grid in winter months?) AC coupled is most efficient for PV direct to loads or to grid.

I'm happy with AC coupled GT PV inverters and battery inverters which support them and act as UPS for grid connection. It is a larger system and more expensive. Make sure any GT PV inverters are UL-1741-SA with frequency-watts, so they AC couple nicely.

For string systems, all PV strings wired in parallel have to be same voltage. They can be different orientation. Slight differences in partial shading are OK (power/voltage curve is flat on top) but large percentage difference would cause extra losses. Full shading of one string (e.g. on other roof face) is fine.

For partial shading of multiple parallel strings, best if all get similar amounts of shading, so effectively become shorter strings.
Avoid hard shadows on part of a string when other parts have full sun - bypass diodes may not tolerate full current for an extended time.

Batteries are the most expensive part, so more economical to have an "open" design where various brands can be used instead of one captive to inverter manufacturer.