Checking Numbers for an Off-Grid Backup Solar Solution
Hey all, thanks for all the helpful advice since I started following the sub. I've read through the stickied posts and run my numbers, but I've got some knowledge gaps and would like to have those with experience check my numbers before I buy cables and BOC so I'm not burning my house down.
The goal is to run a standard refrigerator for no fewer than 24 hours on a manually-switched backup (read: extension cable plugged into an inverter) as a starting point, with the eventual goal of whole-home grid tie-in. That second part is not what I am concerned with right now, and I plan to expand as needed once I have the small system running and managed.
Right now, I have:
4x 100W Thunderbolt Solar mobile PV panels with SAE connectors (got them at a solid deal, planning on getting SAE to MC4 adapters or cutting and crimping MC4 on there). 18VDC/100W/5.56A rated, 21.6VOC;
1x EPEVER Tracer4210AN G3 MPPT Solar Charge Controller. 12/24V 40A operation, max PV input of 92VOC, max charging power 1040W;
4x PowerSync LFP12.8-100G31 100Ah/12.8V LiFePO4 batteries;
1x Giandel PS-3000KAR-24 3000W/24v Pure Sine Wave inverter.
Load is expected to be 120vac at approximately 250W in your standard fridge duty cycles.
My intention is to run the batteries in series/parallel to end up with a 200Ah/24V system. This is because I'm trying to keep the battery cables at or under 6 AWG, which is the maximum physical size the MPPT can accept. I intend to run 8 AWG to the panels, which have a one-way cable run to the MPPT of 68 feet.
My missing information is this:
- Is it advisable to wire the panels in series/parallel to keep overall voltage underneath the MPPT input voltage max of 92VOC? This would get me to 43.2VOC at 11.12A, but sticking to pure series would get me uncomfortably close to that 92VOC limit at 86.4VOC.
- The charge controller has an output for load. Is it advisable to connect the inverter to this output, or should I connect it to the battery circuit via buss bar instead? I'm imagining the cables are going to need to be beefier than 6 AWG for this application, but I'm less than certain on the math.
- Is it advisable to connect two lugs to the battery posts for the sake of connecting the batteries, or would additional buss bars be preferred?
- Are DC disconnects or breakers the general standard between the MPPT and the batteries? Everyone has an opinion and I'm having trouble reading between the lines.
- Is there anything I've missed here? Any additional advice?
Thanks a lot, I know questions like this can be annoying when the information is out there, but having just enough experience to connect things and not enough to know when it's in a dangerous edge case is something I've learned about the hard way. Much appreciated!