Balance of system is everything on the invoice that is not a panel, an inverter, or a battery: the wire, breakers, disconnects, combiners, and attachment hardware that turn boxes into a working system. Cheap parts, expensive consequences. Each one has a single job, and knowing the job is what lets you read a one-line diagram, understand your own quote, and catch the missing part before install day.
Wire: the part that is really three parts
A solar job carries at least three different wire families. PV wire runs from the modules to the inverter's DC inputs, built to live in sunlight and heat on a roof; our house standard is 10 AWG copper minimum on string runs. Building wire in conduit handles the AC runs between the inverter, the service, and the panels. Battery cable is the short, fat DC link between the bank and the inverter, sized from the inverter maker's cable table, and on a big off-grid build that can mean 2/0 copper.
One standard rides every job we design: all conductors copper, no aluminum. That is a shop rule, tighter than code requires. How thick each wire must be (ampacity, derates, voltage drop) is its own deep subject with its own tables. The thing to carry from this guide is simpler: a conductor is always sized to a table, never to a guess.
Breakers and fuses: the sacrifice parts
An overcurrent protective device (a breaker or a fuse) exists to die before the wire does. Too much current makes heat; the breaker opens the circuit before the heat becomes a fire. On a real job they show up everywhere: a 90A breaker on the inverter's grid run, a 200A main in the transfer equipment, and a 250A breaker built right into the face of the 314Ah wallmount battery.
Some are parts you buy. Some ship inside the equipment. A materials list has to know which is which, because nothing burns a morning like driving to a job with a breaker the battery already had.
One wrinkle worth knowing: a string design can legitimately need no string fuses at all. With at most two strings per MPPT tracker, the current one string could backfeed into another stays under what the module itself can safely carry, so there is nothing for a fuse to protect. Overcurrent protection is a per-circuit decision with reasons, and sometimes the reason is "none needed."
Disconnects: the part for the person who comes later
A disconnect is a switch whose job is letting a human kill a circuit visibly and lock it off: the fire crew, the utility lineman, the electrician servicing the inverter ten years from now. A typical whole-home design carries a lockable, visible-break AC disconnect at the service, and many inverters build the PV disconnect into the unit itself. The concept to carry: a system must be shut-off-able by someone who has never seen it before, at the places they would look. Where disconnects are required on your job is a code question your permit set answers.
Combiners: many circuits become one
A combiner lands several circuits on one bus so a single fatter run continues on. The classic DC combiner has mostly vanished from residential work, because hybrid inverters land strings directly on their MPPT inputs. Three trackers and five string inputs on a FlexBOSS21 is a lot of combining for free.
The idea persists in two other places. On the AC side, an off-grid build with two inverters combines their outputs, one breaker each, in an aggregate panel that feeds the loads panel. And on the battery side, EG4's internal busbars parallel up to six wallmount units cable to cable; past six, an external fused busbar takes over, per the manual. Same concept in all three places: many into one, with protection at the joint.
The smalls, and why they decide your install day
Roof attachments, breakers, conduit fittings, grounding lugs, wire: the smalls are the least glamorous lines on a materials list and the most common reason an install stalls. The big boxes get tracked carefully because they cost thousands. An $8 part gets forgotten because it costs $8, and then the crew stands around while somebody drives to the supply house.
Any real rating that lands on a materials list (a breaker's amps, a wire's gauge, a disconnect's rating) comes from the job's design documents or the part's own datasheet. Nothing gets specced from memory, and an unknown is a field-verify line, never a guess. If you are reviewing your own quote, try annotating every small line with the part's one-sentence job: this breaker protects this run, this disconnect is for this person, these attachments hold this rail. Any line you cannot annotate is a question worth asking before install day. Build a system and the smalls come on the list from the start.
Sources
- EG4 WallMount Indoor 314Ah Spec Sheet, v1.0.4
- EG4 FlexBOSS21 Spec Sheet, v1.2.9