Reference · How a solar system is built
Every part of a solar installation, and what it is for
Modules, inverters, batteries and energy storage, racking and trackers, and the balance of system — each drawn with its parts labelled, with a practical example of the scale it comes in, and a note on which of those parts the US compliance rules ask about.
Figures are typical, rounded values given for scale, not specifications. Nothing here is a regulatory number — those are published, dated and cited, on the rates and thresholds page.
The whole system at a glance
Sunlight becomes direct current in the modules, is collected and converted to alternating current, and is either used, stored, or sent to the grid. Every other part exists to hold, connect, protect or control that flow.
- 1Modules (panels) Turn light into DC electricity. Wired in series into strings.
- 2Racking or tracker Holds the modules at the right angle and survives wind, snow and decades of weather.
- 3DC wiring and combiner Collects the strings, fuses them, and carries the DC to the inverter.
- 4Inverter Converts DC to grid-ready AC and is the brain that keeps the array at its best operating point.
- 5Battery storage Stores energy to use after sunset, ride through outages, or shift when power is exported.
- 6Meter and main panel Where the building or site connects, and where import and export are measured.
- 7Grid connection The utility interconnection — with its own approvals and protection requirements.
A typical home system: 20–25 modules of around 400 W each (roughly 8–10 kW), one string inverter, and often a 10–15 kWh battery. A utility site scales the same picture up: thousands of modules per megawatt, central inverters, and containerised storage.
Every test on this platform works line by line through a bill of materials. The three tests ask different questions of the same list: who manufactured each part (prohibited foreign entity), where each part was made (domestic content), and what each imported part is classified as (tariffs).
Modules
A module is a laminated sandwich. The cells do the work; every other layer exists to keep water, air and mechanical stress away from them for 25 years or more.
Drag to rotate · hover a layer · slide to pull apart
- 1Frame and backrail Aluminium edge that stiffens the laminate and gives the clamps something to hold.
- 2Front glass Tempered, low-iron glass: lets light through, keeps hail and weather out.
- 3Encapsulant A polymer layer (EVA or POE) on each side of the cells, bonding the stack together and sealing it.
- 4Cells Thin wafers of silicon with a junction that turns light into current. The part that decides the module's power.
- 5Bus ribbons Metal strips that carry current from the cells to the junction box.
- 6Backsheet or back glass The rear barrier; glass-glass modules use a second pane instead of a polymer sheet.
- 7Junction box Sealed enclosure on the back where the ribbons terminate, with bypass diodes inside.
- 8Edge seals and pottants Sealants around the perimeter and inside the junction box against moisture ingress.
- 9Bypass diodes Inside the junction box: let current route around a shaded or failed group of cells so one shadow does not switch off the whole module.
A residential module of about 400–450 W measures roughly 1.7 m × 1.1 m and weighs about 20 kg. At around 450 W, a megawatt of DC capacity takes somewhere over two thousand modules.
The IRS elective safe-harbor tables break a module into exactly these parts — cells, front glass, encapsulant, backsheet or back glass, frame and backrail, junction box, bypass diodes, bus ribbons, edge seals, pottants — plus an allowance for production, and assign each a share of cost. For customs, cells are classified under HTS 8541.42 and assembled modules under 8541.43.
Inverters
Modules make direct current at a voltage that wanders with sunlight and temperature. The inverter turns that into alternating current at the exact voltage and frequency the grid demands, and it decides moment by moment how hard to load the array.
- 1Enclosure Weatherproof housing; for large units a skid or cabinet.
- 2DC inputs and MPPT Each maximum-power-point tracker adjusts its load so its strings deliver the most power available right now.
- 3DC-to-DC stage Power electronics that condition the incoming DC before conversion.
- 4DC-to-AC stage The switching bridge that synthesises the AC waveform. Both stages are printed circuit board assemblies (PCBAs).
- 5Thermal management Heat sinks and fans. Power electronics that run hot run short.
- 6AC output To the building's main panel, or through a transformer to the grid.
- 7Control board and monitoring Grid protection, communications, and the data that tells you how the plant is performing.
Three families: a string inverter (one box for a home or a block of an array), microinverters (one per module, common on shaded or complex roofs), and central inverters (megawatt-class units at utility sites). Arrays are usually sized with more DC than the inverter's AC rating — a DC-to-AC ratio of roughly 1.2 to 1.3 is common at utility scale — so the inverter runs full for more of the day.
The safe-harbor tables list an inverter's PCBAs, electrical parts, enclosure or skids and thermal management separately, with different breakdowns for ground-mount, rooftop string and rooftop microinverter (MLPE) systems.
Batteries and energy storage
A battery energy storage system is not one thing but a hierarchy: cells grouped into modules, modules stacked into racks, racks housed in an enclosure — with electronics at every level watching temperature, voltage and state of charge.
- 1Cell The basic electrochemical unit. Lithium iron phosphate (LFP) dominates stationary storage.
- 2Module A group of cells in a case with its own sensors and connections.
- 3Battery management system (BMS) Electronics that balance the cells and keep each one inside its safe voltage and temperature window.
- 4Rack Modules in series and parallel to reach the working voltage and capacity.
- 5Power conversion system (PCS) The bidirectional inverter: charges from DC or AC and discharges to AC.
- 6Energy management system (EMS) Decides when to charge and discharge — for self-consumption, backup, peak shaving or grid services.
- 7Thermal management Air or liquid cooling. Battery life and safety both depend on it.
- 8Fire detection and suppression Gas and smoke sensing, ventilation and suppression, required by fire codes for larger systems.
- 9Enclosure or container A wall-mounted cabinet at home; at utility scale, containers holding megawatt-hours each.
A home battery of 10–15 kWh will typically carry essential loads through a night. Lithium-ion systems return on the order of 85–90 % of the energy put into them. Utility storage arrives as pre-built containers, sited beside the inverters and interconnection.
Storage projects can qualify for the same clean-electricity credits, and the prohibited-foreign-entity test applies to them with their own thresholds. The domestic-content safe-harbor tables loaded on this platform cover solar PV only, so a storage project currently gets an indeterminate domestic-content result that names the missing table rather than a guess.
Racking and trackers
The structure is the part nobody photographs and everybody depends on. It has to hold the modules at the right angle, resist wind uplift and snow load, and do so for the life of the plant.
- 1Piles and foundations Driven steel piles, screws or ballast, depending on soil and site.
- 2Posts Vertical members carrying the structure above the ground.
- 3Rails and purlins Horizontal members the modules are clamped to.
- 4Clamps and structural fasteners Mid and end clamps, bolts and bonding hardware that hold modules and ground the frames.
- 5Torque tube On a tracker: the long rotating tube the modules ride on.
- 6Drive system and actuator Motor and gearbox that turn the tube to follow the sun through the day.
- 7Bearings Let the tube rotate on top of each post.
- 8Dampers Absorb wind-driven oscillation so the array does not flutter itself apart.
- 9Tracker controller Electronics that command the drive, including stowing flat when wind speeds rise.
- 10Roof attachments Flashed mounts or clamps that fix rooftop rails to rafters or standing seams without leaks.
Fixed-tilt racking is simpler and cheaper; a single-axis tracker follows the sun east to west and, in sunny locations, often adds on the order of 15–25 % more energy over a year. Rooftop systems use rails on flashed attachments, or rail-less clamps on standing-seam metal.
Trackers have their own line in the safe-harbor tables — torque tube, rails, drive system, actuator, controller, dampers, structural fasteners and production — and so does roof racking (rails and structural fasteners). Fixed-tilt ground racking is treated as steel or iron, under the separate steel-and-iron test rather than the manufactured-product percentage.
Balance of system
Everything that is not a module, inverter, battery or structure. Individually small; collectively a large share of the cost and most of the safety.
- 1Combiner box Brings strings together in parallel, with a fuse per string.
- 2DC disconnect A switch that isolates the array for maintenance or emergency.
- 3AC disconnect The same on the AC side; utilities usually require one they can lock.
- 4Transformer Steps the inverter's output up to the distribution or transmission voltage.
- 5Meter Measures energy delivered and, with net metering, energy taken from the grid.
- 6Grounding and bonding Electrodes and conductors that keep every metal part at earth potential.
- 7Monitoring and communications Data from inverters, meters and sensors to the plant controller and the owner.
- 8Weather station and sensors Irradiance and temperature, so performance can be judged against what the sun actually delivered.
- 9Cabling, conduit and trenching DC and AC conductors, connectors and their protection. On a large site, kilometres of it.
A residential system's BOS fits on one wall: disconnects, a combiner in the inverter, a meter and a breaker. A utility site adds medium-voltage transformers, switchgear, a substation and a supervisory control system.
Balance-of-system items are where sourcing questions most often go unrecorded. For the prohibited-foreign-entity test every manufactured item in the bill of materials counts, so the transformer and the cable need a maker on file just as the modules do.
Now put numbers to it
The parts named above are the lines of a bill of materials. The rest of this reference is about what the rules do with them.