How a Solar PV System Works

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Here's one that matters more than its name lets on — How a Solar PV System Works. If you remember one thing, make it this: Modules make DC power whenever light hits them, the inverter turns it into AC the building and the grid can use, and everything else in the system — strings, disconnects, rapid shutdown, the interconnection — exists to move that power safely and shut it off when it must be. Do this right and you can read a one-line diagram and know what every box does.

The module

A PV cell is a semiconductor wafer, usually crystalline silicon, that produces a voltage when light strikes it. Cells are wired in series inside a module (what most people call a panel) behind tempered glass and a frame. A module's label lists its ratings at standard test conditions: open-circuit voltage (Voc), short-circuit current (Isc), maximum power voltage and current, and rated wattage. Two things trainees must internalize: output rises and falls with irradiance (how much light hits the glass), and voltage rises as temperature falls — a cold, bright morning is when a string's voltage is highest.

Strings and the DC side

Modules are connected in series into a string, which adds their voltages while the current stays the same. Strings may be paralleled to add current. The DC conductors — listed PV wire on the roof, then a wiring method in conduit where they enter the building — run to the inverter through a DC disconnect. NEC Article 690 limits DC system voltage to 600 volts on one- and two-family dwellings; commercial systems commonly run 1000 volts and utility-scale systems 1500 volts.

Inverters

The inverter converts DC to AC and is the brain of the system. Three approaches:

• String inverter — one inverter, usually at ground level, handles one or more strings. Simple and efficient, but one shaded module drags down its whole string.
• Microinverters — a small inverter under each module converts to AC on the roof. Each module runs independently, and there is no high-voltage DC on the roof.
• DC optimizers — electronics at each module condition the DC and feed a central inverter, giving module-level control with a string-style inverter. Microinverters and optimizers are module-level power electronics (MLPE), and they dominate residential work largely because of shade tolerance and rapid shutdown. Inverters are listed to UL 1741, and modern versions carry the grid-support functions required by IEEE 1547.

The AC side and the interconnection

AC from the inverter passes through an AC disconnect (utilities typically require one they can lock) to the point of interconnection — usually a dedicated breaker in the main panel (a load-side connection) or a tap between the meter and the main breaker (a supply-side connection). NEC Article 705 governs how power sources connect to a building's electrical system. Under net metering, the utility credits power the system exports; the exact rules vary by state and utility and are changing in many markets. No system may operate until the utility grants permission to operate.

Rapid shutdown and disconnects

Firefighters cannot turn off the sun. NEC 690.12 rapid shutdown requires that, when shutdown is initiated, controlled conductors outside the array boundary drop to 30 volts or less within 30 seconds, and conductors inside the array either meet a module-level limit or use a listed PV hazard control system. MLPE meets this at each module; string systems use shutdown devices on the roof. 690.13 and 690.15 require disconnecting means for the system and for isolating equipment, and labels under 690.56(C) tell responders where the shutdown is and what it does.

Batteries

A battery energy storage system (ESS) stores PV power for backup or self-consumption. It may be DC-coupled (sharing a hybrid inverter with the PV) or AC-coupled (its own inverter on the AC side). ESS installations fall under NEC Article 706, must be listed to UL 9540, and have location, clearance, and quantity limits in the fire code.

Going Deeper (Intermediate)

String design is where theory meets code. 690.7 requires the maximum DC voltage to be calculated at the lowest expected ambient temperature using the module's temperature coefficient — a string that is fine at 77°F can exceed the inverter's input limit, or 600 volts, on a cold morning. 690.8 sizes conductors and overcurrent protection using 125 percent of Isc, and conductor ampacity is adjusted for the rooftop temperatures PV wire actually sees. Designers also balance the DC-to-AC ratio: oversizing the array relative to the inverter captures more energy on average but "clips" output on the brightest days.

Advanced / Pro-Level

Pros understand the grid side: UL 1741 SB inverters certified to IEEE 1547-2018 ride through voltage and frequency disturbances and can be told by the utility to limit export, which is why utility applications ask for inverter model numbers and settings. They understand anti-islanding — the inverter shuts down when the grid goes down, so a grid-tied system without a battery provides no backup. They read a one-line diagram and immediately check module count against voltage limits, the breaker against the panel's 120 percent rule under 705.12, and the location of every disconnect against what the utility and AHJ will want to see.

Practice Challenge

A homeowner says, "The power went out last night and my solar did nothing. Is it broken?" The system is a grid-tied string inverter with no battery. What do you tell them? (Answer: it worked as designed — a listed grid-tied inverter must shut down when the grid fails (anti-islanding) to protect line workers and the grid; backup requires an energy storage system with a transfer or backup interface, which is a separate design under NEC 706 and the inverter's listing.)

In Practice

An installer who understands Voc and temperature catches a string that is one module too long before it reaches the roof. One who doesn't learns about it from an inverter fault code in January.

Common Mistakes to Avoid

• Forgetting that voltage rises in the cold and sizing strings at room-temperature ratings
• Treating the DC side as "off" because the inverter is off
• Confusing a grid-tied system with a backup system
• Choosing a string inverter on a shaded roof without optimizers

Takeaway: Modules make DC power whenever light hits them, the inverter turns it into AC the building and the grid can use, and everything else in the system — strings, disconnects, rapid shutdown, the interconnection — exists to move that power safely and shut it off when it must be.

Educational overview only — not hands-on installation or electrical training, a safety program, or a substitute for work under a licensed electrician or NABCEP-certified professional. Follow the NEC as adopted in your jurisdiction, the building and fire codes, OSHA, the manufacturers' instructions, and your utility's interconnection requirements.