A solar panel turns light directly into electricity, with no moving parts, no fuel, and no combustion — just a semiconductor at work when photons land on it. Understanding how matters for a practical reason: it explains why the same panel produces very different amounts of power on a cold bright morning and a hot hazy afternoon, and why its real output is almost always below the wattage printed on the back.

This article builds the mechanism from the ground up, then uses it to read a panel's rating honestly. It pairs well with energy vs. power and AC vs. DC, since a panel is a DC source.

The photovoltaic effect

A solar cell is a thin sandwich of semiconductor — usually silicon — arranged as a p-n junction: a layer with a slight surplus of mobile electrons (n-type) joined to a layer with a slight deficit (p-type). Where they meet, a permanent internal electric field forms across the junction.

When light strikes the cell, some photons are absorbed and hand their energy to the semiconductor, knocking electrons loose and creating electron-hole pairs — a free negative charge and the positive "hole" it left behind. The junction's built-in field then does the crucial work: it sweeps the freed electrons to the n-side and the holes to the p-side. As an academic photovoltaics reference puts it, "the collection of light-generated carriers by the p-n junction causes a movement of electrons to the n-type side and holes to the p-type side of the junction." That separation of charge builds a voltage across the cell.

Connect the two sides through an external circuit and the electrons flow through it — doing useful work on the way — before recombining on the other side. That flow is an electric current, and because it always moves in one direction, a solar cell produces direct current (DC). The cell keeps generating for as long as light falls on it.

Voltage and current come from different things

The single most useful idea for reading a panel is that its voltage and its current are set by different physics:

  • Voltage comes from the junction itself. A single silicon cell produces only about 0.5–0.6 V, and that figure barely depends on how large the cell is. It is a property of the material, not the area.
  • Current comes from how many charge carriers the light frees, which depends on the cell's area and the light intensity falling on it. A bigger cell in brighter light makes more current.

That is why panels are built the way they are. To reach a useful voltage, many cells are wired in series inside a module (the panel you can hold): about 0.5 V per cell, stacked up. Modules are then combined into an array to reach the voltage and current a system needs. Keeping voltage-per-cell and current-per-area straight will save you from most confusion later.

Irradiance sets the current

The amount of sunlight landing on a surface is called irradiance, measured in watts per square metre (W/m²). Because current tracks the light, irradiance is the first thing that governs output: to a good first approximation, a cell's current rises in proportion to irradiance. Halve the light and you roughly halve the current, and so roughly halve the power. A panel at 500 W/m² produces on the order of half what it does at 1 000 W/m², all else equal. This is why a passing cloud drops output almost immediately, and why orientation and shading matter so much: they change how much light actually reaches the cells.

What the nameplate really means

A panel's rated wattage is not a promise of everyday output. It is measured under a single, standardized laboratory setting called Standard Test Conditions (STC), defined by manufacturers and the relevant IEC standards as:

  • 1 000 W/m² irradiance,
  • a cell temperature of 25 °C (77 °F), and
  • an AM1.5 spectrum (the sun's spectrum through a defined thickness of atmosphere).

STC exists so that panels from different makers can be compared on equal terms. But those exact conditions rarely occur where a panel actually works: outdoors, irradiance is usually below 1 000 W/m², and a module sitting in the sun commonly heats well past 25 °C — often to 50 °C (122 °F) or more. The nameplate is best understood as a ceiling measured in a lab, not a typical yield.

Temperature lowers the output

More light helps, but more heat hurts. As a silicon cell warms above 25 °C, its output falls, because the voltage each cell produces drops with temperature. For silicon:

  • open-circuit voltage falls by about 2.2 mV per °C per cell,
  • short-circuit current rises only slightly (about +0.06% per °C), and
  • maximum power drops by roughly 0.4–0.5% per °C.

Every datasheet states this as a Pmax temperature coefficient — one real module, for instance, lists −0.29%/°C. Put it to work on a 400 W (STC) panel whose cells reach 45 °C (113 °F), 20 °C above the rating point, using a typical −0.4%/°C:

Loss   = 0.4%/°C × 20 °C = 8%
Output ≈ 400 W × (1 − 0.08) = 368 W

Check it directly: 8% of 400 W = 32 W, and 400 − 32 = 368 W. So temperature alone has taken roughly 8% off the nameplate before you even account for below-1 000 W/m² irradiance, imperfect angle, or dust. It also explains a real-world quirk: a cold, clear winter day can produce more power per panel than a hot summer one, because the cells run cooler even if the sun is lower.

Voltage, current, and the operating point

A module does not produce one fixed voltage and current; it produces a range of combinations described by its current-voltage (I-V) curve. Two endpoints frame it:

  • Open-circuit voltage (Voc) — the voltage with no current drawn. Power here is zero (nothing is flowing).
  • Short-circuit current (Isc) — the current with the terminals shorted and no voltage. Power here is zero too.

Somewhere between them is the maximum power point (MPP), where voltage × current is greatest. A panel connected naively to a battery rarely sits at that sweet spot, which is exactly why charge controllers exist. A maximum-power-point-tracking (MPPT) controller continuously adjusts the operating point to keep the module near its MPP, extracting more energy than a simpler controller would — the subject of a later components article.

A quick safety note

A solar module is electrically live whenever light falls on it, and you cannot switch it off by unplugging the battery. Disconnecting the load does not de-energize the panel; it simply pushes it toward its open-circuit voltage. Series strings of modules can reach hazardous DC voltages. Treat panels as energized in any daylight, learn your string voltage before wiring anything, and leave array wiring, isolation, and roof work to the methods your local code and the manufacturer specify.

Key takeaways

  • A solar cell converts light to DC electricity via the photovoltaic effect: photons free charge carriers, and a p-n junction's field separates them to make a voltage.
  • Voltage comes from the junction (~0.5–0.6 V per silicon cell, area-independent); current comes from cell area and irradiance.
  • The nameplate is an STC lab figure (1 000 W/m², 25 °C, AM1.5); real output is usually lower.
  • More light raises output, but more heat lowers it (~0.4–0.5%/°C for silicon), and a module works best when kept near its maximum power point.

Where to go next