What is a Solar PV Module?

A solar cell converts sunlight directly into direct-current electricity, but one cell provides limited voltage and power. A module packages an interconnected group of PV solar cells behind protective materials and provides a practical output voltage. Modules can operate individually or connect with other modules to form an array. The array, mounting hardware, wiring and power electronics form part of a complete solar electric power system.
An individual PV cell is therefore the conversion device, while the module is the packaged field unit. Its rated power depends on cell technology, cell area, cell count and electrical configuration. Modern modules are not limited to one fixed wattage range.

Cells connected in series carry the same current and their voltages add. Manufacturers use series strings to reach the module’s intended operating voltage. Parallel strings can increase current, although the exact internal arrangement depends on the module design.

For an idealised example, six cells that each operate at 0.5 V would provide about 3 V in series. Actual voltage changes with irradiance, cell temperature, load and cell technology, so designers use the manufacturer’s I-V data rather than a fixed per-cell value.

Ratings of Solar Module

Solar module output changes with irradiance, spectrum and cell temperature. Standard Test Conditions provide a common rating basis: 1,000 W/m² irradiance, an AM1.5 reference spectrum and 25°C cell temperature. A data sheet normally lists open-circuit voltage (V_oc), short-circuit current (I_sc) and maximum rated power (W_p) at these conditions.

The values V_oc, I_sc and W_p describe measured operating points; they are not a statement that all three occur at once or that the module can deliver them under every condition. The standard uses a cell temperature of 25^oC and irradiance of 1,000 W/m².
These values, together with the reference spectrum, define Standard Test Conditions at 25^oC and 1,000 W/m².
Outdoor conditions rarely remain at STC. Module output changes through the day with sunlight, shading, module temperature, angle of incidence and other site conditions.

V-I Characteristic of Solar Module

An I-V curve plots terminal voltage on the horizontal axis and currents on the vertical axis for one stated irradiance and cell temperature. Each point shows the current available at a particular operating voltage. The curve runs from the short-circuit point at zero voltage to the open-circuit point at zero current, with the maximum-power point between them.

Short Circuit Current of PV Module

Short-circuit current is the current measured when the module terminals are connected through a near-zero-resistance path, so terminal voltage is approximately zero. The data-sheet symbol is Isc. This test point requires suitable equipment and safe procedures.
Isc usually rises with irradiance and also depends on illuminated area, temperature, spectrum and cell technology. A larger value alone does not mean that one module is better because modules can differ in area, voltage and power rating.
For cell comparisons, current density is written as J_sc.
Hence,
Here, A is the illuminated area under the stated conditions, including irradiance of 1,000 W/m². Use the module data sheet when comparing complete products.

Open Circuit Voltage (V_oc)

Open-circuit voltage, written as V_oc, is measured while the module supplies no external current. It marks the zero-current end of the I-V curve. Its value depends on cell technology, the number of series-connected cells, irradiance and cell temperature. A higher value alone does not show superior quality because modules are designed for different system voltages.

Maximum Power Point

The maximum-power point is the voltage-current pair that gives the greatest electrical power under the stated test conditions. Rated maximum power is commonly written as W_m or W_p.
A module’s operating point follows its I-V curve between V_oc and I_sc, but the connected load or power converter determines the actual voltage and current.
Electrical power equals voltage multiplied by current. At short circuit, voltage is approximately zero, so output power is also approximately zero even though current reaches I_sc. The current at maximum power is I_m.
At the open-circuit point, a solar cell supplies approximately zero current, so output power is again approximately zero even though voltage reaches V_oc. The voltage at maximum power is V_m. Maximum module power is therefore

The current and voltage at this point are called the current and voltage at maximum power. A maximum-power-point tracker adjusts the electrical load as irradiance and temperature change.

Fill Factor of a Solar Module

The fill factor compares maximum power (P_m = V_m x I_m) with the product of open-circuit voltage (V_oc) and short-circuit current (I_sc).

A larger fill factor gives the I-V curve a squarer shape and indicates that more of the Voc × Isc product is available at the maximum-power point. Compare fill factor only under the same test conditions.

Efficiency of Solar Module

Module efficiency is the ratio of maximum electrical output power to the optical power incident on the module’s stated area. At STC, irradiance is 1,000 W/m², so incident power is 1,000A W when A is the area in square metres.
Therefore, efficiency,

Number of Cells in Module

The cell count and wiring arrangement depend on the module’s required voltage, current, power and physical layout. Older 36-cell crystalline-silicon modules were commonly designed to charge a nominal 12 V battery through suitable control equipment. Modern modules use many different formats, and the data-sheet maximum-power voltage V_m must be matched to the system.
The number of series-connected solar cells mainly sets module voltage, while parallel paths affect current capacity. The resulting open-circuit voltage V_oc is the sum of the active series elements under the stated conditions.
V_oc also changes with cell technology, temperature and irradiance. The table summarises why cell voltage cannot be reduced to one universal value.

A crystalline-silicon cell has a design-dependent open-circuit voltage, so a module calculation must use the manufacturer’s cell or module data. The rated V_oc applies at 25^oC cell temperature. Above 25^oC, crystalline-silicon voltage drops, but the change is specified as a temperature coefficient rather than one fixed 0.08 V value.
The module voltage at an expected operating temperature must therefore be calculated from its data-sheet coefficient.

A 15 V open-circuit rating is not a current universal module standard.
For a proposed design, the required number of series solar cells follows from the target voltage and the cell voltage at the design conditions.

A 36-cell layout is one established battery-charging format, not a requirement for every solar module.