Solar Panels Characteristics

Current, Voltage and Power Curves of a Solar (PV) Panel

Solar Illustration
Fig.1. Current, Voltage and Power Curves

A Current (I) versus Voltage (V) Curve of a PV / Solar Module (“I-V” Curve) shows the possible combinations of its current and voltage outputs. A typical I-V curve for a 12 V Module is shown at Fig. 1 above.

The power in a DC electrical circuit is the product of the voltage and the current. Mathematically,

Power (P) in Watts (W) = The Current (I) in Amperes (A) X the Voltage (V) in Volts (V) i.e. W = V X A

A Solar (PV) Cell or a Panel / Module produces its maximum current when there is no resistance in the circuit, i.e. when there is a short circuit between its Positive and Negative terminals. This maximum current is known as the Short Circuit Current and is abbreviated as Isc. When the Cell / Panel (Module) is shorted, the voltage in the circuit is zero.

Conversely, the maximum voltage occurs when there is a break in the circuit. This is called the Open Circuit Voltage (Voc). Under this condition, the resistance is infinitely high and there is no current, since the circuit is incomplete. Typical value of the open-circuit voltage is located about 0.5 – 0.6 V for Crystalline Cells and 0.6 – 0.9 V for Amorphous Cells. These two extremes in load resistance, and the whole range of conditions in between them, are depicted on the I-V Curve. Current, expressed in Amps, is on the vertical Y-axis. Voltage, in Volts, is on the horizontal X-axis.

The power available from a photovoltaic device at any point along the curve is just the product of Current (I) in Amps (A) and Voltage (V) in Volts (V) at that point and is expressed in Watts. At the short circuit current point, the power output is zero, since the voltage is zero. At the open circuit voltage point, the power output is also zero, but this time it is because the current is zero.

Maximum Power Point and Rated Power of Solar (PV) Panel

There is a point on the knee of the I-V Curve where the maximum power output is located and this point is called the Maximum Power Point (MPP). The voltage and current at this Maximum Power Point are designated as Vmp and Imp.

The values of Vmp and Imp can be estimated from Voc and Isc as follows: Vmp = (0.75 – 0.9) Voc Imp = (0.85 – 0.95) Isc

The rated power of the PV / Solar Module in Watts (Pmax) is derived from the above values of voltage Vmp and current Imp at this Maximum Power Point (MPP):

Rated power in Watts, Pmax = Vmp x Imp

Example of I-V Curve and Ratings of a 12 V Solar (PV) Panel

Solar Illustration
Fig.2. Example of I-V Curve and Ratings of a 12 V PV / Solar Panel

The I-V Curve for a typical 12 Volt PV / Solar Panel is shown at Fig.2 above

This Maximum Power Point in the example curve given above is where Vmp is 17 Volts, and the current Imp is 2.5 amps. Therefore, the rated or the maximum power Wmax in watts is 17 Volts times 2.5 Amps, or 42.5 Watts.

Standard Test Conditions (STC) for Specifying PV / Solar Modules

The I-V curve is also used to compare the performance of PV / Solar Modules. The curve is, therefore generated based on the performance under Standard Test Conditions (STC) of sunlight and device temperature of 25 °C. It assumes there is no shading on the device. Standard sunlight conditions on a clear day are assumed to be 1,000 Watts of solar energy per square meter (1000 W/m2 or 1 kW/m2). This is sometimes called one sun, or a peak sun. Less than one sun will reduce the current output of the PV device by a proportional amount. For example, if only one-half sun (500 W/m2) is available, the amount of output current is roughly cut in half.