# Power Output of PV Panels - Part 1

Solar panels are available in many sizes, shapes, power ratings and voltages.

What determines the output power of a solar panel?

• The area of each cell in the panel;
• The number of cells in the panel;
• The amount of light falling on the solar panel;
• The angle that the light strikes the panel;
• The temperature of the solar panel.

What solar panels should we use?

• What are we using the solar panel for?  If using it for, charging batteries in a 12V system, 24V panels may not be the best choice. Other battery systems may have voltages of 24, 26, 48 or higher voltages.
• What is the voltage needed – for Grid feed, this is determined by the system design parameters and the chosen inverter, for battery systems this can be the battery terminal voltage
• If using a low voltage system and greater power is needed than one panel can provide, additional panels can be added in parallel.
• Grid connect systems usually operate with all panels in series, to minimise the current (and the wire size), and if the inverter can cope with higher voltages, it may be possible to install additional panel(s) in series.

Series Connections – Connecting the positive terminal of one cell to the negative terminal of the next cell.  The output is taken from the negative terminal of the first cell and the positive terminal of the second cell.  This adds the voltages of the cells in series, but the current available is the cell current. (Also called a “string” of cells).  Refer example below.

Parallel connections – Connecting the positive terminal of each cell to the positive terminal of the other cells and the negative terminal of the each cell to the negative terminal of the other cells cell.  The output is taken from any cell negative terminal and any cell positive terminal.  This adds the cell currents, but the voltage of the combination is the cell voltage.  Refer example below.

For example, five 0.5V, 1A solar cells can be arranged in series or parallel as shown below:
In addition, solar panels can be connected in series and parallel at the same time, to increase the power from the solar array.

Most silicon solar cells provide an open circuit voltage of around 0.5 volts. The current developed by solar cells depends on the area of the cell, larger cells can capture more light to convert to electricity.

## Measuring Panel Output

The power developed by a solar panel is not simple to measure, because the voltage varies with the amount of current drawn from the cell.  There is a point where the current is maximum for the voltage at the terminals. This is known as the Maximum Power Point of the panel.  This is the rated power of the panels.

A 120W solar panel can supply a maximum power of close to 120W in Laboratory conditions.  In real life conditions, it is rare for solar panels to sit at 25°C when in full sunshine, so the actual power output is usually significantly less than the rated power.

To determine the Maximum Power, measurements made in accordance with international standards and procedures wunder the following conditions:

• illumination of 1 kW/m2 (1 sun) at spectral distribution of AM 1.5 (ASTM E892 global spectral irradiance);
• cell temperature of 25°C.

A graph is made of the Voltage as Current is varied.

This is the Volts and Current for a BP 80W solar panel.

This panel has 36 silicon solar cells in series to provide a nominal voltage of 12V (open circuit voltage of 18 – 24 volts)

Maximum power occurs at a voltage of 217.6V (at 25°C cell temperature)

As can be seen, the Voltage is reasonably constant until the current gets too great, then the voltage falls rapidly as current is increased.

Also apparent from the graph is the significant impact of increasing temperature

The open circuit voltage drops from about 22 V at 25°C to about 18 V at 75°C.  The power available from the panel drops in proportion to the voltage, so you will lose significant output at high operating temperatures.

NOTE:  These temperatures are not the air temperature, but the cell temperature in the solar panel.

For further discussion on this topic see Power Output of PV Panels - Part 2.

References:
Wikipedia: http://en.wikipedia.org/wiki/Solar_cell