Name plate power output details

The capacity of a Solar PV Panel is as what is given in its name plate. The output will invariably be given in DC power output in watts. The watt output of an array of PV panels has to be converted to a practical unit which is kw (DC) and for this purpose the total watts (DC) of the PV module array is added up and divided by 1000.

Sadly this out put too does not give an indication of the ultimate power available to you for useful work due to several factors which derate the output.

Affect of location of the building and the name plate capacity

The name plate detail of generated DC power is measured at Standard Test Conditions (STC). Now this does not tally most of the time with the highly variable conditions of a particular locality of an average building.

The STC is laid down simply to arrange a common platform for the sake of convenient comparison. Strangely two different STCs can exist depending on the country and authority responsible. A widely used STC is the standards laid down by the NRFL laboratory of the US Department of Energy.

This standard specifies:

• A Solar Irradiance of 1000 watts/m2

• A module (panel) temperature of 25 0 C

• A default PV Panel surface area of 35 m 2

• A default DC Power output of 4 kw

Local Factors causing derating

The numerous component derate factors are as follows.

• PV module nameplate DC rating – The name plate details do not coincide with field measured readings due to degradation and other reasons.

• Combined Inverter and transformer efficiency – The conversion of DC to AC is a combined action of the inverter (which is a solid state device) and the transformer and naturally there are significant power losses during the path from the PV Modules to the building distribution board.

• Mismatch – The manufacturers of the PV modules indicate in their catalogues a performance tolerance. The performances of each module vary within the limits of tolerance and when interconnected may not produce the total name plate output.

• Diodes and connections – The diodes ensure the flow of current is unidirectional in the PV panels. Also the multitude of electrical connections imposes resistance causing energy losses. Both these contribute to derating of a system.

• DC wiring – These account for the resistive losses in DC wiring between the PV panel installation and the inverter.

• AC wiring-The derating occur due to losses taking place in the AC wiring between the inverter and the grid connection.

• Soiling – Dirt such as dust, snow, bird droppings, etc settling on the PV modules derate the performance of the installation.

• System availability –The down time of the PV array and that of the utility authorities due to maintenance etc. affect the output of the system.

• Shading – The PV array performance is easily affected by surrounding buildings, vegetation etc.

• Sun-tracking –When a tracking mechanism (generally not used in small capacity installations due to cost) is employed, the precise orientation of the Solar Modules is important to maintain the optimum efficiency.

• Age - Over the years the efficiency of a PV array is affected and in about 10 years of operation a derating of about 10% may occur.