Clean Energy Council (“CEC”) design guidelines specify a number of derating factors that we must consider when designing an installation. These must be taken into consideration so that you will have a realistic understanding of just how much energy your new solar array will produce once installed. It is very important to understand that the headline figure arrived at by simply multiplying the number of panels by the wattage, will NOT be the amount produced and exported to the grid.
In order to ensure that actual energy production will meet or exceed your expectations and requirements, it is essential that we, as designers, make our calculations based on these factors. See below for what we need to take into account:-
The inverter efficiency is obtained from the manufacturer and with the inverter types we recommend and install, typically will vary between 94% and 96% .
Designers of electrical systems always need to consider Voltage Drop. This occurs whenever electricity travels along a cable and it is caused by the resistance of the cable. Effectively, the voltage going into a cable will gradually decrease as it travels along it. To counteract this effect, the length and type of cable needs to be considered. Put simply, as the length of a cable increases, so will its size.
In a grid connect solar power system, voltage drop will occur between the array of panels and the inverter (the DC side) and between the Inverter and the switchboard (the AC side). This will be reflected as a loss in power from system.
This loss should therefore be as small as possible but in any event the maximum allowable under Australian Standards is 5% and CEC Guidelines is 1%. ESP Projects always uses appropriately sized AC and Solar DC cable to ensure this figure is below the prescribed maximum.
The output of a PV module is specified in watts, with a manufacturing tolerance of (normally) +/- 3 to 5%. and is based on a cell temperature of 25 degrees C.
EG : We are installing a typical 260W PV module, the actual output will be in the range of 252-267W (ie 260w +/- 3%). Our calculations are always based on the minimum amount to ensure we establish a conservative baseline for actual output.
The output of a PV module can be reduced as a result of a build up of dirt on the surface of the module. The actual value of this derating will be dependent on the actual location but in some city locations this could be high due to the amount of pollution in the air. Clean Energy Council guidelines recommend the use of a derating factor of 3 to 5% .
All panel output figures are confirmed under Standard Test Conditions at 25 degrees Celsius. This is the effective cell temperature whilst under test conditions, not the ambient temperature . The average cell temperature will be higher than the ambient because of the glass on the front of the module being heated by the sun. All panel manufacturers detail temperature voltage and/or power temperature coefficients in their product specification sheets.
If we are designing a system to meet the total energy demand, the amount of shading must be taken into account when calculating the total amount of sunlight available to a solar array. This adjusted figure is then used in the design.
When the roof is not orientated true north and/or not at the optimum inclination (27.5 degrees for Brisbane) the output from the array will be less than the maximum possible. ESP Projects utilises Clean Energy Council (CEC) calculation tables to derive average annual daily total irradiation for orientation and tilt. The CEC provides us with information on the expected output of a system (% of the maximum possible output ) when it is located on a roof that is not facing the true north (+/- 5°) and for various tilt angles.