Frequently Asked Questions
Solar Module & System Performance
When determining the correct system for installation to residential premises, it is important to realize the difference between the output of a solar module when quoted at Standard Test Conditions (STC) and what the real life performance of that solar module is when consideration is given to the following factors:
- Manufacturer’s Output Tolerance: the power output for all solar modules is specified in watts with a typical power tolerance of +/- 3% and a solar cell operating temperature of 25 degrees Celsius. Cell temperature should not be confused with operating ambient air temperature. For example an ambient temperature of say 30 degrees C, translates to an approximate 50 degree C operating cell temperature. Therefore a typical 190W solar module could have an actual nominal rating of 184.3W.
- De-rating due to Dirt: Any air borne dirt or bird dung accumulation on the solar module over a period of time will have a de-rating effect on the solar modules’ output. This will vary due to location but an acceptable figure of 5% from the already de-rated figure of 184.3W would not be uncommon. This then would further de-rate the solar module to 175.09W.
- De-rating due to Temperature: As mentioned in Australian Standard AS4059.2, and above in 1, cell performance will alter at higher working temperatures by a set formula. For our mono-crystalline solar modules, for every degree above 25 degrees C the output power will drop by 0.43% as a direct result of the drop in output voltage of the solar module. Using the accepted formula of ftemp = 1-[y x (Tcell eff y –Tstc)], a solar module operating at 30 degrees C will drop power by a further 12.9%. Therefore the previously de-rated figure of 175.09W will now drop to 152.50W as the effective power at the inverter.
- Inverter Efficiency: The final factor for what is actually produced as AC power for either home consumption or export to the grid is the inverter efficiency. This important factor is one reason Renewable Energy International, selects only the most efficient inverters available. Our current range of inverters have a maximum efficiency of between 96.8% to 97.6% and a Euro efficiency of between 95.8% and 97.1%. As the Euro efficiency is a more realistic gauge, this takes 3.55% (average figure) from the 152.50W solar module de-rating now with an effective AC power of 147.08W.
Note the above calculations assume a solar insolation figure of 1000W/m2
John Hall’s Rule of Thumb has always been “Nominal rating less 20%” for quick calculations on effective peak solar module performance.
So a 190W module less 20% equals 152W or if the +/-3% is applied, then (190W -3%)-20% = 147.44W. This matches the previous calculations.
Annual Performance Differences due to Direction of the Solar Array
It is also important to understand the performance variation due to the direction the solar array is facing (azimuth). In the southern hemisphere, all solar modules face north (towards the equator) for maximum year round performance.
Any variation to due north has a slight change to the annual performance of the solar array; but by how much?
Using an example of 16 x 185W solar modules on a 20 degree roof, Perth weather data and a transformerless inverter with a Euro efficiency of 96.3%, the annual performance data shown in the table below is observed.
As can be seen there is very little difference between NW all the way through to NE on an annualised basis.
For solar arrays that are installed West or East an annualised power drop of 10% is not considered to be of a magnitude that should preclude an installation.
| Direction | Annual Units (kWh) | Variation in Annual Power |
|---|---|---|
| North | 4997 | 0% |
| North West | 4842 | 2.7% |
| West | 4479 | 10% |
| East | 4499 | 9.6% |
| North East | 4856 | 2.43% |
Annual Performance Differences due to Roof Angle
Using the same solar power system described above, the table below shows the annualised performance difference at differing roof angles, when the solar array faces due north.
The other major consideration in regards to angle of inclination also relates to the accumulation of dirt and bird dung and the ability of natural rainfall to ensure the glass surface of the solar modules stay relatively clean and not hold the dirt.
| Roof Angle | Annual Units (kWh) | Variation in Annual Power |
|---|---|---|
| 35° | 4891 | -2% |
| 30° | 4951 | -1% |
| 25° | 4980 | +1% |
| 20° | 4977 | 0% |
| 15° | 4944 | -1% |
| 10° | 4876 | -2% |