About Photovoltaics

About Photovoltaics | Solar Energy | Typical System | Energy Production

Energy Production

A PV module or system has a rated power output, measured in Watts peak (Wp). This is the output of the module as measured with 1000W/m2 insolation (with an AM1.5 spectrum of light) at 25 degrees C. This can be considered to be the maximum output of the module in ideal sunny conditions. In practice, the solar energy is typically less than 1000W/m2 and the temperature of the module is higher than 25 degrees C, leading to lower output on average. (although, in some countries such as Australia, solar modules may give more than their rated power)

As a first step to estimate the energy output over a year, the rated power of the system and the insolation at the site are needed. Typical insolation levels are as follows:

Note: NASA has a database containing insolation data for all locations at http://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi?uid=3030

The annual power output can be found by simply multiplying the insolation by the rated power output. For example a 50kWp system located in Rome would give an output of 1535 x 50 = 76,750kWh/year.

This calculation gives a first estimate of the power output. However, we may need to compensate for a number of factors. Firstly, the PV system may not be orientated at the ideal angle to collect the incoming solar energy. The table below shows the percentage of power output with different system angles and orientations. (Assuming a system in the Northern Hemisphere - a system in the Southern Hemisphere should face North). Also note that a completely horizontal system is not a good idea since dust will collect on the modules and will not be washed off by rain. Typically a minimum angle of 5 degrees is preferable.

Other factors affecting the energy output to consider include:

• Shading by surrounding buildings and trees or other parts of the building such as chimneys
• Dust may be an issue depending on the system's location
• The temperature of the location - the efficiency of crystalline solar modules reduces as temperature increases whereas thin-film modules generally maintain their efficiency
• There will be energy losses in the inverter and cabling (approximately 5-10%)

For example, for a 75kW system located in New York, mounted on a South facing façade with no shading or dust we may estimate the energy output to be 75kWp x 1300kWh/m2/year x 0.7 (orientation) x 0.9 (system losses) = 61,425kWh/year. (ignoring any temperature effects)

A PV system typically has a guaranteed lifetime of 25 years and an expected design lifetime of at least 50 years. So the total energy output from the system above would be approximately 3,071,250kWh.*

Carbon Emission Reductions

Once the energy output is known, it is simple to calculate the carbon reduction. Typical average CO2 emissions per kWh of electricity generation are around 0.5-1kg/kWh. So the above system would give a CO2 reduction of 3071 tonnes of CO2 over its lifetime assuming it reduces emissions by 1kgCO2/kWh.

* Note: The annual power output can be found by simply multiplying the insolation by the rated power output. For example a 50kWp system located in Rome would give an output of 1535 x 50 = 76,750kWh/year.