converters, racking systems and various components) make up about half of the total costs of installations. For merchant solar power stations, where the electricity is being sold into the electricity transmission network, the cost of solar energy will need to match the wholesale electricity price. This point is sometimes called 'wholesale grid parity' or 'busbar parity'. Some photovoltaic systems, such as rooftop installations, can supply power directly to an electricity user. In these cases, the installation can be competitive when the output cost matches the price at which the user pays for their electricity consumption. This situation is sometimes called 'retail grid parity', 'socket parity' or 'dynamic grid parity'. Research carried out by UN-Energy in 2012 suggests areas of sunny countries with high electricity prices, such as Italy, Spain and Australia, and areas using diesel generators, have reached retail grid parity. Standards Standards generally used in photovoltaic modules: IEC 61215 (crystalline silicon performance), 61646 (thin film performance) and 61730 (all modules, safety), 61853 (Photovoltaic module performance testing & energy rating) ISO 9488 Solar energy—Vocabulary. UL 1703 from Underwriters Laboratories UL 1741 from Underwriters Laboratories UL 2703 from Underwriters Laboratories CE mark Electrical Safety Tester (EST) Series (EST-460, EST-22V, EST-22H, EST-110). Applications There are many practical applications for the use of solar panels or photovoltaics. It can first be used in agriculture as a power source for irrigation. In health care solar panels can be used to refrigerate medical supplies. It can also be used for infrastructure. PV modules are used in photovoltaic systems and include a large variety of electric devices: Solar canals Photovoltaic power stations Rooftop solar PV systems Standalone PV systems Solar hybrid power systems Concentrated photovoltaics Floating solar; water-borne solar panels Solar planes Solar-powered water purification Solar-pumped lasers Solar vehicles Solar panels on spacecraft and space stations Limitations Impact on electricity network With the increasing levels of rooftop photovoltaic systems, the energy flow becomes 2-way. When there is more local generation than consumption, electricity is exported to the grid. However, an electricity network traditionally is not designed to deal with the 2- way energy transfer. Therefore, some technical issues may occur. For example, in Queensland Australia, more than 30% of households used rooftop PV by the end of 2017. The duck curve appeared often for a lot of communities from 2015 onwards. An over-voltage issue may result as the electricity flows from PV households back to the