**4. Availability and technical limitations**

One of the important assessing factors to generate power from renewable energy sources is the availability and their technical limitation. Each resource has some limitations; photovoltaic has limitation to generate power only because heat energy from sun can only be received during the day time, except cloudy season. For wind turbine, speed should not increase beyond 25 m/s; otherwise, turbine will be damaged. Also, low speed of wind, that is, <3 m/s, will not be sufficient for the generation of electric power. Geothermal has good ability to generate power throughout the day for 24 h but is geography limited according to the presence of resources. Hydro-electric power plants are easy to start, stop, and operate within minutes; hence, they are considered as one of the highest available, reliable, and flexible renewable energy resources. From efficiency point of view, hydroelectric is classified at the top of the list, and then wind energy, photovoltaic, and geothermal are lowest efficient renewable energy resources. Because of availability of cells in different categories, the efficiency of photovoltaic is very much variable [7]. According to the efficiency, different energy sources are categorized in **Table 6**.


**Table 6.**

*Efficiency of electricity generation [7].*
