Low-grade energy: Based on the thermodynamic concepts, an energy source can be called as high-grade or low-grade, depending the ease with which it can be converted into other forms. Thus electrical energy is called a high-grade energy, as it is very easy to convert almost all of it into other energy forms such as thermal energy (say by using an electrical heater). Whereas, it is not possible to convert thermal energy completely into electrical energy (typical efficiencies of thermal power plants are around 30 percent), hence thermal energy is called a low-grade energy. Naturally, high-grade energy sources are more expensive compared to low-grade energy sources.