Small hydro is the development of hydroelectric power on a scale suitable for local community and industry, or to contribute to distributed generation in a regional electricity grid. Exact definitions vary, but a "small hydro" project is less than 50 megawatts (MW), and can be further subdivide by scale into "mini" (<1MW), "micro" (<100 kW), "pico" (<10 kW). In contrast many hydroelectric projects are of enormous size, such as the generating plant at the Three Gorges Dam at 22,500 megawatts or the vast multiple projects of the Tennessee Valley Authority. Small hydro projects may be built in isolated areas that would be uneconomic to serve from a national electricity grid, or in areas where a national grid does not exist. Description The use of the term "small hydro" varies considerably around the world, the maximum limit is usually somewhere between 10 and 30 MW. While a minimum limit is not usually set, the US National Hydropower Association specifies a minimum limit of 5 MW. In California, hydroelectric generating stations with a maximum capacity of less than 30 MW are classified as small, and are eligible for inclusion in the state's renewable portfolio standard, while hydroelectric generating stations with a higher capacity are classified as large and are not considered renewable. The "small hydro" description may be stretched up to in the United States, Canada and China. In India, hydro projects up to 25 MW station capacities have been categorized as Small Hydro Power (SHP) projects. Small hydro can be further subdivided into mini hydro, usually defined as 100 to 1,000 kilowatts (kW), and micro hydro which is 5 to 100 kW. Micro hydro is usually the application of hydroelectric power sized for smaller communities, single families or small enterprise. The smallest installations are pico hydro, below 5 kW. Since small hydro projects usually have correspondingly small civil construction work and little or no reservoir, they are seen as having a relatively low environmental impact compared to large hydro. Growth According to a report by REN21, during 2008 small hydro installations grew by 28% over year 2005 to raise the total world small hydro capacity to 85 gigawatts (GW). Over 70% of this was in China (with 65 GW), followed by Japan (3.5 GW), the United States (3 GW) and India (2 GW). Global growth is expected to be 2.8% annually until the mid-2020s when capacity will be about 150 gigawatts.