Norway is a heavy producer of renewable energy because of hydropower. Over 99% of the electricity production in mainland Norway is from 31 GW hydropower plants (86 TWh reservoir capacity, storing water from summer to winter). The average hydropower is 133 TWh/year (135.3 TWh in 2007). There is also a large potential in wind power, offshore wind power and wave power, as well as production of bio-energy from wood. Norway has limited resources in solar energy, but is one of the world's largest producers of solar grade silicon and silicon solar cells.