Renewable energy commercialization:

Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy, accounting for 16 percent of global electricity generation – 3,427 terawatt-hours of electricity production in 2010, and is expected to increase about 3.1% each year for the next 25 years. Hydroelectric plants have the advantage of being long-lived and many existing plants have operated for more than 100 years.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Electricity production is expected to increase by what percentage each year for the next 15 years?
unanswerable