The production from hydroelectric power plants varies from year to year depending on run-off into, and the storage capacity of, reservoirs, domestic demand for electricity, and the levels of import from or export to neighbouring countries as determined by the prices prevailing in the Nordic energy market. In Norway, usable run-off for power generation can vary by more than +/- 25 % compared with that for a normal year.

Electricity generated by hydroelectric power is a renewable resource. Power production releases no damaging emissions into the atmosphere.

Norway has the highest levels of hydroelectric power generation among the western European countries, and is the sixth largest producer of hydroelectricity in the world.

In 2001, global hydroelectric power production totalled 2646 TWh. Canada is the largest hydroelectric power producer with 333 TWh, followed by China with 277 TWh, Brazil with 268 TWh, the USA with 223 TWh, Russia with 176 TWh, and Norway with 124 TWh.

Hydroelectric power contributed 2% of total global primary energy supplies during 2001.