Tonstad power plant is located 750 metres inside a mountain at the town of Tonstad in Sirdal Municipality in Vest-Agder County.

Ownership: Statkraft is an indirect co-owner through its ownership in Skagerak Energi, Agder Energi and Sira-Kvina kraftselskap
Production: 3800 GWh
Power: 960 MW

The power plant is the largest in Norway measured in production.

The first two generating units of 160 MW each were put into operation in 1968, and two units of the same size were added in 1971. The fifth and final generating unit of 320 MW began operation in 1988, with Crown Princess Sonja and Crown Prince Harald performing the official startup.

All units are equipped with Francis turbines. The height of fall is 450 metres.

Lake Homstølvatnet is the intake reservoir for the power plant, and both the Sira and Kvina watercourses are affected by this development. Water from the plant is discharged into Lake Sirdalsfjorden and from there by a canal to Lake Lundevatn and Åna-Sira power plant, which is the last plant in the watercourse – almost at the sea. Parts of the regulated water area for these plants extends into Bygland and Kvinesdal municipalities.

The development of Tonstad power plant began in 1965 and in 1966 the head office of Sira-Kvina kraftselskap was established at Tonstad in Sirdal Municipality, where a plant administration was also established. The large development presented major challenges, resulting in new technology being adopted. Blasting of the head race tunnel to Tonstad power plant posed special problems, because a weak zone in the surrounding rock had to be temporarily frozen so the rock mass would not collapse.

About five million cubic metres of rock were excavated and used for the construction of dams, roads and weirs. The 60 weirs in the watercourses help to counteract the consequences of reduced water flow in certain stretches of the river as a result of regulation.

Through its ownership in Skagerak Energi AS, Agder Energi AS and Sira-Kvina kraftselskap, Statkraft is co-owner of this power plant. Sira-Kvina kraftselskap is responsible for daily operations, and manages the licences and obligations in terms of river management and regulating of the watercourse.