The power station at the lower reservoir has four water turbines, which can generate at full capacity within 60 seconds of the need arising.

The plant is operated by First Hydro, a UK company owned by a joint venture of International Power and Mitsui & Co.,[1] and has an average efficiency of 72–73%, i.e. it uses 39% more electricity (when pumping the water back up to the Llyn Stwlan) than it actually produces.

[3] Llyn Stwlan had previously been enlarged in 1898 by the Yale Electricity Company, who built a dam and used the water to generate power at Dolwen, located near Tanygrisiau, below the lower reservoir.

The levels of Tanygrisiau Reservoir are carefully monitored, to ensure that there is always sufficient spare capacity to hold the balance of the water left in Llyn Stwlan.

In order to accommodate the vertical shafts of the machines, 140,000 tons on rock were excavated, and the building extends 34 m (112 ft) below the operating floor, which is at ground level.

The outer wall is faced with local stone,[10] of a type which has been used for a number of significant buildings in the Vale of Ffestiniog, but it proved difficult to find a source.

A disused quarry was tracked down with help from retired quarrymen, but there was insufficient stone to complete the project, and it was problematic to use modern extraction methods.

The gates are 14.5 ft (4.4 m) square, and can be dropped into position to prevent water entering the tunnels to the power station.

[7] The equipment, supplied by the English Electric Company, installed in the main building consists of four vertically mounted motor/generator units, rated at 90 MW each, which are located on the alternator floor.

[12] Water is pumped from Tanygrisiau Reservoir to Llyn Stwlan during the night, when there would normally be insufficient demand for the electricity produced by relatively efficient steam plant.

A particular problem is the huge rise in demand at the end of popular television programmes, and the station can be brought online very quickly to cover this,[13] providing an additional 360 MW of power in under a minute.

Thus the enabling Act of Parliament included an amenity clause, which included provision of a landscape consultant, who would liaise with the National Parks Commission, to ensure that issues connected with the disposal of spoil, the planting and felling of trees and bushes, and the reinstatement of land affected by the construction, were adequately addressed.

Local planning authorities were consulted, concerning the location and external appearance of all aspects of the project, and the Royal Fine Art Commission was also involved in the process.

[15] Restoration of the Ffestiniog Railway as a tourist railway had begun in 1952, but the construction of the power station would sever the link between Porthmadog and Blaenau Ffestiniog by flooding the northern portal of the Moelwyn Tunnel and the trackbed to its north, and constructing access roads along the trackbed to the south of Tanygrisiau station.

Based on the cost of the east side route, the Lands Tribunal ruled that equivalent reinstatement would only be justified if passenger number were to rise from 76,000 to 196,000, which was not deemed to be "within the bounds of the possible", despite the fact that these figures were reached in 1967.

[21] Pressure was applied to the CEGB during the final hearings at the Lands Tribunal, to allow a west side route, and the Ffestiniog Railway were awarded £65,000, an interest payment of £30,000, and legal costs.

[23] Whereas most of the reinstated route, which included a spiral at Dduallt and a new Moelwyn Tunnel, was built by volunteers, the section immediately behind the power station was built by Sir Alfred McAlpine (Northern) Ltd, as it included four buried bridges to carry the trackbed over the inlet penstocks to the power station.

It was worked sporadically between the 1820s and 1897, but was connected to the Ffestiniog Railway by a spectacular series of seven inclines, descending some 335 metres (1,099 ft).

They therefore drained both reservoirs, and Cookes began removing about 250 tons of explosives per week, which were taken to Penrhyndeudraeth or by rail from Blaenau Ffestiniog railway station to various other ICI Nobel works in the United Kingdom.