Water filling to the Upper Kothmale reservoir began on Thursday has been completed within 12 hours and the reservoir is now ready to supply water to the generators, Sri Lanka 's power authority, Ceylon Electricity Board (CEB) said.

The 65-acre reservoir that has a capacity of 800,000 cubic meters opens to a 13-kilometer tunnel that takes water to the generators.

Upper Kothmale tunnel is the longest tunnel of a hydropower reservoir in Sri Lanka.

The Rs. 53 billion project that was constructed with the assistance of the government of Japan will add 150 MW of electricity to the national power grid.

When completed the run of river hydropower project with an installed capacity of 150MW consisting of two 75MW units, will produce 409GWh per year.

The project now in near completion is to commence generation of hydropower from January 2012.