The Tupperware Co. has begun construction of a $2 million plant that will generate electricity from the power of the Blackstone River and save the firm about $400,000 a year.

The hydroelectric plant is being built on the site of the old Blackstone Cotton Mill power plant, which closed in 1946 when generating electricity from water power became more expensive than buying it from the local utility.

James G. Hagan, Tupperware vice president, said the energy cost savings from the hydroelectric plant will help the manufacturing firm stay competitive despite its location in the Northeast, where energy costs are high. The plant will also save 21,350 barrels of oil a year, he said.

Tupperware, which manufactures housewares and plastic containers and employs 1,000 persons, is a division of Dart Industries Inc., which had $1.8 billion in sales in 1978.

When the hydroelectric plant is completed next July, it will generate about 10.5 million kilowatt hours of electricity a year, about 40 percent of the amount Tupperware uses for lighting and machinery, Hagan said.

In a state where surging streams once provided the power for the nation's first factories, Tupperware will become the only manufacturing firm to rely on water power in about 30 years.

But trends are changing. In nearby Woonsocket, for example, voters will decide Tuesday whether to borrow $2.7 million for the construction of a hydroelectric plant on the Blackstone River. The plant would provide electricity for the town hall, schools and other municipal purposes.

And in Pawtucket, where the Blackstone first began powering Samuel Slater's textile factory in 1793, the Blackstone Valley Electric Co. is considering reopening a long-closed hydroelectric plant.

Allowing for construction costs, Tupperware estimates it can produce electricty for 1.5 cents per kilowatt hour instead of the 4.5 cents per kilowatt hour it now pays, according to a company spokesman. Once construction costs are paid off, the plant will produce electricity at virtually no cost, he added. He said the plant is expected to operate for at least 50 years.

Jay Ryder of Halliwell Associates, an engineering firm assisting in the project, said new turbines, generators and other equipment are being installed in a renovated brick building where power was generated for the Blackstone cotton mill beginning in 1904.

The plant will be completely automated, and will need only occasional maintenance and cleaning of "trash racks" covering water intakes, Ryder said.

The turbines will be turned by water that spills over a 200-foot-wide, 12-foot-high granite dam built for the old mill about 1900, according to Hagan. The dam diverts water through a series of ponds, dropping the water level 30 feet and increasing the water's speed before it passes past the plant site and is returned to the main channel.

The construction will be financed with$1 million in industrial revenue bonds backed by the Rhode Island Port Authority and $1 million in private funds, Hagan said. Because port authority bonds are tax-exempt, the company will pay interest on them at only about half the 12 to 14 percent rate of regular corporate bonds.

In addition to saving money on energy, Tupperware will be able to take investment tax credits on its federal income taxes, Ryder said. The federal income tax credit for hydroelectric projects is 10 percent of the project cost, but Congress is considering legislation this month to increase the credit to 30 percent, Ryder said.