Citing a 25 to 30 percent slowdown in the growth of energy consumption, Virginia Electric & Power Company yesterday slashed its forecast of how much electricity its customers will use in the next decade.

The new estimate reduces Vepco's projection of how much power its customers will need in 1989 by 1 million kilowatts -- equivalent to the output of a entire nuclear power plant.

If the estimate proves to be correct, it could mean savings of hundreds of millions of dollars for vepco customers, because the company won't have to build as many new power plants as it now plans.

The growth rate of power demand by Vepco's customers has slowed dramatically since the original Arab oil boycott in 1973-74. At that time Vepco was the fastest growing utility in the nation, with peak demand climbing 11 percent a year.

Now, Vepco estimates the annual growth rate has dropped to 3.5 per cent for winter peak demand and 2.8 percent for summer peak demand.

A year ago Vepco's winter peak was growing at a 4.5 percent a year rate and summer demand was climbing 4 percent a year.

Peak demand is the amount of electricity Vepco's customers use on the hottest day of the summer or the coldest day of the winter. The power company has to build enough generators to supply its customers on the days when all the air conditioners or all the electric heaters are turned on full blast.

Vepco said rapidly increasing energy prices and soaring inflation are the reasons demand for its electricity is not growing as fast as it used to.

Those two factors, the company said, "will cause more people to give their energy consumption careful scrutiny and increase conservation efforts."

Recent high interest rates have slowed both industrial expansion and new home construction in Vepco's service area, said William W. Berry, executive vice president.

"A decline in construction activities obviously will mean fewer new electric customers and is a factor in our new forecast," Berry added.

Demand for electricity for heating is also slowing because of increases in the amount of natural gas available, the Vepco executive noted. Until a moratorium on new gas hook-ups was lifted last year, virtually all new homes built in Virginia had to be heated electrically.

Vepco's lates estimate is that peak power demand during the winter will increase from this years' 7.4 million kilowatts to 10.39 million kilowatts in 1989 and summer peak demand will go from 7.93 million kilowatts to 10.36 million.

A year ago, Vepco was projecting peak demand of 11.4 to 11.5 million kilowatts at the end of a decade.

"Obviously this lower load forecast will affect our planned construction program," Berry said, but he would not specify how Vepco plans to cut construction.

The million-kilowat reduction in the latest 1989 estimate is equivalent to the output of one of Vepco's nuclear power plants or a very large coal-burning generator.

Vepco has started construction on a pair of million-kilowatt power plants at its North Anna nuclear facility where it already has two nuclear plants. Originially both North Anna 3 and North Anna 4 were to be nuclear plants, but the company is studying switching them to coal.

Also on Vepco's drawing board is another coal-burning plant about the same size that was to be built by 1988 or 1989.

No site has been chosen for that plant and the new forecast could mean it will never be needed.

Because of past reductions in its growth forecast, Vepco is negotiating to sell off part of its several of its plants to other companies.