A new concept in energy conservation that allows people to cut electric bills by using appliances late at night or early in the morning is being implemented by Virginia Electric and Power Co. to the delight of some homeowners and the skepticism of critics.

Vepco's new "time-of-usage" program will charge customers more for electricity used during the 10 a.m. to 10 p.m. peak demand period weekdays than for that used during other hours when it is cheapest for the utility to produce electricity.

But the utility's critics charge that it has dragged its feet for years in implementing the program and is doing so in a way that makes it difficult for participants in the program to make it work.

The 12-hour peak demand period is too long, the critics say. Other states with experimental programs use much shorter periods, making it easier for participants to use electricity during off-hours.

And the critics wonder why the utility hasn't started the program for its big commercial and industrial users as many other states have - since savings could be bigger with such customers for roughly the same investment in metering equipment.

"Their time-of-usage experiment is a fraud, the company didn't want it," said John T. Schell, formerly an attorney for the Consumer Congress of the Commonwealth of Virginia.

Schell said the Congress spent three years pressing Vepco to try the program it is now undertaking. He said the company doen't want to push the program for fear of losing revenue on its plant investment.

Vepco vice president William W. Berry replied that the company's time-of-usage program is the leading residential program in the nation. The State Corporation Commission has not authorized it for the big customers, he said, who fear they would not be flexible enough to avoid high peak period rates.

Berry said company officials "anguished over" setting the long peak period hours, but thought that shorter hours might create new false peaks. The peak-period may be shortened later, he said.

Some 17,000 eager customers applied for the program when it was announced, and 2,000 of these were chosen. In the coming months, special new meters will be installed in their homes that will measure not only how much electricity they use but also when they use it.

The customers will be briefed by company officials. Then they can set about what may well become America's most popular new family game in the decades ahead - saving energy.

Berry said that if the program is successful, 20,000 customers a year would be added to its starting in 1981.

Time-of-usage programs like Vepco's can save the nation more than $13 billion between now and 1985 by renew generating plants, according to Howard Perry, a utility expert in the U.S. Department of Energy.

At the same time, he said, the programs that are springing up in Virginia and other states could eventually save a quarter of a million barrels of oil a day that would otherwise have been burned in such new plants.

President Carter's energy program originally mandated the programs for the nation's largest utilities, but this has been softened in Congress to a requirement that such programs be considered.

Time-of-usage programs save energy by reducing what utilities call their "peak load" requirements - the generating capacity they must have to meet the maximum electricty demand from their customers. For example, a peak demand time would be early evening when people come home from work and turn on the lights, television, diswasher and other utilities at the same time the air conditioning is running.

But a few hour later, when everyone is in bed, the company is left with enormous generating capacity that is idle because electricity cannot easily be stored. Convincing customers to put off their demand until such periods - turning the diswasher on when going to bed, for example - is what utility people call "shaving the peaks."

A time-of-usage program is aimed at getting customers to shave the peaks in their household usage not only by using appliances outside peak periods, but also by the avoidance of "stacking" - using many appliances simultaneously - during the peak period itself.

At the heart of the criticisms of the Vepco program is the argument that Vepco's $600-million-a-year plant building program is unnecessarily large. In large part to maintain the program the company has had to request a $246 million rate increase that would boast rates for its 1.2 million customers by 20 percent.

Schell said that by building a $755 million, "peak shaving" pump storage plant in western Virginia and large nuclear power generating facilities faster than necessary, Vepco is seeking to preempt the energy market far into the future.

Berry denied that the company is overbuilding. He said plant construction is designed to meet a 5.5 per cent annual increase in demand, and that the success of a large time-of-usage program has been factored into these projections.