Nelson Modrall is the King of Conservation in The Knolls, a neighborhood hotbed of penny-pinching a d patriotism in the Lake Ridge subdivision near Woodbridge, Va.

Modrall, his wife and their three children have kept their utility bills from skyrocketing with a daily, concerted conservation effort. Last year the total energy cost for their all-electric home was less than that of some of the gas-heated neighboring houses.

Mrs. Modrall said it was nice that President Carter asked the rest of the country to do what they have been doing for a couple of years - conserve. It means "taking a little trouble and exercising a little restraint," Mrs. Modrall said.

Modrall, an Army major, leads his family in taking advantage of every energy-saving device they can think of in the interests of the twin virtues of family economy and energy conservation.

"When we were stationed in Germany in the mid-1960s," Mrs. Modrall said, "we lived on the local economy and had to get the landlady to turn on the water heater when we wanted hot water. We thought that was absolutely barbaric. Now we turn the water heater off when we aren't using it - which is most of the time, probably 20 hours a day."

The water heater also rests snugly in an insulation blanket so that when it is running more heat goes to the water and less escapes through the tank walls.

Other energy-saving methods used by the Modralls include:

Turning off the dishwasher once the wash and rinse cycles are completed. Just by opening the machine's door, the dishes are air-dried.

Placing a glass door across the fireplace to keep cold air out in the winter and warm air out in the summer when the air conditioning is on.

Capping electric outlets to keep air out. "You can feel the cold air coming in the winter," Mrs. Modrall said.

Using air conditioning only in extreme heat.

Reinsulating the attic.

Maintaining the thermostat in the mid-sixties during the day in the winter and at no more than 60 a t night.

Many community residents use some of those measures. In addition, they - and the Modralls - also recommend these other measures: setting the water heater thermostat lower, installing storm windows switching off lights when leaving a room, closing off unused rooms, burning wood in fireplaces, keeping drapes closed in cold weather unless the sun is shining, drawing blinds in the summer, running a fan in the attic in hot weather, washing clothes in cold water, reducing the use of the oven and of small appliances.

A Vepco spokesman said company experts feel that shutting off water heaters saves only a negligible amount of electricity - but the idea of doing so encourages a family to use less hot water, which is the real conservation measure.

In an informal survey of energy costs and conservation practices. The Post interviewed the residents on two-block-long Mulberry Court in The Knolls. The homes along one block are all-electric, while those in the other block are gas-heated.

The Modralls live around the corner on Woodfern Court, but were interviewed because of their reputation in the neighborhood as champion conservers.

They style and size of house appear to be key factors in energy consumption. Four different house styles were built in the Mulberry-Woodfern neighborhood. Their names, bestowed by builder Edward R. Carr, are: Taurus, with about 3000 square feet of living space: Scorpio, about 2800; Capricorn, about 2500; and Aquarius, 2200.

The Taurus model has cathedral ceilings in its living room and family room-kitchen areas; Scorpio is essentially a three-story house and Aquarius has bo basement.

The other variable factors were the intensity with which each family approached conservation: none were found who claimed to be making no efforts.

The Modralls turned out to be the champions indeed. In 1976, they kept their electric bill for an all-electric house Aquarius model to $815, an average of about $68 per month.

Their closest competitors were Charles and Rosemary Luckett in a gas-heated Aquarius on the corner of Mulberry and Woodfern. Their total gas and electric bills for 1976 averaged $70 per month.

The all-electric Aquarius on Mulberry closest to the Modralls in utility cost is occupied by George and Anne Eakin, who averaged $84 per month in 1976. However, the Eakins were beaten by people in larger homes: George and Virginia Frank in a Capricorn averaged $77 and Larry and Anne Rissler in a Scorpio averaged $78.

And Fred and Virginia Federici, in an all-electric Taurus, at $85, averaged only a dollar more a month than the Eakins.

Most residents interviewed said it was difficult to see the results of their conservation efforts, particularly in the first quarter of this year, because of the unusually cold winter, a rate increase levied by the Virginia Electric and Power Co. and sharp increases in fuel adjustment charges.

But the Modralls and the Lucketts still led the field - joined by Robert and Joan Degrant across the street from the Lucketts. The Lucketts and Degrants, both with gas heat, edged the Modralls out of the championship for the quarter. The three-month totals: Lucketts and Degrants, each $353; Modralls, $363.

Monthly averages thus soared from $68 to $121 for the Modralls and from $70 to $118 for the Lucketts.

The residents unanimously feel that their utility bills would have been appreciably higher without the conservation measures taken.

The fuel adjustment charge was the main complaint about utility costs. It is levied by Vepco as a "pass-through" charge in which the increases in fuel prices are passed on directly on the consumer as a separate item on the bill.

The charges have been higher this year because Vepco was forced to buy more oil to replace the power that had been supplied by its two nuclear units at Surry on the James River. The nuclear units, which operate for less cost than other plants, were shut down most of the last quarter of 1976 and part of early 1977 for repairs and maintenance.

The fuel adjustment charges were particularly noticeable because Vepco customers received fuel adjustment credits during most of 1976 when Surry's operation significantly reduced the need to burn coal and oil to generate electricity, a Vepco spokesman said.

The State Corporation Commission sets a fuel cost base absorbed by the regular rates consumers pay. If fuel costs to Vepco go above that, the pass-through charge is levied on consumers. If the fuel cost is less, consumers get the benefit and receive credits on their bills.

Under the accounting procedure used, the fuel adjustment charge or credit is figured on the average fuel cost for the three months preceding the month of a given bill - separated by a month. For example, the large fuel adjustment charges The Knolls residents received in February were based on fuel costs to Vepco in the last quarter of 1976 when the nuclear units at Surry were down and Vepco had to buy extra coal and oil.

Despite the general success of the neighborhood in exercising conservation measures, most homeowners appear to be still searching for more ways to save.

"We're patriotic people, no doubt about it," said Rosemary Luckett, "and we paid attention to the President, but we were starting to conserve two years ago because of the costs."

Modrall typifies the attitude toward conservation: "There was a covenant in this community against hanging laundry out to dry. Imagine that! The greatest source of free energy hanging up there in the sky and going to waste while everybody uses clothes driers, which, according to Vepco, use more electricity than any other single unit except the heating-cooling units and the water heater."

According to Marguerita J. Smith, associate manager of the Lake Ridge Parks and Recreation Association, Inc., the controlling body of the subdivision, that covenant has been amended because of the energy situation and residents now may hang their clothes out to dry as long as they have retractable clotheslines and take down the clothes when dry.