If America woke up tomorrow morning to the brave, new world of Jimmy Carter, it might be surprised to discover that the future is not nearly as bleak as he made it sound.

The President has cast this business of energy conservation like a U.S. version of "The Battle of Britain." Lots of scary talk about sacrifice and changing lifestyles.

But, if this were 1985 and all of Carter's goals had come to pass, the mainstream lifestyles of Americans would not look much different, according to a scattering of experts consulted yesterday.

America will still be driving a multitude of cars, long distances. Still piling the kids into camper vans on the weekends. Still enjoying air conditioning and well-heated homes.Still enjoying the good life that has flourished post-World War II. A lot of this will cost more - but much of it will be cheaper in the long run. Indeed, some experts believe the future lifestyle more efficient, more humane in scale, will turn out to be a lot more pleasant than the oil-guzzling society of the present.

The President, thus, is being criticized for over-scare. Some predict that his valued credibility is going to suffer as a result.

"The sheep in wolf's clothing," said Ralph Nader, the tart-tongued consumer advocate.

"It's like you go to the dentist as a kid," Nader said, "and he says, 'Charlie, this is really going to hurt you, really going to hurt.' Then suddenly it's over and it doesn't hurt at all and you feel great and you really like the dentist. This is the reverse of the politics of heightened expectations."

Ted Eck, chief economist for Standard Oil of Indiana in Chicago, whose perspective could not be more distant from Nader's, expressed a similar measure of Carter's remedies:

"If you woke up 10 years from now, you wouldn't see much difference in how traffic moves around the Loop I don't see a brave new prescription for a radically different lifestyle. It's not a blueprint - it's a wish book."

Indeed, some people believe that Carter sold the wrong message - by scaring citizens with the implications of energy conservation, instead of emphasizing the long-term positive benefits - not just for the national economy - but for their own lives.

"I don't think any of thest programs are going to make life more difficult for people," said Gary DeLoss, energy strategist for Nader's Public Interest Research Group. "Good conservation can actually improve lifestyles."

For instance, John McGinty of the American Institute of Architects forecasts a bright and exciting future in housing as architects adapt their designs to the changes energy conservation demands.

"There will be dramatic changes in architecture, but for the better," said McGinty. "I don't think we're talking about human discomfort or even economic penalties in the long run."

All sides of the issue agree on this much: getting from here to 1985 is the problem, politically and economically. There are great questions of environmental protection and equity among fuel consumers which the Carter administration has not really settled in the public statements so far. Many also insist that the long-term arguments over nuclear vs. coal vs. oil production are still on the table, essentially unresolved.

Put all those arguments aside for a moment and imagine, with the experts, what the future might look like if President Carter's goals are realized:

Cars will change. So will houses. So weill the electric appliances in our kitchens. So, perhaps, will the way electric power is generated for home and industry. You will not have to buren coal in your basement or unplug your air conditioner - though you may have become a bit wiser about when you turn it on.

If you want a clue as to how this future hardware and housing might look, look backward, not forward. Though houses, office buildings and all automobiles and trucks will incorporate new, more efficient, technology, they may resemble what America once enjoyed. Car sized close to what sold in the early 1950s. Houses designed on the popular regional models of old - the New England salt her for instance - intended to adapt to local weather, rather than overpower it with mechanical heating and cooling.

Our cars will be shorter, ligher, probably too slow to satisfy the muscle-car jockeys who want maximum power. But the interior dimensions won't be dramatically smaller. Detroit has already been steered in this direction by Congress and, against its long-established instincts, the industry is already planning a future of diesel engines, electronic fuel injection and other efficiencies.

If Carter's proposed 50-cent gasoline tax is enacted by Congress (a very shaky assumption at the moment), it would pinch everyone whose lifestyle is committed to long-distance driving - the folks who live in small towns, for instance, and drive 50 or 60 miles to work in a city. It would shorten the range of everyone who has to count pennies. That hurts, no questions.

The gas-guzzler tax proposed by Carter will add another powerful incentive for car nuyers to move down the line to the smaller, more efficient models. By 1985, if the Detroit makes these more efficient cars and Americans will buy them, the vast U.S. fleet should approach the goal - an average of 27.5 miles per gallon.

Eck of Standard Oil of Indiana predicts that, because Carter did not solve long-range production needs, the world supply of oil will eventually force more drastic changes on Americans. "Ten years from now," he warned, "another President is going to make the real sky-si-falling speech."

One big variable is Americans' habit of buying what they really want to buy, even when the price is jacked up on them. Gasoline prices soared in 1974, for instance, but altered America's car-buying tastes only modestly. Coffee prices have more than doubled in the last year - without reducing coffee consumption.

In the transportation field, for instance, the recreation vehicles, now nearly 10 million of them, suffered a sharp setback in 1974. Sales were off 40 per cent, according to the industry association, mainly because gasoline stations closed on Sunday, not because their fuel efficiency is low. In the last three years, the RV market has soared again, up 30 per cent in 1976, up a predicted 17 per cent this year.

Steve Hines, of the Recreation Vehicle Industry Association, predicts a bright future because many familes, right or wrong, have concluded that RV's provide a better, even cheaper vacation than getting on an airplane or staying in a motel.

In housing, McGinty predicts taht better design, solar power and more respect for the elements of nature will "make housing more affordable." Though solar panels now increase the initial investment for a house, mass production will bring that down some and the long-term savings can run as high as 60 to 80 per cent of contemporary home energy costs. In time, he suggested, they may even start putting up office buildings again with windows that open.

"I think its going to be exciting," he said. "More beautiful, more humane. We're going to think more about natural comfort instead of resorting to mechanical solutions. I don't think we're going to have any suffering - I think we're just going to burn less oil."