People in the Arabian desert survived for centuries without air conditioning, but it has taken only a few decades for it to become so widespread that it is starting to threaten the national purse.

Saudi Arabia is in the throes of an energy crisis, and to officials who are mounting a nationwide conservation campaign, there is a new paradigm in this ostensibly energy-rich society: Raise the thermostat and sweat a little.

The alternative, they warn, is to pony up an estimated $120 billion to triple the country's electrical capacity and keep pace with skyrocketing demand. It is a steep bill, even in a country accustomed to spending freely, and one that officials are hoping to avoid.

"This is something we have to get used to," said Abdul Rahman Tuwaijri, deputy minister for electricity, who is leading conservation efforts in a country where people are known to leave their air conditioners running when they leave home for the summer, to protect their wallpaper from the intense desert heat. At some peak periods, he said, air conditioning accounts for as much as 70 percent of the country's electricity consumption.

"We overdid it," he said. "I say to all communities, we should think about this as a duty. . . . Even if we have billions and billions of dollars, I should do 100 percent my duty to conserve."

Tuwaijri is a man of his word. At the start of a recent interview, he began to sit down, then scuttled across his office to turn off a light. His office is noticeably warmer than many others. He said he has trained his children to tell on each other when someone leaves a light on at home. The informant gets part of the offender's allowance.

But extending such discipline across a country with a quarter of the world's known oil reserves is another matter.

As oil prices rose in the 1970s, and this once sleepy kingdom became a worldwide symbol of overnight wealth and power, consumer goods began to pour in. Along with air conditioning, Saudis stocked up on radios, televisions, electric ovens and all the other trappings of modern society.

With electricity rates heavily subsidized by the state, there has been little incentive to worry about the cost of using all the new gadgets, and with so much oil in the ground, there seemed little need to worry about how much was being consumed by the expanding number of power plants.

Coupled with a rapidly increasing population and industrial growth, however, the Saudi appetite for modern conveniences, air conditioning in particular, has stretched the power system to its limits.

Already, on the hottest days, Saudis are encouraged to follow "load-management" guidelines to conserve power. Industries are being asked to shift production schedules to hours of the day when demand is lightest. Homeowners are requested to keep their ovens, irons and water pumps off at midday, and their thermostats set as high as possible.

Occasionally, though, demand becomes so heavy that the country has to cut power to industry and other heavy users to avoid overloading the system.

More ominous, Tuwaijri said, demand is projected to grow so rapidly that Saudi Arabia will have to triple its electricity output in the next 20 years, requiring a massive investment in new plants.

Hard realities like that, coupled with slumping government cash flow caused by low oil prices, have generated plans to sharply increase some electricity rates so that they roughly cover the cost of production -- a significant cultural and economic shift.

But if Tuwaijri has his way, some of those costs can be avoided by simply convincing Saudis that even if they do live in one of the world's great oil patches, they shouldn't waste the national treasure trying to maintain igloo-like temperatures in their homes and offices.

In a campaign reminiscent of U.S. efforts to curb energy use at the height of the Saudi- and OPEC-led oil embargo in the early 1970s, Tuwaijri said he is using schools, mosques, radio and television to tell people that saving power is not only economical but good for the country -- and even stipulated by Islam.

"Our religion asks us only to use things that are beneficial. We want people to use only the needed things," Tuwaijri said. "It is not miserly behavior. It is brave behavior."

CAPTION: To persuade Saudis to reform their power-wasting ways, government posters juxtapose a man dutifully adjusting his air conditioner and a guilty household.