American households, which use only one-quarter of the energy consumed nationwide, accounted for more than half of the nation's reduction in energy use from 1978 through 1980, according to a new Department of Energy analysis.

During that three-year period, American families achieved a 17 percent reduction in energy used in their homes and cars, a "dramatic, striking" result, said J. Erich Evered, energy information administrator.

The millions of individual decisions to turn down thermostats, insulate houses and apartments and switch to more fuel-efficient heating equipment and cars--and a relatively mild winter in 1980--were responsible for the energy savings, he said.

Overall, the nation consumed 75.9 quadrillion British thermal units (BTUs) of energy in 1980 (there are just over 125,000 BTUs in a gallon of gasoline).

The reduction from 1978 totaled 2.3 quadrillion BTUs, or "quads," as this measure is commonly called. Energy saving by households accounted for 1.3 quads, while business and industry reduced energy use by 1 quad.

Evered said he expected to see the same trends continue although perhaps at a slower rate. "We've just had a drop in the bucket in terms of conservation spending."

And he noted that the effects of the recent recession have probably increased the drop in consumption by industry since the years surveyed by DOE.

Despite major reduction in demand, it was not enough to keep up with rising energy costs.

The average household expenditure for energy rose from $724 to $917 during the three-year period, an increase of nearly 27 percent despite declining use. Total household energy expenditures rose 36 percent, from $55 billion in 1978 to $75 billion in 1980.

However, there was further evidence yesterday that reduced demand for petroleum prices is pushing prices down as two oil companies announced that they will reduce the price of a barrel of crude oil by $1, or approximately 3 percent.

Ashland Oil Inc. said it will reduce by $1 a barrel the price it intends to pay for crude oil because of the drop in value of products refined from the crude.

Cities Service Co. cut prices at which it sells its crude by $1 a barrel except for Kansas sweet crude, which was reduced by 50 cents, citing essentially the same reasons. Many oil producers had raised their prices in October.

Along with the evidence of reduced consumption, the study found major changes in the types of fuel American are using to heat and light their homes.

One of the biggest changes was the increase in the use of wood as a home heating fuel. In 1981, more than 5 million homes, or about 6 percent of the total, used wood as their primary heating source, Evered said.

In 1978, only 1.9 million households used wood as the primary fuel. Wood is now the fourth most frequently used household fuel, replacing liquefied natural gas. Most of the use is in rural areas.

The biggest declines in fuel use involved natural gas and fuel oil. Both total and average household consumption of natural gas fell during the period, although the percentage of homes using natural gas as a main heating fuel was essentially unchanged.

In the case of fuel oil, there were declines in all three indicators, the total amount used, the average amount used and the percentage of homes that used fuel oil as a principal heating source.

The survey found that houses using fuel oil as the main heating fuel had the highest total energy expenditures. The average was $1,458, up from $959 in 1978. The lowest expenditures were in homes using electricity for heating, which increased to $801 in 1980 from $662 in 1978.

Evered noted that some of that difference was accounted for by regional disparities. Most of the electric homes are in the South and are smaller than fuel oil heated homes, many of which are in the colder Northeast.

He cited several factors that contributed to residential conservation. Among them were a milder winter in 1980, increased use of renewable fuels such as wood, conservation activities, improved thermal efficiency in new homes and appliances and changed energy consumption patterns.

The study found wide variations in consumption patterns.

The highest average expenditures, not surprisingly, were found in the Northeast, where the average was $1,268. The lowest was $604 in the West.

During the period surveyed, high-income households consumed more energy than low-income households but they also reduced their energy consumption by a larger percentage than did the low-income households.