The planks on energy and environmental policy in the Democratic platform adopted here establish a clear choice for voters concerned with these issues in this year's presidential campaign.

The Republicans put their faith in the free market, the oil companies and nuclear power. The Democrats seek to encourage government-sponsored research into new energy sources and higher taxation on the oil companies, and are skeptical about nuclear power.

Both parties endorse decontrol of domestic oil and gas prices, though the Republicans trumpet the benefits of substantially higher domestic prices, while the Democrats prefer not to dwell on them.

The Democratic platform says, "We must make conservation and renewable energy our nation's priorities for the future." It endorses heavy investments in research on solar energy and synthetic fuels, as well as expensive new measures to encourage conservation, including "a massive residential energy conservation grant program."

The differences between the Republican and Democratic energy planks are symbolized by a single issue: the 55 mph speed limit. The Democrats endorse it, noting that if just one of four drivers who now exceed that limit would respect it, the country could save 100,000 barrels of oil per day. The Republicans say individual states should be permitted to abolish the 55 mph limit if they want to.

The Democratic platform contains several passages urging much greater reliance on domestic coal. "We must lead the western world in developing a program for increased use of coal in Europe, Japan and the developing nations," the platform says. It sets a goal for the use of solar energy, recommending that it "account for 20 percent of our total energy by the year 2000."

On the environment, the Democrats make a polite nod in the direction of anti-regulatory sentiments that prevail in the Republican platform, but then insist that regulation continue.

The platform says, "We must not lose sight of the fact that the benefits of these regulations (protecting the environment) far outweigh their costs. We must work to reform regulation without deforming it."

The Democrats fully endorse the Environmental Protection Agency and pledge to fund it adequately. They endorse "strict adherence to automobile pollution standards." (Ronald Reagan has suggested relaxing these standards to help American automakers compete better against foreign producers.)

While endorsing offshore oil exploration and many new energy development efforts, the Democrats say that "all of our energy development efforts should be carried out without sacrificing environmental quality." The Republicans criticize putting many potential new sources of energy out of reach by imposing environmental controls that preclude their exploitation.

On the politically sensitive issue of water resources, the Democrats adopted a cautiously worded plank saying, in effect, that new water projects should be pursued with care. "The Democratic Party strongly supports the desalinization of sea water," the platform declares.