Fields of energy-churning windmills sprout in California

When this decade began, there was no such thing as a wind farm. Now there are enough of them that, by the end of this month, more than 4,000 modern windmills will be erect and spinning in California's blowy mountain passes.

These new-age windmills have sprouted like giant, bare-limbed weeds almost entirely within the past two years, most of them during 1983. They are going up, according to Thomas Gray, director of the American Wind Energy Association, just as fast as they can be manufactured.

Since the first wind farm was set up three years ago in New England, California has become the premier wind-farming country, accounting for about 95 percent of the nation's wind farms.

There are passes in California's myriad mountain ranges with the necessary steady and strong winds. At a wind field in the Tehachapi Mountains operated by Zond Systems Inc., for instance, wind speeds averaged more than 20 miles per hour for the first six months of this year.

Wind farms are fields full of windmills, spaced far enough apart to avoid poaching each other's wind. Modern wind machines are made to spin fast, up to 600 revolutions per minute. For this reason, the blades look as lean as airplane propellers, unlike the many-bladed traditional windmills designed to run rural water pumps or Dutch grain mills at low speed.

Perched on top of 60- to 80-foot poles, guy-wired for stability, these turbines spin electricity onto the power grid of the regional utility company, which is required by federal law to buy it at ''highest avoided cost.'' This formula amounts to whatever the power would have cost the utility if produced by oil-fired generators.

California will have a windmill capacity of more than 300 megawatts by the end of 1983, according to the California Energy Commission. This is but a trickle to the almost 40,000 megawatts of electrical current California currently consumes.

But the energy commission has projected that by the year 2000, the state will have 4,000 megawatts of windpower capacity, about 8 percent of the state's projected electrical appetite.

So far, wind farms are the product of state and federal tax credits, which amount to about 50 percent for the investor who buys into a farm.

The farms generally sell working wind machines to outside investors. Zond operates turbines for investors who own anywhere from one-quarter of a windmill to 11 machines.

The wind-power business is already beginning to shake out some of the weaker companies, and the loss of the tax credits is expected to cast a chill on the industry and leave a handful of the largest alive and well.

The 15 percent federal energy tax credit is scheduled to expire at the end of 1985.

Wind-power optimists like Mr. Gray estimate that several years from now, late in the decade, wind farms will be financially competitive without tax credits. In some areas, he says, they already are.

But it is a very young industry. ''Most of them are still working the bugs out of their equipment,'' he says.