IT'S GONE LARGELY unnoticed, but there's been a profound transformation in one area of energy policy: spending on energy research, development and demonstration.

In 1973, the federal budget contained a grand total of $4 million for R&D in solar energy. That sum quadrupled the next year, and then tripled in each of the following two years. For 1981, President Carter has proposed a solar budget of $1.517 billion -- almost a 400-fold increase in eight years. That spending will go well beyond research: a large proportion is devoted to demonstration and implementation programs, with $450 million planned for tax credits and exemptions and a low-interest solar bank.

Spending for conservation has undergone even more explosive growth. In 1973, budget authority for energy conservation totaled $6 million; for next year, the president proposes $2.8 billion -- almost a 500-fold increase. Only about 10 percent of that amount will be spent for research on new conservation technologies. The rest will go for grants and tax credits to offset the cost of energy-saving investments, mass-transit and automobile-efficiency programs, public education campaigns, conservation efforts in homes and commerical buildings, and much more.

On the other hand, spending for nuclear R&D, which traditionally consumed most of the available budget dollars, has dropped. Acting on its belief that breeder rectors won't be economically competitive for at least 40 years, the administration cut that previously untouchable budget in half -- from more than half a billion dollars in 1980 to $320 million for 1981. Spending for the outdated and unnecessary Clinch River breeder demonstartion plant has finally been dropped altogether. The total nuclear fission budget is down about 15 percent in just one year.

So far, the need for increased defense spending and the size of the windfall profits tax on oil have dominated debate over the 1981 budget. But the dramatic changes in energy spending -- in just one year, the solar budget is up 30 percent, conservation up 50 percent -- may well emerge as being of equal, or greater, importance.