The overall stock market was fabulous last year, but as investors focused on climate change, renewable energy stocks did even better.

Consider that while the SPDR S&P 500 Exchange-Traded Fund Trust, which tracks the benchmark S&P 500, returned 18.37 percent in 2020, the Invesco Solar E.T.F., which tracks an index of solar energy stocks, soared 233.95 percent, according to Morningstar Direct. The Invesco WilderHill E.T.F., which invests more broadly in alternative energy of various types, rose 204.83 percent.

Returns like those are so strong that they are unlikely to be replicated: It is possible that the stocks of companies engaged in carbon-free energy production are already in a bubble. Jason Bloom, head of fixed income and alternative E.T.F.s for Invesco, describes the sector this way: “I would call it rational optimism in view of improving fundamentals.”

The International Energy Agency recently called solar-generated energy the “cheapest” electricity source in many countries. In the United States, it accounts for just 3 percent of energy output, but it is increasing rapidly. Wind power, which now supplies roughly 8 percent of domestic energy, has also been growing. There is plenty of room for expansion for many renewable energy companies.

The results of the presidential election have already bolstered the returns of these companies, too. While President Trump has promoted the use of fossil fuels like coal, President-elect Joseph R. Biden Jr. has advocated a $2 trillion climate plan to “achieve a carbon pollution-free power sector by 2035.” His plan, not yet fully detailed, includes a variety of investment inducements and tax breaks.

That would be more ambitious than the once-trendsetting 2045 goal of carbon-free energy production set by California. Already, solar power accounts for 18 percent of electricity generation at the utility Southern California Edison, said Erica Bowman, the company’s director of resource and environmental planning and strategy.

Garvin Jabusch, chief investment officer for Green Alpha Advisors, an alternative energy investor, notes that the cost of generating electricity from solar energy is 90 percent lower than 10 years ago. Mr. Jabusch expects alternative energy prices to decline further with expanded demand. Mr. Jabusch favors companies that are “growing production capacity,” like First Solar, which has opened a new plant in Lake Township, Ohio, to expand production of its solar panels.

For all its promise, investment in solar and wind power is limited by the laws of nature: Solar units can produce electricity only when the sun is shining, and wind turbines need wind.

For the most part, Southern California Edison backs up its solar power with electricity generated by natural gas. But the utility recently contracted for nearly 600 megawatts of lithium ion battery storage so it can store excess electricity produced under ideal weather conditions.

“Battery prices are down 90 percent over the last five to eight years,” Ms. Bowman said. “As we transition to a cleaner grid, solar generation coupled with battery storage is the cost-effective solution for California,” she added.

Hydrogen fuel cells, which produce electricity by combining hydrogen and oxygen, have emerged as a possible near-term solution for use in trucking and shipping, says Mr. Bloom. But such applications will require a costly expansion of the hydrogen gas filling station network, said Steve Capanna, director of U.S. climate policy and analysis for the Environmental Defense Fund. Right now, he said, beyond “a handful in California,” there aren’t many such stations.

Buying shares of renewable energy stocks now requires a degree of faith, because they are so expensive, partly because of the low interest rates engineered by the Federal Reserve, which have helped to drive the overall stock market higher. Fed support may be the biggest reason the market has withstood all the grim economic news of the coronavirus to continue its seemingly unending valuation advance.

Paul Coster, a JPMorgan analyst, said that the high prices in the renewables sector are based on solid achievement. “It’s not like the dot-com era,” he said. “These are real actors with real technology.” He added, “We’re living in this wonderful moment in time when virtue and self-interest coincide.”

Perhaps, Mr. Coster mused, there are still good reasons to own some of these stocks. He cited FuelCell Energy, which has negative cash flow and has consistently reported quarterly earnings losses. Mr. Coster said investors may want to project out several years.

By 2025, he said, it’s “feasible” that FuelCell Energy would have $60 million in earnings before interest, taxes, depreciation and amortization, justifying a rich, growth stock valuation. Even so, the company’s shares more than doubled in the last month, and on Jan. 14, Mr. Coster warned that at current prices, the stock was already “richly valued.”