President Biden is still hoping to get “big chunks” of his Build Back Better agenda passed, and that could mean $500 billion in climate spending, including subsidies for green electricity. Although Sen. Joe Manchin is opposed to a plan to create and enforce clean-power mandates, he seems open to shoveling out money instead.

“The climate thing,” he said in early January, “is one that we probably can come to an agreement much easier than anything else.” Typical: Whenever Congress wants to cut a deal, spending is a least common...

Chairman Joe Manchin, D-W.Va., speaks during a Senate Energy and Natural Resources Committee hearing on pending nominations in Dirksen Building on Tuesday, Feb.8.

Photo: Tom Williams/CQ Roll Call/Zuma Press

President Biden is still hoping to get “big chunks” of his Build Back Better agenda passed, and that could mean $500 billion in climate spending, including subsidies for green electricity. Although Sen. Joe Manchin is opposed to a plan to create and enforce clean-power mandates, he seems open to shoveling out money instead.

“The climate thing,” he said in early January, “is one that we probably can come to an agreement much easier than anything else.” Typical: Whenever Congress wants to cut a deal, spending is a least common denominator. But given the way energy markets work, it would be a mistake to think of this as mere corporate welfare.

The House version of Build Back Better features billions of dollars in subsidies for renewables, including to extend and expand the Production Tax Credit. If you guessed Democrats want to transmogrify this “tax credit” into something that isn’t a tax credit, you’re correct. The BBB plan, Windpower Monthly reports, would also let renewable generators “receive ‘direct payment,’ instead of a tax credit, which is invaluable when the project owner does not have enough tax liability.”

Direct payments to wind farms might sound like the usual unappetizing green pork. But electricity markets in most of the U.S. are facilitated via Regional Transmission Organizations (RTOs) and Independent System Operators (ISOs). Generators bid in at their marginal cost of production, and wind and solar don’t need to buy fuel. The highest bid needed to meet demand is what every producer gets paid.

The more that Congress amps up renewable subsidies, the more fossil-fuel generation risks getting forced out of such electricity markets. This might interest Mr. Manchin, given his state’s coal and gas jobs. But it’s important more broadly because fossil-fuel generators produce readily available electricity that’s needed once the skies turn overcast or the wind turbines shut down in a cold snap, as disastrously happened in Texas last year.

Even without new subsidies via Build Back Better, the advance of renewables is raising big questions. “What happens when price is no longer an effective tool for fulfilling the tasks that RTOs were created to complete?” asks a paper co-written last year by Tony Clark, a former Republican member of the Federal Energy Regulatory Commission. “We don’t believe it will be possible going forward to ignore how the kilowatt hour was generated and simply conduct an auction among all kilowatt hours however derived.”

Energy storage could help solve the reliability problem by letting wind and solar electricity be saved for a calm rainy day, so to speak. But we’re still waiting on the technology breakthroughs needed to make it happen at scale. Another issue is the massive construction of transmission lines that the left imagines will carry power from outlying wind and solar farms. How are they going to be funded, and what if locals try to block those big transmission towers? There are big issues here that the Senate Energy Committee, led by Mr. Manchin, might want to chew over.

In the here and now, though, millions of people need and expect the heat pump to whirr when they hit the thermostat, no matter the freak cold front arriving outside. Progressives want evermore subsidies for renewable power, and if Congress views this idea as merely green spending or at worst corporate welfare, it would be sleepwalking into bigger problems.

Journal Editorial Report: If it can happen in San Francisco, can it happen anywhere? Images: WSJ/San Francisco Chronicle/Getty Images Composite: Mark Kelly The Wall Street Journal Interactive Edition