BIBLIS, Germany — Not since the grim period after World War II has Germany had significant blackouts, but it is now bracing for that possibility after shutting down half its nuclear reactors practically overnight.

Nuclear plants have long generated nearly a quarter of Germany’s electricity. But after the tsunami and earthquake that sent radiation spewing from Fukushima, half a world away, the government disconnected the 8 oldest of Germany’s 17 reactors — including the two in this drab factory town — within days. Three months later, with a new plan to power the country without nuclear energy and a growing reliance on renewable energy, Parliament voted to close them permanently. There are plans to retire the remaining nine reactors by 2022.

As a result, electricity producers are scrambling to ensure an adequate supply. Customers and companies are nervous about whether their lights and assembly lines will stay up and running this winter. Economists and politicians argue over how much prices will rise.

“It’s easy to say, ‘Let’s just go for renewables,’ and I’m quite sure we can someday do without nuclear, but this is too abrupt,” said Joachim Knebel, chief scientist at Germany’s prestigious Karlsruhe Institute of Technology. He characterized the government’s shutdown decision as “emotional” and pointed out that on most days, Germany has survived this experiment only by importing electricity from neighboring France and the Czech Republic, which generate much of their power with nuclear reactors.

Then there are real concerns that the plan will jettison efforts to rein in manmade global warming, since whatever nuclear energy’s shortcomings, it is low in emissions. If Germany, the world’s fourth-largest economy, falls back on dirty coal-burning plants or uncertain supplies of natural gas from Russia, isn’t it trading a potential risk for a real one?

The world is watching Germany’s extreme energy makeover, as politicians from New York to Rome have floated their own plans to shut or shelve reactors.

The International Energy Agency, generally a fan of Germany’s green-leaning energy policy, has been critical. Laszlo Varro, head of the agency’s gas, coal and power markets division, called the plan “very, very ambitious, though it is not impossible, since Germany is rich and technically sophisticated.”

Even if Germany succeeds in producing the electricity it needs, “the nuclear moratorium is very bad news in terms of climate policy,” Mr. Varro said. “We are not far from losing that battle, and losing nuclear makes that unnecessarily difficult.”

The government counters that it is prepared to make huge investments in improving energy efficiency in homes and factories as well as in new clean power sources and transmission lines. So far, there have been no blackouts.

But Jürgen Grossmann, chief executive of the German energy giant RWE, which owns two closed reactors here in Biblis, about 40 miles south of Frankfurt, expressed skepticism. “Germany, in a very rash decision, decided to experiment on ourselves,” he said. “The politics are overruling the technical arguments.”

The Nuclear Equation

Germany’s planners believed they could forgo nuclear energy in large part because of the country’s remarkable progress in renewable energy, which now accounts for 17 percent of its electricity output, a number the government estimates will double in 10 years. On days when the offshore wind turbines spin full tilt, Germany produces more electricity from renewable sources than it uses, according to European energy monitors.

Germany has “exceeded everyone’s expectations on renewable power,” said Mr. Varro, showing that it could be cost effective and reliable.

Until it closed the reactors, Germany was Europe’s leading energy exporter.

With a total of 133 gigawatts of installed generating capacity in place at the start of this year, “there was really a huge amount of space to shut off nuclear plants,” Harry Lehmann, a director general of the German Federal Environment Agency and one of Germany’s leading policy makers on energy and environment, said of the road map he helped develop. The country needs about 90.5 gigawatts of generating capacity on hand to fill a typical national demand of about 80 gigawatts a day. So the 25 gigawatts that nuclear power contributed would not be missed — at least within its borders.

To be prudent, the plan calls for the creation of 23 gigawatts of gas- and coal-powered plants by 2020. Why? Because renewable plants don’t produce nearly to capacity if the air is calm or the sky is cloudy, and there is currently limited capacity to store or transport electricity, energy experts say.

New coal and gas plants will use the cleanest technology available and should not aggravate climate change, government officials said, because they will operate within the European carbon-trading system in which plants that exceed the allowed emissions cap have to buy carbon credits from companies whose activities are environmentally beneficial, thus evening out the environmental ledger.