A proposal by Gov. Tim Walz to make Minnesota's electricity providers generate all their energy from clean sources, like solar and wind, by 2050 drew immediate criticism Monday for being too costly and unreliable.

"The new policies will ensure reliable, affordable and sustainable electricity in Minnesota," said Walz, a Democrat. "They'll also give the state a cleaner, healthier environment and a strong clean-energy economy."

Environmental organizations and student advocates cheered the move. Republicans and some utility companies said it would raise energy prices for consumers and put too much pressure on an energy system that so far hasn't proved to be reliable.

"Renewable resources are not available all the time. You know the sun doesn't shine and the wind doesn't blow 24/7," said Joni Livingston of Missouri River Energy Services, which provides electricity in 25 communities around Minnesota.

Walz's idea differs slightly from a similar requirement state lawmakers recently proposed. The legislators' measure set a series of benchmarks that companies would have to meet in the years leading up to 2050. Walz said he left those out, giving companies more flexibility as they work toward 100 percent. He said he hopes they will "blow by" the benchmarks legislators want to set and achieve the goal even earlier than 2050.

The additional flexibility is helpful as utilities come up with strategies based on the resources available to them, said Livingston. Nonetheless, she said she is worried about the limits of renewable energy.

Governor Tim Walz and Lieutenant Governor Flanagan held a press conference to announce major energy and climate policy initiatives.

"Achieving that goal is going to be very challenging and it's going to be very expensive. That's really if it can be accomplished at all," she said.

Xcel Energy, the state's largest energy provider, has already set its own goal of going carbon-free by 2050. The bill legislators previously proposed would push the company to move faster, with a 2045 deadline. Walz's plan doesn't include that more ambitious time frame for Xcel.

The company said in a statement that it shares Walz's goal of zero-carbon electricity and looks forward to working with the administration, lawmakers and stakeholders on next steps. Xcel emphasized that the state needs to move forward in a way that doesn't raise customers' bills.

The full cost of implementing the changes in Walz's proposal was not yet clear.

Xcel relies on nuclear generating plants to help cut greenhouse gas emissions. Nuclear energy is one of the potential clean energy sources allowed under the governor's plan, Commerce Commissioner Steve Kelley said. But he said he's not sure whether the two nuclear power plants in Minnesota will still be around by 2050.

Senate Majority Leader Paul Gazelka, R-Nisswa, said it is important that Walz is talking about nuclear power. Consistent power-generation sources need to be part of the equation, he said, because when it's 22 below, wind power won't work.

Not all utilities have access to nuclear power, Livingston stressed. Missouri River Energy Services gets a small amount of nuclear power from a plant in Wisconsin, she said, in addition to other renewable sources like hydropower, solar and wind.

It is critical to ensure Minnesotans have reliable energy sources in situations like a polar vortex, Rep. Chris Swedzinski, R-Ghent, said in a statement.

"Governor Walz's extreme energy proposals would cause Minnesotans' energy bills to skyrocket, force the closure of reliable and cost-effective power plants, and puts Minnesota all-in on technology that simply cannot provide the reliable power you need to keep the lights on and heat your home in the winter," said Swedzinski, the Republican lead on the House energy and climate committee.

When extreme cold weather hits, it can cause performance issues not just with wind turbines but with coal and natural gas, noted Michael Noble, executive director of the clean energy advocacy group Fresh Energy. Additional technical improvements to clean energy will be necessary, he said, but he believes a reliable carbon-free electric system is inevitable.

Noble described Walz's goal as "the first and easiest part" of a multistep move away from fossil fuels. Once the power sector moves to clean energy, it can electrify other sectors, like transportation, he said.

The governor and others at the Capitol are pushing for Minnesota to build on the work started under former Republican Gov. Tim Pawlenty. In 2007, the state set the goal of having utilities rely on renewable energy sources for at least a quarter of their energy by 2025 — a goal they have already surpassed.

Solar-energy company employees in hard hats and environmental advocates were part of the crowd that stood behind Walz as he announced the goal Monday. The group promoted the shift as a way to create more clean-energy jobs and combat climate change.

In addition to the 2050 goal, Walz's proposal states that anytime a utility company replaces or adds new power generation, it should prioritize clean energy. It would also expand a program that helps homeowners and businesses increase energy efficiency and help more low-income residents get assistance for energy-saving projects.

A committee will hear the bill next week, said Rep. Jamie Long, D-Minneapolis.