European utilities are pushing to build power plants that would burn natural gas now and then be switched to burn clean hydrogen down the road. But the technology they’re depending on wastes energy and opens the door for accusations of greenwashing.

RWE, SSE and Equinor say the swap will be easy once green hydrogen becomes plentiful and cheaper. The concept is so popular that Germany, Europe’s biggest energy user, is counting on the clean fuel to help it achieve 100% renewable-power production by 2035.

A significant obstacle to those plans is hydrogen’s inefficiency. By the time the gas is made, stored and burned to make electricity again, there’s nearly 70% less energy than at the start — and the cost has tripled. Plus, there may not be enough clean tech to produce that hydrogen, leaving an opening for fossil fuels.