The idea that giant batteries may someday revolutionize electrical grids has long enthralled clean-power advocates and environmentalists. Now it's attracting bankers with the money to make it happen.

Lenders including Investec PLC, Mitsubishi UFJ Financial Group Inc. and Prudential Financial Inc. are looking to finance large-scale energy-storage projects from California to Germany, marking a coming-of-age moment for the fledgling industry. The systems help utilities solve a long-standing clean-power conundrum: managing the unpredictable output from wind and solar farms, and retaining electricity until it's needed.

Battery costs have declined 40 percent since 2014 and regulators are mandating storage technology be added to the grid. That's encouraging utilities to offer longer contracts and developers are expected build $2.5 billion in systems globally this year. These trends are changing the risk profile, giving lenders confidence in batteries in much the same way that power-purchase agreements opened banks' doors years ago for wind and solar power.