One of the barriers to widespread adoption of renewable energy sources like solar and wind power is that it must be stored on the power grid for use when the sun isn’t out or it’s not windy. Currently, battery storage at grid-scale is too expensive. But two Stanford engineers have come up with a much more budget-friendly solution: urine batteries.

More accurately, they’re batteries that use urea, found in most fertilizers and mammal urine, as an electrolyte.

“So essentially, what you have is a battery made with some of the cheapest and most abundant materials you can find on Earth,” said Hongjie Dai, one of the urea battery’s developers.

In 2015, Dai’s lab created a rechargeable aluminum battery, but the electrolyte it used made it 100 times more expensive to make than the urea version. The main goal is to use the urea battery for wide-scale grid storage, hopefully making renewable energy sources cheaper than fossil fuels.