Three percent of our nation’s energy goes to pump and treat water, so cutting back water use conserves energy and reduces greenhouse gas pollution. Demand type water heaters are an option. I don’t have one so I simply put an empty bowl under the shower head, allowing it to fill until the water runs hot enough. Then I use the bowl of cold water for plants.
https://19january2017snapshot.epa.gov/climatechange/what-you-can-do-home_.html
https://www.energy.gov/energysaver/heat-and-cool/water-heating/tankless-or-demand-type-water-heaters