Gravity is always on. If theres enough water and/or enough slope, you can take advantage of this fact about gravity and make cheap electricity.
With lots of flow but not much drop a water wheel can work.
With lots of drop, but less flow a turbine may be a better option.
Both can be built, salvaged(like a used basement sump pump in reverse, or bought new and purpose built.
The often work well in conjunction with solar. In the winter its dark, cloudy, and rainy. You typically use more electricity in the winter, which is fine because the rainfall should have your water source flowing well. In the summer its sunny, the days are long, and dry. Your hydro may not be performing as well but your usage is typically down and the solar fills the void.
He who sweats most in times of peace,
bleeds least in times of war.
I'm just a poor boy, I need no sympathy, because I'm easy come, easy go, little high, little low, little ad