Sustainable energy is energy which is self-renewing, since it is part of natural systems. It does not get depleted like oil or gas reserves.
Examples of sustainable energy are sources like wind or solar power. Power output varies as the natural conditions (sun, wind) change. The wind does not always blow, and changes speed. The sun does not always shine, and moves through the sky.
The systems which harvest this energy must relate to this variety, yet honor the integrity of the users, whose needs also vary through time. The systems often do it by: 1. Creating links, as well as using the value of some and grouping, and, 2. Making independent connections, for smooth transitions. More about both follows:
1. Links between solar collection and use are made via batteries and/or the energy grid. The wind is often blowing somewhere, so linked systems of many windmills generate more continuous, ongoing power as a group.