Is there a single adjective that means "does not use much energy" / "not using much energy"? 

**I do not mean "efficient"**. 

Efficient means something that does not waste much energy - something efficient could use a lot of energy for a specific purpose, yet not waste much energy on actions that do not achieve that purpose/not waste energy as heat or sound or whatever. 

However, something \*word I seek\* may or may not be efficient, i.e may waste very little, or very much energy, in trying to perform a task, but does not/will not **use** or **consume** much energy in trying to perform the task - whether some of this energy is wasted or not.