What are typical techniques that we could do at the software level to reduce energy consumption? Or is energy consumption mainly achieved through hardware design?

From my understanding, we are given the fixed power and our goal is to increase performance by increasing energy efficiency. One thing we can try is to specialize the hardware and this will give better energy efficiency. This specialized processor is expected to be more efficient than the general-purpose processor.

There was a (somewhat controversial) paper published last year ranking the energy efficiency of several programming languages. Unsurprisingly, C is the most energy efficient!

https://greenlab.di.uminho.pt/wp-content/uploads/2017/10/sleFinal.pdf

Is there any way we can vary the amount of power we have at our disposal? Or is that a constant fixed value?

@yonkus I think what they mean here by power being "fixed" is that the hardware (transistors) constrains how much power we can dissipate(in the form of heat) without causing a failure in the circuitry.