Simulation of a prospective ultra-lower power electronics system can help you confirm if a design is feasible. This step can also help prevent premature optimization and futile optimization of the wrong areas of the design. 

The main goal of simulation is to understand the power used in various modes, how long is spent in each mode, and any energy harvesting that may be available. I have developed some helpful open source tools for this, which you can find on my other Hackaday.io project here: 

Embedded System Power and Energy Estimation Tool