How we can use P4 for power saving.
Your question is fairly general, so my reply will also be 
A non-programmable switch or NIC in a network will, in many cases, implement a “kitchen sink” kind of feature set chosen by the ASIC developer, based upon what some combination of their most common customers want. This is likely to be more than any individual one of the customers wants.
A P4-programmable network device opens up the possibility to customize the data plane features and table sizes to exactly what a particular network deployment needs, with little or no extra features there. That might help reduce the power utilization of that device, too, but really you need measurements to find out whether that is the case for any particular deployment. Even non-programmable devices often have capabilities to adjust the sizes of tables for various features, and reducing the size of tables and “disabling” or “powering down” the part of the memory that isn’t needed is often the biggest power saving you can achieve in many cases.
I would also add that on some programmable targets, such as Intel Tofino there are additional ways to save power. The details can be learned in Intel Connectivity Academy, but suffice it to say that different P4 constructs and the underlying hardware primitives have different power consumption.
Therefore, the programmer has the ability to write their program in a variety of ways depending on the requirements and as a result, the programs can have different power profiles even while they all might be implementing the same algorithm.
We also do provide a visualization tool (P4 Insight, aka P4i) that allows the programmer to visually examine the power consumption of their program, find the P4 objects that consume the most power, etc.
Happy hacking,
Vladimir