According to [1], the most important factor for the power consumption of code is how long the code takes to run. Code that spreads over multiple cores is generally more power efficient than code that runs sequentially, because the power consumption of multiple cores grows less than linearly (that is, it requires less than twice as much power to run two cores as it does one core).

Therefore if parallelising code reduces the runtime of that code, it is almost always more energy efficient to do so. Obviously if this is important in a particular context, it's probably worth measuring it in that context (e.g. embedded devices), but I suspect this is true more often than it isn't true.

[1]: https://arxiv.org/abs/2410.05460

> Multithreading does not make code more efficient. It still takes the same amount of work and power (slightly more).

In addition to my sibling comments I would like to point out that multithreading quite often can save power. Typically the power consumption of an all core load is within 2x the power consumption of a single core load, while being many times faster assuming your task parallelizes well. This makes sense b/c a fully loaded cpu core still needs all the L3 cache mechanisms, all the DRAM controller mechanisms, etc to run at full speed. A fully idle system on the other hand can consume very little power if it idles well(which admittedly many cpus do not idle on low power).

Edit:

I would also add that if your system is running a single threaded database, and a single threaded web server, that still leaves over a hundred of underutilized cores on many modern server class cpus.

Multithreading can made an application more responsive and more performant to the end user. If multithreading causes an end user to have to wait less, the code is more performant.