I decided to run a test.
![Image](https://forums.factorio.com/images/ext/7a2a2ea80d4e3e086e9be84ae0a871d2.jpg)
Each inserter gets 5 MJ of energy, and has to use that to fill a box with as much coal as possible.
Results
Inserter: 726 coal
Long-handed inserter: 736 coal
Fast inserter: 756 coal
Smart inserter: 544 coal
The results are very close, but the fast inserter uses the least power!
Interestingly, inserters do not run at full power all the time. The graph looks like this.
![Image](https://forums.factorio.com/images/ext/1293ea694befeedac7fcc42c5329f322.png)