Monitoring power consumption on Mac

As of Mavericks, this is provided by the OS. Activity monitor has a tab devoted to displaying the energy use of running (and recently running) apps.

Also, the battery icon shows applications that use significant energy on Mavericks and newer.

iStat Menus

let's you monitor your current computer power usage. Unfortunately, it does not break it down by app.

If you try to catch an app or process that is exceptionally energy-hungry, to the point it annoys MacOS, you can also open the Mac console application (in /Applications/Utilities folder) and select the "Diagnostic Reports" group on the side-bar. high-energy incidents are recorded there, many times with quite detailed information. Each report is for one energy-impact incident. For example

shows that the 'zoom' application was imposing high energy impact several times (dozens of times to be exact...) This technique is useful for retrospective analysis - not for what's going on NOW on the machine (where Activity Monitor is your friend) but for the long run - to see what apps are better removed from your Mac altogether.