<p>I'm trying to write a small app that monitors how much power is left in a notebook battery and I'd like to know which Win32 function I could use to accomplish that.</p>

For Vista and up you can use
RegisterPowerSettingNotification
For earlier functions see the
Power Management Functions
in this section of the MSDN page "Power Management Functions: Windows Server 2003 and Earlier"
You can see example code of the Vista method on
codeproject
.