While it is hard to directly access the Windows Power Management API, there are other API calls that can do this for you. The following one-liner returns the current power status of your machine. If you are using a notebook and have no AC power, it can tell you the remaining battery power:
Add-Type -AssemblyName System.Windows.Forms [System.Windows.Forms.SystemInformation]::PowerStatus
The result looks similar to this:
PowerLineStatus : Offline BatteryChargeStatus : 0 BatteryFullLifetime : -1 BatteryLifePercent : 0,45 BatteryLifeRemaining : 13498
