procedure TForm1.Button1Click(Sender: TObject);
   var
     SysPowerStatus: TSystemPowerStatus;
   begin
     GetSystemPowerStatus(SysPowerStatus);
     if Boolean(SysPowerStatus.ACLineStatus) then
     begin
       ShowMessage('System running on AC.');
     end
     else
     begin
       ShowMessage('System running on battery.'); 
       ShowMessage(Format('Battery power left: %d percent.', [SysPowerStatus.BatteryLifePercent]));
     end;
   end;
	             		

Источник: www.swissdelphicenter.ch