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;