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;