# powerMonitor > Monitor power state changes. You can only use it in the main process. You should not use this module until the `ready` event of the `app` module is emitted. For example: ```javascript app.on('ready', function() { require('electron').powerMonitor.on('suspend', function() { console.log('The system is going to sleep'); }); }); ``` ## Events The `power-monitor` module emits the following events: ### Event: 'suspend' Emitted when the system is suspending. ### Event: 'resume' Emitted when system is resuming. ### Event: 'on-ac' _Windows_ Emitted when the system changes to AC power. ### Event: 'on-battery' _Windows_ Emitted when system changes to battery power.