gistfile1.js

var os          = require('os'),
    prevTimes;
    
function getCpuTimes() {
    var cpus    = os.cpus(),
        cpusl   = cpus.length,
        times   = {
            user: 0
          , sys : 0
          , nice: 0
          , idle: 0
        };
        
    cpus.forEach(function(cpu) {
        times.user += cpu.times.user;
        times.sys += cpu.times.sys;
        times.nice += cpu.times.nice;
        times.idle += cpu.times.idle;
    });
    
    times.user = times.user / cpusl;
    times.sys = times.sys / cpusl;
    times.nice = times.nice / cpusl;
    times.idle = times.idle / cpusl;
    
    return times;
}

prevTimes = getCpuTimes();

function getCpuUsage() {
    var times   = getCpuTimes(),
        user    = prevTimes.user - times.user,
        sys     = prevTimes.sys - times.sys,
        nice    = prevTimes.nice - times.nice,
        idle    = prevTimes.idle - times.idle;
        
    prevTimes = times;
    
    return (user + sys + nice) / idle * 100;    
}

setInterval(function() {
    console.log('cpu usage percentage: ' + getCpuUsage());
}, 1000);