如何估算太坊交易的gas消耗量

var from = web3.eth.accounts[1]
var to = web3.eth.accounts[2]
var quantity = web3.eth.estimateGas({
  from:from,
  to:to,
  value:50000000000000})
console.log('大约要消耗gas：',quantity)
var amount = quantity * web3.eth.gasPrice
console.log('转出方的账户余额大约要减少：',amount,'(wei)')

pragma solidity ^0.4.8;
contract EzTest {
    uint public num;
    function setNum(uint newNum) {
        num = newNum;
    }
}

//inst 是EzTest合约的实例对象
inst.setNum.estimateGas(4, {from: web3.eth.accounts[1]})