Rate of use, not rate of change - that would indicate a derivative.
Power x Time = Energy
Energy/Time = Power
That wattmeter is designed for only 3.6-25V, it is not good for household voltage (110V-220V).
You can buy a watt-hour meter cheaply -
Amazon.com: P3 International P4460 Kill A Watt EZ Electricity Usage Monitor: Electronics
If you want to build the wattmeter project, you can modify the code to find kwh. Read the wattage every second, add the watts up, divide by 3,600 to get watt-hours, divide again by 1,000 to get kwh.