Trying to measure kW and kWh
You can use a clamp meter to measure current through a single conductor.

Current is the number of electrons flowing through the wire per unit time, and is measured in amperes. If you know the voltage, you can use that to calculate power (watts):

Power [watts] = Current [amps] X Voltage [volts]


If this is a standard residential power system you can assume 110V or 220V for the voltage, or you can measure it with a voltmeter/multimeter. If you want kW instead of watts, you divide by 1000. (The formula on your website includes a "Power Factor" which is based on the appliance being used. In most cases it's close to 1, so you can just leave it out unless you need to be super-precise or happen to know that you have a low power factor.)

The major limitation of a clamp meter in measuring household power is that it can only be clamped around a single conducting wire, not the entire romex cable. If you clamp around the entire cable you will get a reading of 0, since the two conductors have opposite currents and they cancel each other out. This limitation means the meter is probably only practical inside the circuit breaker box or an electrical box, where space and safety concerns may make it a bad idea.