Owl Energy Monitors
Most energy monitors which use a clamp-on current sensor do not actually measure energy. They measure the current in a wire using the clamp-on current sensor and multiply the current by the mains supply voltage. You can usually set a voltage to closely match your actual supply voltage in the set-up menu.
If you set the Owl display to ‘Energy’, the display reads in Watts or Kilowatts. Technically, the reading is Volt Amps (VA) not Watts.
For a purely resistive load – an immersion heater, kettle or other heater – there will be no difference between Watts and Volt Amps. But as soon as you use loads such as motors (vacuum cleaner, drill, fan, refrigerator etc) or power supplies (television, computer, chargers etc) the load is no longer purely resistive and the current will be out of phase with the voltage – a so-called reactive load. Multiplying the current by the voltage will result in a reading which is higher than the actual wattage; how much higher depends on the nature of the load. It can be 100% incorrect for a purely reactive load.
In a typical house with the usual mix of loads (chargers, lamps, kettle, television ... ) the inaccuracy over time is not too great. In my house, the reading is about 10% high. The inaccuracy tends to increase significantly at low loads. In general smaller loads are more reactive, so the Owl may be reading 100W when the actual power is around 10W. Furthermore the specification of the Owl monitor states a much lower accuracy at low loads.