In brief, wattmeters tend to operate by measuring continuously instantaneous voltage (volts) and current (amperes) to give accurate information of energy units used, expressed usually in KWh.
OR-WAT-419 Power Meter from ORNO is a digital Power Usage Monitor which is designed to analyze the energy consumption of electric devices. It can be easily connected between a mains socket and the power plug of the tested appliance. Additionally, large LCD displays and simple configuration of settings helps to check quickly and effortlessly the amount of power consumed by an individual device or multiple appliances.
For more details and specifications please use the link: https://www.tme.eu/