the rate of producing, transferring, or using energy most commonly associated with electricity. Power is measured in watts and often expressed in kilowatts (kW) or megawatts (MW). Also known as "real" or "active" power.
Power readings, often taken with a meter, are the proof things are happening.
It can be difficult to understand in an applied way, for example the difference between active power (measured in watts) and reactive power (measured in vars, or volt-amps reactive) caused by inductive or capacitive loads.
It's important to learn about the basic SI units related to electrical power and energy. Power can be thought of as the ability to do work (e.g. it takes 20 watts to power a 20-W light bulb). Power is directly proportional to voltage and current as expressed in the power law equation: P=I*V (P=power, I=intensity or current, V=voltage)