Where power is a primary concern for the design under discussion. Use the "low-power" tag when that applies.
Power is the rate of energy consumption, transfer, or storage. In general, power can be divided into real (or active) power and reactive power. Real power measures energy that is used to do useful work or lost as heat. Reactive power measures energy that is temporarily stored, generally in capacitance and inductance, before being returned to the grid. The combination of the two is complex power.
Reactive power is generally considered undesirable, as it is not used for useful work, but increases the design requirement of the transmission system.
Units of real power are Watts (joules per second). Units of reactive power are VARs (volt-amp reactive). Units of complex power are VA (volt-amps).
See power-supply for questions on converting power.