power does no work at the load, it still must be supplied by the power source. Conductors, transformers and generators must be sized to carry the total current, not just the current that does useful work. Failure to provide for the supply of sufficient reactive power in electrical grids can lead to lowered voltage levels and, under certain operating conditions, to the complete collapse of the network or blackout. Another consequence is that adding the apparent power for two loads will not accurately give the total power unless they have the same phase difference between current and voltage (the same power factor). Conventionally, capacitors are treated as if they generate reactive power, and inductors are treated as if they consume it. If a capacitor and an inductor are placed in parallel, then the currents flowing through the capacitor and the inductor tend to cancel rather than add. This is the fundamental mechanism for controlling the power factor in electric power transmission; capacitors (or inductors) are inserted in a circuit to partially compensate for reactive power 'consumed' ('generated') by the load. Purely capacitive circuits supply reactive power with the current waveform leading the voltage waveform by 90 degrees, while purely inductive circuits absorb reactive power with the current waveform lagging the voltage waveform by 90 degrees. The result of this is that capacitive and inductive circuit elements tend to cancel each other out. Engineers use the following terms to describe energy flow in a system (and assign each of them a different unit to differentiate between them): Active power,<ref>Definition of Active Power in the International Electrotechnical Vocabulary </ref> P, or real power: watt (W); Reactive power, Q: volt-ampere reactive (var); Complex power, S: volt-ampere (VA); Apparent power, |S|: the magnitude of complex power S: volt-ampere (VA); Phase of voltage relative to current, φ: the angle of difference (in degrees) between current and voltage; . Current lagging voltage (quadrant I vector), current leading voltage (quadrant IV vector). These are all denoted in the adjacent diagram (called a power triangle). In the diagram, P is the active power, Q is the reactive power (in this case positive), S is the complex power and the length of S is the apparent power. Reactive power does not do any work, so it is represented as the imaginary axis of the vector diagram. Active power does do work, so it is the real axis. The unit