Power in an electrical circuit is measure of how much work is being done by the circuit and is calculated by Joules Law:
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| S = V ⋅ I |
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This is measured in Volt-Amps or VA and is called the "Apparent Power" of the system. Sometimes you will see this as
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| P= V ⋅ I |
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But this is not strictly true. Useful work in a system is measured by the power consumed by the system and is measured in "Watts" (W) and is given the symbol P. In a DC system, apparent power, S is always the same as the real power, P, which is why you will see 'P' used incorrectly.
In an AC system, due to the impedance (capacitance and inductance added), the true power is different from the apparent power. The impedance of the circuit is what causes the Reactive Power (Q). The relationship between Apparent Power, True Power and Reactive Power can be shown diagrammatically:
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| Cos(θ)=P/S, or P=S ⋅ cos(θ) |
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Cos(θ) is known as the Power Factor and is the definitive relationship between Apparent Power and Real Power. More on this later.
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