An $Active Power$ is, under periodic conditions, the mean value, taken over one period $T$, of the instantaneous power $p$. In complex notation, $P = \mathbf{Re} \; \underline{S}$, where $\underline{S}$ is $\textit{complex power}$.
$P = \frac{1}{T}\int_{0}^{T} pdt$, where $T$ is the period and $p$ is instantaneous power.
Applicable units are those of quantitykind:ElectricPower
Active Power