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 $\textit{complex power}$.
Applicable units are those of quantitykind:ElectricPower
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=31891
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=131-11-42
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD003
$P = \frac{1}{T}\int_{0}^{T} pdt$, where $T$ is the period and $p$ is instantaneous power.