For a two-terminal element or a two-terminal circuit with terminals A and B,
$\textit{Instantaneous Power}$ is the product of the voltage $u_{AB}$ between the terminals and the electric current i in the element or circuit:
$$p = u_{AB} \cdot i$$
Where $u_{AB}$ is the line integral of the electric field strength from A to B,
and where the electric current in the element or circuit is taken positive if its direction is from A to B and negative in the opposite case.
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For an n-terminal circuit, it is the sum of the instantaneous powers relative to the n - 1 pairs of terminals when one of the terminals is chosen as a common terminal for the pairs.
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For a polyphase element, it is the sum of the instantaneous powers in all phase elements of a polyphase element.
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For a polyphase line consisting of m line conductors and one neutral conductor, it is the sum of the m instantaneous powers expressed for each line conductor by the product of the polyphase line-to-neutral voltage and the corresponding line current.
Applicable units are those of quantitykind:ElectricPower
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=31891
http://en.wikipedia.org/wiki/Power
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=131-11-30
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=131-11-31
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=141-02-14
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=141-03-10
$p = ui$, where $u$ is instantaneous voltage and $i$ is instantaneous electric current.