= Distributed energy resource
A <distributed energy resource> is a localized source or store of electrical <energy> connected near the consuming part of an <electric power system>, commonly through <electric power distribution>. Examples include a nearby <electrical generator>, an array of <solar cells>, and an <electric battery> with its power-conversion equipment. <Energy storage> can either absorb or deliver <power>; calling it a resource does not make it a net source of <energy> over a complete charge-discharge cycle.<Microgrid Overview by the U.S. Department of Energy>[Printed p.1, Definitions: distributed energy resources; key components]{ref}
For an original boundary calculation, a site consumes $50\,\mathrm{kW}$, its <solar cells> supply $30\,\mathrm{kW}$ at the stated output boundary, and an <electric battery> supplies $10\,\mathrm{kW}$. Neglecting site losses, the imported <power> is
$$
P_{\rm import}=P_{\rm load}-P_{\rm solar}-P_{\rm battery}=10\,\mathrm{kW}.
$$
If the same <electric battery> instead charges at $10\,\mathrm{kW}$, its signed supply is $-10\,\mathrm{kW}$ and import becomes $30\,\mathrm{kW}$. If solar output rises to $70\,\mathrm{kW}$ while storage is idle, the algebra gives $-20\,\mathrm{kW}$ import: an export, provided the connection and controls permit that operating point.
The location matters physically. Local generation can reduce upstream <electric current> when it offsets downstream consumption, but sufficiently large export can reverse the <electric current>. In a simplified resistive two-terminal branch, $V_s-V_r=RI$ for signed <electric current> from supply to site; negative $I$ makes the receiving-site <electric potential difference> higher. This is an illustrative <direct current> relation, not a complete <alternating current> feeder calculation.
A group of <distributed energy resources> becomes a <microgrid> only with an appropriate boundary and coordinated operation. Installed generating capacity alone does not establish that disconnected operation can regulate the local supply. A useful test records import or export together with each resource's signed <power>, so an apparent reduction in imported <energy> is not confused with a change in the consuming <electrical loads>.
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