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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.^[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 isIf 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.

Electrical load shedding deliberately reduces connected electrical load when the available supply or operating constraints cannot support the original demand. It can disconnect selected loads or reduce their requested power. The immediate goal is a feasible remaining electric power system, with an explicit account of which service is no longer supplied.

For an original microgrid example, generation and usable storage output total $80\mathrm{kW}$, requested load is $95\mathrm{kW}$, and losses are stipulated as $5\mathrm{kW}$. At least $20\mathrm{kW}$ of load must be removed in the fixed-loss model:Suppose the optional loads are indivisible blocks of 8, 12 and 15 kilowatts. Disconnecting the 8 and 12 kilowatt blocks meets the deficit exactly, while disconnecting only the largest block does not. The choice also depends on which service must be retained; arithmetic capacity alone supplies no priority rule.

Timing matters as well as the final power balance. A $20\mathrm{kW}$ deficit sustained for $0.4\mathrm{s}$ consumes $8\mathrm{kJ}$ from internal energy storage before a load action takes effect. In the rotating-machine model of electric power balance during a demand step, that draw reduces rotational kinetic energy and can reduce generated frequency. A final balanced state does not prove that the intervening trajectory stayed within operating limits.

Automatic low-frequency schemes are assessed using dynamic models, not only static subtraction. WECC's primary assessment simulated generation deficits at several load conditions and monitored frequency decline, minimum frequency and recovery. It explicitly distinguishes a simulation study from observed field response.^[ref] The report's settings and scenarios are particular to that assessment, not universal instructions for another network.

A proposed small-system test imposes a known supply shortfall, records detection and disconnection times, and measures served load, source power, terminal electric potential difference and frequency through the event. Check subsequent restoration separately: reconnecting all removed loads at once can recreate the original demand step. The power-rating limit versus energy autonomy in a microgrid distinguishes a shortfall in instantaneous output from depletion over a longer interval; both can motivate different load-management decisions.

Electric power balance during a demand step tracks where the additional energy comes from when an electrical load suddenly requests more power. At an explicitly chosen system boundary, conservation of energy giveswhere $E_s$ includes the energy storage inside that boundary. A negative right-hand side means that stored energy is decreasing. An external electric power system, an electric battery and a rotating electrical generator supply different physical buffers; they should not all be counted as independent incoming supplies if some are already included in $E_s$.

For an original lumped rotating-machine example, take moment of inertia $J=10\mathrm{kg}{\mathrm{m}}^2$ and initial angular velocity ${\omega }_0=100\mathrm{rad}{\mathrm{s}}^{-1}$. Suppose mechanical input initially remains unchanged while electrical output increases by $2\mathrm{kW}$, with losses already accounted for. The rotational kinetic energy then obeysAfter $0.5\mathrm{s}$, $\omega \approx 98.995\mathrm{rad}{\mathrm{s}}^{-1}$: the extra $1\mathrm{kJ}$ came from rotation. This stipulated model assumes the extra delivered power can be maintained during that interval; it does not predict the load's own response to falling electric potential difference or frequency.

For a rotating electrical generator with $p$ magnetic pole pairs, the generated electrical frequency is $f=p\omega /(2\pi )$. A speed change therefore links the energy balance to a measurable waveform change. The DOE derivation relates generated frequency to pole count and rotational speed.^[ref] This rotating-machine argument does not describe every electronically controlled supply.

A proposed demand-step test records the imposed electrical load, source and storage power, terminal electric potential difference, frequency and timing of control action. The important distinction is between an initial buffer and a sustainable new operating point. If available continuous power remains too small, increasing stored energy alone does not remove that limit; electrical load shedding can instead reduce the requested output.

Variability reduces firm capacity and creates balancing costs, but does not impose a universal tiny upper limit on the energy fraction from wind power. As an illustrative scale calculation, $10\mathrm{GW}$ installed at capacity factor $0.30$ produces $26.3\mathrm{TWh}$ annually, about $8.8\%$ of an assumed $300\mathrm{TWh}$ annual demand. Larger installations can supply larger fractions if the rest of the system is adapted.

Geographical spread, forecasting, interconnection, flexible generation, demand response and energy storage manage fluctuations on different timescales. They are not interchangeable: a battery that balances seconds does not necessarily cover a week of low wind. Correlated weather, curtailment, network constraints and reserve requirements become more important at high penetration. Backup capacity need not burn fuel continuously at the wind plant's nameplate output; energy and capacity should be accounted for separately. The defensible conclusion is that significant penetration requires system planning and has costs, not that variability alone confines wind to a negligible contribution.^[ref]