Distributed Energy Resources (DERs) are active devices that supply energy (or load reductions). Yet, in contrast to traditional generation sources, DERs are installed within the electricity distribution (rather than on the electricity transmission) system. In fact, DERs can also be thought of as dispersed energy resources; consider, for example, emergency generators owned by individual customers. DERs include a wide variety of generation sources, such as fuel cells, micro-turbines, photovoltaics, and hybrid power plants, as well as more traditional sources, such as diesel engines and steam turbines. However, DERs also include electricity storage technologies such as batteries, flywheels, ultra capacitors and superconducting magnetic energy storage. Finally, end-use load controls that manage demand are also considered DERs. No specific size range has been defined for DERs, but most distribution systems would have difficulty accommodating distributed generating resources larger than 10 MW/MVA at any single location; many systems may have even lower limits.