AC. Alternating current. Electric current that periodically reverses direction. Direct current (DC) flows in only one direction. Alternating current runs household appliances, whereas flashlight batteries provide direct current.

ATS. Automatic transfer switch. A “gatekeeper” switch that can automatically connect or disconnect the MicroGrid from the utility grid. If the grid fails and the MicroGrid is producing electricity, the ATS can route power onto the grid.

Base load. The minimal level of demand on an electrical grid over 24 hours.

CA ISO. California Independent Service Operator. The independent, non-profit transmission authority that oversees the movement of bulk electric power long distances and also operates a marketplace for commercial generators to sell power to utilities; a new rule allows private small generation owners to aggregate and sell power.

CCHP. Combined cooling, heating and power system.

Converter: A device that (usually) converts alternating current (AC) into direct current (DC).

CPUC. California Public Utilities Commission. The agency that regulates privately owned public utilities in the state, including electric power.

Charge controller. Device connected between a solar array and battery to control the rate of charge.

DC. Direct current. Electric current that flows in just one direction. Alternating current (AC) periodically reverses direction. Direct current is generated by solar cells and is used to charge batteries. DC may be converted to AC with an inverter.

DER. Distributed energy resource (distributed generation or storage) are typically small, decentralized, renewable energy sources or storage devices. Microgrids are considered DER systems because they connect to the periphery of the macrogrid.

DERP. Distributed energy resource provider.

DRP. Distribution resource plan. A requirement of the CPUC (state law AB 327) that investor-owned utilities develop plans to accommodate more DERPs in their transmission and distribution operations; six ancillary services are defined in the DRP, and the term comes from CA ISOs initiative to accommodate the types of services microgrids could provide. Allowing DERPs to aggregate DER to meet the ISO’s 0.5 MW minimum participation requirement enables entry of residential to corporate resources into energy wholesale markets.

Disconnect. A “smart” safety device that can interrupt a circuit.

Fuel cell. A device that converts chemical energy into electricity from the reaction of hydrogen with oxygen to form water.

Hertz (Hz). A unit of electrical frequency, defined as one cycle per second.

Inverter. A device that (usually) converts direct current (DC) into alternating current (AC). A number of different inverters are deployed at the farm to determine how well each one integrates with the MicroGrid.

ISO. Independent service operator.

Kilowatt (kW). A measure of 1,000 watts of power; the rate at which electric power is generated or consumed.

Kilowatt-hour (kWh). A measure of 1,000 watt-hours of energy; the generation or consumption of 1,000 watts of electric power for one hour.

Load. A component or part of a circuit that consumes electric power.

Management system. The communication and command/control components of the MicroGrid.

Megawatt (MW). A measure of 1 million watts of power; the rate at which electric power is generated or consumed.

Megawatt-hour (MWh). A measure of 1 million watt-hours of energy; the consumption of 1 million watts of power for one hour.

Microgrid. (U.S. Dept. of Energy definition) “A group of interconnected loads and distributed energy resources (DER) with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid (and can) connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.”

Micro-inverter. A small inverter placed, for example, on each solar panel, producing AC more cost-effectively.

PCC. Point of common coupling. The point where the customer connects to the utility grid, typically considered at the metering point, service entrance or service transformer. The connection point between the MicroGrid and the utility grid.

Peak. Period(s) of relatively high system demand for electricity.

PV. Photovoltaic. Production of electric current at the junction of two substances exposed to light; converting sunlight into electricity.

Relay. An electrically operated switch to implement a safety function.

Round-trip Efficiency. The ratio of energy put into a storage device to energy taken from it, expressed as a percentage. The lesser the percentage, the great the loss of energy.

Rule 21. A CPUC document, the law for all utilities, that addresses safety concerns, particularly regarding the point of common coupling (PCC), and elements to accomplish compliance. Currently there is no export provision in Rule 21.

Solar panel. A flat, usually rectangular frame comprising a number of photovoltaic cells that generate electrical power from sunlight.

Single phase (1P). A service with two energized wires that typically serves smaller needs of 120V/240V, requires less equipment, and is less expensive to install and maintain.

Switches. Devices that open and close electrical circuits. Smart switches can be programmed to perform tasks on their own, like taking control of a water pump, eliminating the need to put communication in the pump control.

Thermal runaway. A situation in which a rise in temperature alters conditions to cause further increases in temperature that can produce failure or even fire with certain types of batteries.

Three phase (3P). A service with three energized wires that typically serves larger power needs than 120V/240V, is usually required for motors exceeding 10 hp, and is more expensive to install and maintain.

Trigeneration. A system of combined cooling, heating, and power production (CCHP).