Resources

Renewable energy is generally defined as energy that is collected from resources which are naturally replenished on a human timescale, such as sunlight, wind, rain, tides, waves and geothermal heat. Renewable energy often provides energy in four important areas: electricity generation, air and water heating/cooling, transportation and rural (off-grid) energy services.

Watch Renewable Energy 101

The following definitions/terms are applicable to solar, wind, hybrid, energy storage, telecommunications and EV charging stations.

Common Definitions

Alternating Current (AC) – is an electric current in which the flow of electric charge periodically reverses direction. AC is the form in which electric power is delivered to businesses and residences. The usual waveform of alternating current in most electric power circuits is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves.

Ampere (amp) – A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

Ampere-Hour (Ah) – A measure of the flow of current (in amperes) over one hour, used to measure battery capacity.

Base Load Generating Plants – Typically coal or nuclear generating units that are committed and dispatched at a constant or near-constant levels with minimum cycling. They are often the sources of lowest-cost of energy (LCOE) when run at very high capacity factors.

Battery – Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

Battery Available Capacity – The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state-of-charge, age and cut-off voltage.

Battery Capacity – The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

Demand Response – The process of using voluntary load reductions during peak hours. Some grid operators will contract with commercial customers to reduce power usage (or run an on-site generator) during times of overloaded grid conditions.

Direct Current (DC) – is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, power supplies, thermocouples, solar cells or dynamos. The electric current flows in a constant direction, distinguishing it from alternating current (AC). Direct current is used to charge batteries and as power supply for electronic systems.

Dispatching (Economic Dispatch) – A method by which system operators decide how much output should be scheduled from electric producing facilities.

Distributed Systems – Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential PV system is a distributed system.

Frequency – The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz). Most electrical systems in North America utilize a 60Hz frequency.

Gigawatt (GW) – A unit of power equal to 1 billion Watts; 1 million kilowatts or 1,000 megawatts.

Grid – An integrated system of electricity distribution, usually covering a large area.

Grid-Tied – An electrical generation system connected to the utility electric grid. When local electrical generation assets (like solar and wind) output more power than is consumed by the local facility, the excess electricity is passed back to the local grid and consumed by neighboring facilities.

Grid-Fallback – An electrical generation system connected to a local electrical generation asset (like solar and wind), an energy storage system and the utility electric grid. The local electrical generation asset stores the electrical energy into an energy storage system, then returns the electrical energy back to the local devices through an inverter system. When the energy storage system is exhausted, the utility electric grid provides electricity to the local devices.

Grid-Interactive – An electrical generation system connected to a local generation asset (like solar and wind), an energy storage system and the utility electric grid. Local devices utilize the energy generated by the asset, with excess electricity passed back to the local grid and to the energy storage system. When a grid failure occurs, an inverter system provides electricity to critical local devices (similar to an uninterruptable power supply – UPS).

Micro-Grid – A localized grouping of electricity sources and loads that normally operates connected to and synchronous with the traditional centralized grid, but can disconnect and function autonomously as physical and/or economic conditions dictate.

Net Metering – A system in which renewable energy generators are connected to a public-utility power grid and surplus power is transferred onto the grid, allowing customers to offset the cost of power drawn from the utility. A net meter increases the count of kilowatt-hours (meter runs forward) when consuming energy from the utility grid and decreases the count of kilowatt-hours (meter runs backwards) when exporting energy back to the utility grid.

Off-Grid – A stand alone electrical generation system (i.e. not connected to the utility electric grid). Off-Grid systems typically have an energy storage component, a generation component (possibly photovoltaic solar, a wind turbine or fueled generator) and an inverter to convert DC voltage to AC voltage.

Independent System Operator (ISO) – The entity responsible for maintaining system balance, reliability and electricity market operation.

Investment Tax Credit (ITC) – The business energy ITC is a federal tax credit for corporations for installation of solar, wind, fuel cells and geothermal energy sources. See http://energy.gov/savings/business-energy-investment-tax-credit-itc for the current tax credit rates and service dates. The residential renewable ITC is a federal tax credit for homeowners for installation of solar, wind, fuel cells and geothermal energy sources. See http://energy.gov/savings/residential-renewable-energy-tax-credit for the current tax credit rates and service dates.

Kilowatt (kW) – A standard unit of electrical power equal to 1,000 watts, or to the energy consumption at a rate of 1,000 joules per second.

Kilowatt-hour (kWh) – 1,000 watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3,600 kilojoules.

Levelized Cost of Energy (LCOE) – The cost of energy of a renewable energy system that is based on the system’s installed price, its total lifetime cost and its lifetime electricity production.

Load – The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.

Megawatt (MW) – 1,000 kilowatts or 1 million watts; standard measure of electric power plant generating capacity.

Megawatt-hour (MWh) – 1,000 kilowatt-hours or 1 million watt-hours.

National Electric Code (NEC) – Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, “Solar Photovoltaic Systems” which should be followed when installing a PV system.

National Electrical Manufacturers Association (NEMA) – This organization sets standards for some non-electronic products like junction boxes and cabinets.

Ohm – A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

Peak Demand/Load – The maximum energy demand or load in a specified time period.

Power Factor (PF) – The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

Reactive Power – The sine of the phase angle between the current and voltage waveforms in an alternating current system.

Renewable Energy Credits (REC) – Also known as Green tags, Renewable Electricity Certificates, or Tradable Renewable Certificates (TRCs), are tradable, non-tangible energy commodities in the United States that represent proof that 1 megawatt-hour (MWh) of electricity was generated from an eligible renewable energy resource and was fed into the shared system of power lines which transport energy.

Renewable Portfolio Standard (RPS) – A regulation that requires the increased production of energy from renewable energy sources, such as wind, solar, biomass and geothermal. The RPS generally places an obligation on electricity supply companies to produce a specified fraction of their electricity from renewable energy sources.

Reserve Capacity – The amount of generating capacity a central power system must maintain to meet peak loads.

Smart Grid – An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.

Transformer – An electromagnetic device that changes the voltage of alternating current electricity.

Virtual Net Metering – Allows utility customers to share the electricity output from a single power project, typically in proportion to their ownership of the shared system. Each state imposes customer type, energy source, distance and local grid segment limitations on eligibility of virtual net metering.

Volt (V) – The unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

Voltage – The amount of electromotive force, measured in volts, that exists between two points.

Watt – The rate of energy transfers equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).