Glossary
The Electrical Grid
In New England, the electrical grid is controlled by ISO-NE, Independent System Operator- New England. (To learn more about system operators, visit the Regulatory Bodies section of our website.) Below are some of the terms associated with the work they do, as well as some other grid-related definitions.
Ancillary services: Services other than electrical energy generation that generators can provide to help maintain a reliable electricity system. Examples include reserve capacity, load following, frequency regulation, and voltage regulation. These services are used by the system operator as a corrective measure in the event of a disturbance to the electrical grid.
Balancing: Active efforts to ensure that real-time energy demand and supply are equal. In other words, energy must be produced at the same time it is consumed. If the power system falls drastically out of balance, it can lead to blackouts.
Demand Charge: The demand charge is the charge one pays for electricity consumption during peak demand. This is one of three components in a typical commercial electricity bill. A solar photovoltaic system and/or storage can lower demand charges by shifting energy loads off-grid and reducing peak demand, resulting in demand charge savings. Buildings whose electricity demand is highest during the day-- often schools and office buildings-- have the greatest likelihood of seeing demand charge savings from PV.
Demand Response (DR): Demand response offers financial incentives to customers when they reduce energy usage during peak periods (i.e. high usage times). Customers are then rewarded with lower energy rates. Savings from demand response is different from demand charge savings in that demand is reduced, rather than shifting demand away from the grid to solar PV or storage.
Distributed Energy Resources (DERs): Small scale electricity resources that are distributed throughout the grid. DERs are connected to the distribution network and are often renewable resources like batteries and solar panels. They can be located on homes and businesses and can be owned by the electric utility, independent power producers, businesses, and homeowners.
Frequency regulation: Constant frequency is needed on the electrical grid. Fluctuations in frequency are harmful to electrical appliances and steam and water turbines since they are all designed to operate at a specific speed.
Generation: Where power is produced or generated, for example: coal power plants, solar photovoltaic panels, batteries, etc. Another term for these generators is resources.
Independent Power Producer (IPP): An entity that owns or operates generators and will produce energy for public use, but is not an electric utility.
Interconnection: The process of connecting a new project to the electric grid. All projects must submit an interconnection application to the local utility. An application may lead to an impact study, where the project’s impact to the grid, and the cost of accommodating the project to the grid will be analyzed. Solar projects, batteries or any source of generation that will connect to the grid will go through this process.
Load: Anything that requires power to operate, from a toaster to a phone charger to an elevator and beyond! Frequently used interchangeably with demand, which can be thought of as an aggregation of loads. For example, we might think of all the energy you use at home as the building’s load.
Peak demand : The period at which customer demand for electricity is at the highest. During peak periods, high-cost generation resources need to be operating to meet demand. Thus, it is most expensive to generate electricity during peak periods.
SMART (Self-Managing And Reliable Transmission) Grid : An energy system that uses information, two-way communications, and digital technologies to constantly optimize power generation, delivery, and consumption to increase efficiency and reliability.
Spinning Reserves : A service provided by generators already connected to the system where they can respond to sudden changes in supply (e.g. a generator failing) or demand (e.g. large increase in load) to maintain frequency. These generators respond by contributing to generation immediately, or within seconds.
Transmission and Distribution: Refers to the different stages of carrying electricity between generation to users. Electricity moves at a higher voltage level during transmission compared to distribution. Distribution delivers energy to load locations (aka homes and businesses). Voltage is stepped-down from transmission lines to distribution lines at step-down transformers.
Distributed Energy Resources (DERs)
Battery energy storage systems (BESS): A system of batteries that can store energy from various sources (like the grid or renewables) and release the energy at a later time. The computerized control system determines when to store and release energy to minimize the cost to users.
Combined heat and power: A type of distributed generation that produces electricity and thermal energy from the same energy source. The energy that would normally be lost in the form of heat would be received to provide heating and/ or cooling. It is typically used at the industrial, commercial, institutional, and utility scales.
Curtailment: The action of reducing the amount of electricity a generator produces to maintain the balance of power supply and demand. Curtailment is most commonly used with wind and solar power to prevent oversupply of electricity to the grid. Curtailment is critical in avoiding blackouts, as oversupply will lead to an increase in frequency and throw off frequency balance.
Demand response: An incentive for customers to shift their energy consumption away from peak periods. This aids in reducing stress on the grid during periods of high demand. An example of a demand response incentive is time-based-rates where electricity is much cheaper during non-peak periods. Customers can then shift their non-essential energy consumption (like when they run the washing machine or dishwasher) to these periods with lower electricity rates.
Electric vehicle (EV): A vehicle with a battery instead of a gas tank and an electric motor instead of an internal combustion engine. Electric vehicles do not produce tailpipe emissions, but there may be emissions associated with the generation of the electricity used to charge them.
Intermittency/ Variability: A quality of certain distributed generation, like solar and wind power, that results in a non-continuous (although predictable) flow of electricity being generated. This prevents those resources from meeting demand at any given time. However, the use of storage technologies is overcoming this problem, as it allows the excess energy produced to be stored and then released at a later time. While the term intermittency is more commonly used, variability is a more accurate term since, although the flow of electricity is non-continuous it can be forecasted with great accuracy.
Islanding: The act of disconnecting and isolating a part of the power system and operating it independently from the electrical grid. This is done as a protective measure during external widespread grid disturbance and allows for continuous power flow to the loads connected to the island. This capability is fundamental to the function of a microgrid. It can more easily be envisioned as the flipping of a switch when, when flipped, disconnects the power system from the grid.
Microgrid: A small-scale electrical network with interconnected loads and distributed energy resources. It is clearly defined from the larger electricity grid and has a local source of generation, and thus may disconnect and act as an island. A benefit of the microgrid is that it has a local source of generation so electricity remains available in the event of a grid outage. Visit the microgrids page for more information.
Net metering: A billing mechanism that credits solar panel system owners for the electricity they provide to the grid. For example, during the day, the solar panels on a residential house may produce more than the people in the house are using; net metering allows that excess power to be exported to the grid to serve other loads. In return, the household will receive credits for the energy exported, that they can use later to offset the electricity they use from the grid.
Smart meter: An electronic device that records electricity consumption in real time and is able to transmit such information to the utility. Smart meters allow for households to understand their electricity consumption throughout the day. On the other hand, the traditional meter will only provide the person reading it with the amount of electricity consumed since the last time the meter was read. Smart meters provide granular energy consumption data, often in 5, 10, or 15 minute increments. Traditional meters only give building owners the total amount of energy used over the course of a month.
Solar photovoltaic (PV) system: A system of one or more photovoltaic panels combined with an inverter and other electrical components that generate electricity from the sun’s energy. Photovoltaic panels generate direct current (DC) electricity, but transmission lines and most home appliances use alternating current (AC) electricity. The inverter converts the DC power from the photovoltaic panels into AC power. For information on adding solar to your home, visit the solar page.