Energy glossary

Charging an electric vehicle from alternating-current supply, with the vehicle's on-board charger converting AC to DC; typically 1.4–22 kW.

The combined use of the same land for solar power generation and agriculture, such as crops or grazing beneath or between PV arrays.

A heat pump that extracts heat from outdoor air, supplying space heating and hot water (air-to-water) or warm air (air-to-air), and the most widely installed type.

The fraction of incoming solar radiation that a surface reflects, from 0 (absorbs everything) to 1 (reflects everything).

Use of ammonia (NH3), a carbon-free hydrogen carrier, as a fuel for ships, power plants and long-distance hydrogen transport.

The microbial breakdown of organic matter in the absence of oxygen, producing biogas and a nutrient-rich digestate usable as fertiliser.

Services beyond bulk energy that system operators procure to keep the grid stable and secure, such as frequency reserves, voltage control and black start.

An energy unit equal to the approximate heat released by burning one barrel (42 US gallons, 158.99 L) of crude oil, about 5.8 MMBtu or 6.12 GJ.

The minimum level of electricity demand on a grid over a day, season or year, historically served by plants that run continuously at high output.

A vehicle propelled solely by electric motors powered from an on-board rechargeable battery, with no internal combustion engine.

Equipment located on the customer's side of the utility meter, such as rooftop solar, home batteries or EV chargers, which primarily serves on-site load.

A complete grid- or site-connected battery installation, including cells, battery management, power conversion, thermal management and controls.

The theoretical maximum fraction of kinetic energy an open rotor can extract from the wind: 16/27, or about 59.3%.

A PV module that collects light on both its front and rear faces, using a transparent back sheet or glass to harvest ground-reflected and diffuse light.

A geothermal power plant in which hot brine heats a separate low-boiling-point working fluid that drives the turbine, enabling generation from resources of roughly 100 to 180 °C.

A liquid or gaseous fuel derived from biomass, such as ethanol, biodiesel or renewable diesel, used mainly in transport to replace petroleum fuels.

A gas mixture of roughly 50–70% methane and 30–50% CO2 produced by anaerobic digestion of organic matter such as manure, food waste, sewage sludge and energy crops.

Biogas upgraded to more than 95% methane so that it meets natural gas quality standards and can be injected into the gas grid or used as vehicle fuel.

The process of restoring a power system after a total or partial blackout using units that can start without power from the grid.

Hydrogen produced from natural gas by reforming, with most of the resulting CO2 captured and permanently stored.

A light-water nuclear reactor in which coolant boils in the core and the resulting steam drives the turbine directly, without steam generators.

A traditional unit of heat, approximately the energy needed to raise one pound of water by one degree Fahrenheit; 1 BTU (IT) = 1,055.06 J.

The thermal energy extracted from nuclear fuel per unit mass of initial heavy metal, usually expressed in gigawatt-days per tonne (GWd/tHM).

A diode wired in parallel with a group of cells in a PV module that lets string current flow around shaded or faulty cells, preventing hot spots.

The charge or discharge current of a battery relative to its capacity; 1C empties a full battery in one hour, 0.5C in two hours.

The heat released by complete combustion of a unit quantity of fuel; the higher (gross) value includes condensation heat of water vapour, the lower (net) value excludes it.

The fraction of a resource’s nameplate capacity that can be counted on to meet peak demand, used in adequacy planning and capacity markets.

The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.

A mechanism that pays resources for being available to supply power or reduce demand at times of system stress, alongside payments for energy.

A licensed increase in the maximum power output of an existing power plant, most commonly a nuclear reactor, achieved through better measurement or equipment upgrades.

Capital expenditure: the upfront investment to build an energy asset, usually quoted per kW of capacity for power plants or per kWh for storage.

The cumulative amount of CO2 that can still be emitted while keeping global warming below a given limit with a stated probability.

Technologies that separate CO2 from flue gas or process streams, compress it, and inject it into deep geological formations for permanent storage.

The mass of CO2 (or CO2e) emitted per unit of electricity generated or consumed, usually expressed in grams per kilowatt-hour (gCO2/kWh).

A claim that the CO2 emissions attributed to an entity, product or event have been reduced and the remainder compensated, usually by purchasing carbon credits.

A tradable credit representing one tonne of CO2e reduced, avoided or removed by a project outside the buyer’s own operations, used to compensate for the buyer’s emissions.

A cost applied per tonne of CO2e emitted, through a carbon tax or an emissions trading system, so that the climate damage of emissions enters market prices.

An EU levy on the embedded emissions of imported carbon-intensive goods, aligning their carbon cost with that paid by EU producers under the EU ETS.

Simultaneous generation of electricity and useful heat from one fuel input, reaching total efficiencies of 75–90% versus about 35–60% for power-only plants.

A common unit that expresses emissions of different greenhouse gases as the mass of CO2 that would cause the same warming over a chosen time horizon.

A power plant that runs a gas turbine and uses its hot exhaust to raise steam for a second, steam turbine, reaching electrical efficiencies of 55–64%.

A storage technology that compresses air into caverns or vessels with surplus electricity and expands it through a turbine to regenerate power.

Power plants that use mirrors to concentrate direct sunlight onto a receiver, heating a fluid that drives a steam turbine, usually with built-in thermal storage.

A support scheme or contract in which a generator receives the difference between a fixed strike price and a market reference price, paying back when prices exceed it.

A measure of how much and for how long outdoor temperature exceeds a base temperature, used to estimate air-conditioning demand.

The ratio of useful heat (or cooling) delivered by a heat pump to the electrical energy it consumes, measured at a specific operating point.

Deliberate reduction of a generator’s output below what it could produce, usually wind or solar, because of grid, market or system constraints.

The lowest wind speed, measured at hub height, at which a wind turbine starts producing net electrical power, typically 2.5 to 4 m/s for modern machines.

The wind speed at which a turbine shuts down or ramps down to protect itself from excessive loads, typically 25 m/s (10-minute mean) for utility-scale machines.

The number of full charge-discharge cycles a battery can deliver before its capacity falls to a defined end-of-life threshold, usually 70–80% of rated capacity.

A wholesale electricity auction held the day before delivery that sets hourly or quarter-hourly prices and schedules for the following day.

High-power EV charging in which an off-board charger delivers direct current straight to the battery, typically 50–400 kW and above.

Ratio of a PV array DC nameplate capacity at STC to its inverter AC rating; also called inverter loading ratio (ILR).

The gradual annual loss of a PV module power output over its life, typically around 0.5% per year for crystalline silicon.

Changes in electricity consumption by end users, in response to price signals or operator requests, to balance the grid.

The fraction of a battery's capacity that has been discharged, or the maximum fraction a manufacturer allows to be used per cycle.

Solar radiation reaching a horizontal surface after being scattered by molecules, aerosols and clouds, excluding the direct beam from the sun disc.

Engineered systems that extract CO2 directly from ambient air using chemical sorbents or solvents, producing a concentrated stream for storage or use.

The annual rate used to convert future costs and revenues into present value, reflecting the time value of money and investment risk.

Generation whose output can be scheduled, raised or lowered on request by the operator, independently of weather.

A network of insulated pipes distributing hot water or steam from central heat sources to many buildings for space heating and hot water.

Solar radiation arriving in a straight line from the sun disc, measured on a surface held perpendicular to the sun rays.

The shape of net load over a day in solar-rich grids: a deep midday dip as PV output peaks, followed by a steep evening ramp.

Synthetic fuels made from electrolytic hydrogen, sometimes combined with captured CO2 or nitrogen, such as e-methane, e-methanol, e-kerosene and e-ammonia.

An electrochemical device that uses electric current to split water into hydrogen and oxygen; the core equipment of green hydrogen production.

A cap-and-trade market in which a regulator caps total emissions, issues tradable allowances, and requires covered installations to surrender one allowance per tonne emitted.

The amount of energy stored per unit mass (specific energy, Wh/kg or MJ/kg) or per unit volume (Wh/L or MJ/L) of a fuel or storage medium.

Energy consumed per unit of economic output (e.g. MJ per USD of GDP) or per unit of activity, used as a proxy for energy efficiency.

The time an energy system must operate to generate as much energy as was used to manufacture, install and decommission it.

A standardised rating of a building's calculated energy performance, typically on an A–G scale, required in the EU and UK when buildings are sold or rented.

A geothermal reservoir created by stimulating hot, low-permeability rock, usually by hydraulic fracturing or shearing, so that water can be circulated between wells to extract heat.

The ratio of usable energy delivered by an energy source over its lifetime to the energy invested to obtain and deliver it.

The electrical energy an electric vehicle consumes per distance, usually in kWh/100 km or Wh/km (Europe) or miles/kWh and MPGe (US).

An SI unit of energy equal to 10¹⁸ joules (277.8 TWh), used to express national and global primary energy supply.

A policy that guarantees renewable generators a fixed price per kWh exported, usually for 15 to 20 years, independent of market prices.

Energy delivered to end users (households, industry, transport, services) in forms such as electricity, fuels and district heat, after conversion and transmission.

Wind turbines mounted on buoyant platforms moored to the seabed, enabling wind farms in water deeper than about 60 m where fixed foundations become uneconomic.

PV arrays mounted on buoyant platforms on reservoirs, lakes, ponds or sheltered coastal waters instead of on land or roofs.

An electrochemical battery storing energy in liquid electrolytes held in external tanks, so power (stack size) and energy (tank volume) scale independently.

A reaction-type hydro turbine with fixed blades in which water enters radially and leaves axially; the most widely installed design, used for medium heads of about 30 to 700 m.

The automatic change in generation or load that arrests and corrects a frequency deviation after a sudden imbalance on the grid.

An electrochemical device that converts the chemical energy of hydrogen or another fuel directly into electricity and heat, without combustion.

An electric vehicle that generates its electricity on board from hydrogen in a fuel cell, emitting only water vapour at the tailpipe.

The number of hours per year a plant would need to run at rated power to deliver its actual annual output; equal to capacity factor multiplied by 8,760.

The rate at which temperature increases with depth in the Earth's crust, averaging about 25 to 30 °C per kilometre away from tectonic plate boundaries.

Total shortwave solar radiation received on a horizontal surface, the sum of direct beam and diffuse sky components.

The time-integrated radiative forcing of one kilogram of a gas relative to one kilogram of CO2, over a stated period such as 20 or 100 years.

Hydrogen produced by splitting water in an electrolyser powered by renewable electricity, with near-zero direct CO2 emissions.

Hydrogen made from natural gas by steam methane reforming without carbon capture; the dominant and cheapest production route today.

The rate at which alternating current changes direction on a power system, 50 Hz or 60 Hz, which reflects the instantaneous balance of supply and demand.

The point at which electricity from a new source, usually solar or wind, costs the same as or less than power bought from the grid.

An inverter that sets its own voltage and frequency reference, behaving like a voltage source that can stabilise weak grids and provide synthetic inertia.

A heat pump that exchanges heat with the ground through buried loops or boreholes, benefiting from stable soil temperatures for high seasonal efficiency.

A tradable certificate proving that 1 MWh of electricity was generated from a specific source, used to substantiate renewable or low-carbon claims.

A device that uses a refrigeration cycle to move heat from a cooler source (air, ground, water) to a warmer sink, delivering 2–5 units of heat per unit of electricity.

The fuel energy a thermal power plant consumes per kWh of electricity generated; the inverse of efficiency, usually in BTU/kWh or kJ/kWh.

A measure of how much and for how long outdoor temperature falls below a base temperature, used to estimate and normalise building heating demand.

A cell that sandwiches an n-type crystalline silicon wafer between ultra-thin amorphous silicon layers, giving very high voltage and a low temperature coefficient.

The height of a wind turbine's rotor centre above ground or sea level; taller hubs reach stronger, steadier winds and are a main driver of rising capacity factors.

Transmission technology that carries bulk power as direct current, with lower losses over long distances and the ability to link asynchronous grids.

The vertical height difference between the water surface at a hydropower intake and the turbine outlet or tailwater, which together with flow determines available power.

The kinetic energy stored in synchronously rotating generators and motors that resists sudden changes in grid frequency.

A transmission link, AC or HVDC, that connects the grids of two countries, regions or market zones and allows electricity to be traded between them.

Energy lost when the DC power available from a PV array exceeds the inverter AC rating, so the inverter caps output at its maximum.

The discount rate at which a project’s net present value equals zero, representing its effective annual return.

The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).

Solar energy received per unit area over a period, usually expressed in kWh/m² per day or per year; the time integral of irradiance.

The SI unit of energy, work and heat: the work done by a force of one newton over one metre, or one watt sustained for one second.

A propeller-type reaction hydro turbine with adjustable runner blades and wicket gates, suited to low heads of about 2 to 40 m and large flows.

A unit of energy equal to one kilowatt of power sustained for one hour, or 3.6 megajoules; the standard billing unit for electricity.

The DC power rating of a PV module or array under Standard Test Conditions; the conventional unit of installed solar capacity.

The constant price per MWh that recovers all lifetime costs of a power plant, discounted to present value, divided by its discounted lifetime output.

The average cost per MWh discharged over a storage asset’s lifetime, including investment, operation, charging energy and efficiency losses.

The percentage cost reduction of a technology for every doubling of cumulative production; for solar modules, Swanson’s law puts it near 20%.

A lithium-ion cathode chemistry (LiFePO4) prized for safety, long cycle life and low cost, at the expense of lower energy density than nickel-based cells.

Total greenhouse gas emissions of a product or power source across its full life, from raw material extraction and manufacturing through operation to decommissioning.

Brown coal: the lowest-rank coal, with high moisture and low energy content, burned mostly in mine-mouth power plants and the most carbon-intensive major source of electricity.

A rechargeable battery in which lithium ions shuttle between a graphite anode and a metal-oxide or phosphate cathode; the dominant chemistry for EVs and grid storage.

Natural gas cooled to about minus 162 degrees C, shrinking its volume around 600-fold so it can be shipped by tanker between markets without pipelines.

The ratio of average demand to peak demand over a period, indicating how evenly a consumer or system uses electricity.

Adjusting a power plant’s output over minutes to hours to track changes in electricity demand or in variable renewable generation.

Storage able to discharge at rated power for roughly 8 hours or more, covering multi-day lulls and seasonal imbalances that 2–4 hour batteries cannot.

A unit of energy equal to 1,000 kWh or 3.6 GJ; the standard unit for wholesale electricity prices and utility-scale generation and storage.

The ranking of power plants by ascending marginal cost, used to dispatch the cheapest units first and set the market clearing price.

Unintended release of methane from oil, gas and coal supply chains through leaks, venting and incomplete flaring, often expressed as a percentage of gas produced.

A local group of generators, storage and loads with defined boundaries that can operate connected to the main grid or independently in island mode.

A small inverter mounted behind one or a few PV modules that converts their DC output to AC at module level, with individual power point tracking.

One million British thermal units (1.05506 GJ, 293.07 kWh); the standard pricing unit for natural gas and LNG in the United States and Asia.

Silicon grown as a single continuous crystal, usually by the Czochralski method, giving the highest-efficiency and now dominant wafer type for PV.

Control algorithm in an inverter or charge controller that continuously adjusts operating voltage so a PV array delivers its maximum available power.

A fossil fuel composed mainly of methane, used for power generation, industrial and residential heat, and as a chemical feedstock for hydrogen and ammonia.

A country’s climate action plan under the Paris Agreement, setting emissions targets and measures, updated every five years with progressively higher ambition.

Wholesale prices below zero, when generators pay to deliver power because inflexible supply exceeds demand.

A billing arrangement in which electricity exported by a small generator, usually rooftop solar, offsets imports at the retail rate over a billing period.

A state in which anthropogenic greenhouse gas emissions are balanced by anthropogenic removals over a specified period, so net additions to the atmosphere are zero.

A family of lithium-ion cathodes (LiNixMnyCozO2) offering high energy density, widely used in long-range electric vehicles.

Nominal Operating Cell Temperature: the temperature a PV module cells reach under 800 W/m², 20 °C ambient air and 1 m/s wind on an open rack.

The sum of all future cash flows of a project discounted to today, minus the initial investment; a positive NPV indicates value creation.

The splitting of a heavy atomic nucleus such as uranium-235 or plutonium-239 into lighter nuclei after absorbing a neutron, releasing about 200 MeV of energy and further neutrons.

The joining of light nuclei, such as deuterium and tritium, into a heavier nucleus, releasing energy; the process that powers the Sun and an experimental route to electricity.

Wind turbines installed in seas or large lakes, usually on fixed foundations in water up to about 60 m deep, exploiting stronger and steadier winds than onshore sites.

A simple-cycle gas turbine that exhausts hot gas directly to the atmosphere, trading lower efficiency (33–42%) for fast start and low capital cost, used mainly for peaking.

Operating expenditure: recurring costs to run and maintain an energy asset, split into fixed costs per kW-year and variable costs per MWh.

Power generation from the temperature difference between warm tropical surface seawater and cold deep water, typically about 20 to 25 °C, using a heat engine cycle.

A rigorous building performance standard, developed in Germany in the 1990s, achieving very low heating demand through insulation, airtightness and heat-recovery ventilation.

The time needed for the cumulative savings or revenues of an investment to equal its initial cost.

The highest electricity demand recorded on a grid during a given period, which sets the capacity the system must be able to supply.

Daily solar irradiation expressed as the equivalent number of hours at 1,000 W/m²; numerically equal to kWh/m² per day.

An impulse hydro turbine in which high-velocity water jets strike double-cupped buckets on a wheel; the standard choice for high heads above roughly 250 m.

A silicon cell design that adds a passivating dielectric layer to the rear surface, reducing recombination and reflecting light back into the cell.

Ratio of actual AC energy output to the theoretical output implied by plane-of-array irradiation and nameplate capacity; a location-independent quality metric for PV plants.

A solar cell whose absorber is a metal-halide compound with the perovskite crystal structure, offering tunable bandgap, cheap processing and very fast efficiency gains.

The generation of a voltage and electric current in a semiconductor junction when it absorbs photons with energy above the material bandgap.

Rotation of wind turbine blades about their long axis to change the angle of attack, used to regulate power above rated wind speed, start up, and stop the rotor.

A vehicle combining an internal combustion engine with a battery and electric motor that can be charged from the grid, giving a limited all-electric range.

Silicon cast into blocks containing many small crystal grains; cheaper to make than monocrystalline but less efficient, and now largely phased out of PV production.

The relationship between hub-height wind speed and a turbine's electrical output, used with a wind speed distribution to estimate annual energy production.

The ratio of active power to apparent power in an AC circuit, between 0 and 1, indicating how effectively current delivers useful work.

A module-level DC-DC converter that tracks each panel maximum power point and conditions its output before it enters a string inverter.

Power (kW) is the rate of energy flow at an instant; energy (kWh) is power accumulated over time. Capacity is measured in power, output in energy.

A long-term contract under which a buyer agrees to purchase electricity from a specific generator at a set price or price formula.

The most common nuclear reactor type, in which water under high pressure cools the core without boiling and transfers heat to a separate steam circuit.

Energy in its raw natural form before conversion, such as crude oil, coal, natural gas, uranium, biomass, and the electricity or heat from renewables.

Energy storage that pumps water to an upper reservoir when electricity is cheap or abundant and releases it through turbines when power is needed; the largest form of grid storage worldwide.

Thermal resistance of a material layer or assembly, its ability to resist heat flow; the reciprocal of U-value. Higher is better.

The lowest wind speed at which a turbine reaches its nameplate (rated) power output, typically 10 to 13 m/s for modern utility-scale turbines.

The component of AC power that oscillates between source and load without doing net work, needed to sustain magnetic and electric fields and control voltage.

Using an electric drive motor as a generator during deceleration to convert kinetic energy back into electricity stored in the battery.

Replacing ageing generating equipment at an existing site with newer, usually larger and more efficient technology, most commonly applied to wind farms after 15 to 25 years.

The amount by which available or accredited capacity exceeds forecast peak demand, expressed as a percentage of peak.

The share of energy put into a storage system that can be recovered on discharge, measured at a defined boundary (cell, DC, or AC grid connection).

Hydropower that diverts or uses the natural flow of a river with little or no reservoir storage, so output follows river flow rather than demand.

A heat pump's average coefficient of performance over a standardised heating season, weighting test points by how often each outdoor temperature occurs.

Direct greenhouse gas emissions from sources owned or controlled by a reporting organisation, such as boilers, furnaces, vehicles and process emissions.

Indirect emissions from the generation of purchased electricity, steam, heat or cooling consumed by a reporting organisation.

All other indirect emissions in an organisation’s value chain, upstream and downstream, not covered by Scope 1 or Scope 2.

Seasonal Energy Efficiency Ratio: total cooling output over a cooling season divided by electricity used, in BTU/Wh (US) or W/W (EU).

The share of on-site generation, typically rooftop solar, that is used directly by the producer rather than exported to the grid.

The SI unit of effective dose, weighting absorbed radiation energy by radiation type and tissue sensitivity to estimate stochastic health risk; 1 Sv = 1 J/kg.

An electronic meter that records consumption in short intervals and communicates the data remotely to suppliers and network operators.

A nuclear reactor of up to about 300 MWe per module, designed for factory fabrication and modular deployment to cut construction time and financing risk.

A rechargeable battery analogous to lithium-ion but using abundant sodium ions, trading energy density for lower material cost and better cold performance.

Reduction in PV or CSP output caused by dust, pollen, bird droppings, snow or other deposits blocking light from reaching cells or mirrors.

A semiconductor device, usually a doped silicon wafer with metal contacts, that converts sunlight directly into direct-current electricity via the photovoltaic effect.

A device that absorbs sunlight and transfers the heat to a fluid, mainly for domestic hot water, space heating, district heating and industrial process heat.

A motorised mounting structure that rotates PV modules or mirrors to follow the sun, raising the irradiance they receive over the day.

Annual energy produced per unit of installed PV capacity, expressed in kWh/kWp per year; combines solar resource and system performance.

Fuel assemblies removed from a reactor after use, highly radioactive and thermally hot, containing uranium, plutonium, fission products and minor actinides.

The reference conditions for rating PV modules: 1,000 W/m² irradiance, 25 °C cell temperature and an AM1.5G solar spectrum.

The remaining charge in a battery expressed as a percentage of its current full capacity.

A group of PV modules wired in series so that their voltages add up while the current stays that of a single module.

An inverter that converts the DC output of one or more series strings of PV modules into grid-synchronised AC, typically with several independent MPPT inputs.

Drop-in jet fuel made from biomass, waste or captured carbon and hydrogen, certified to blend with fossil Jet A-1 and cut lifecycle emissions.

The area of the circle traced by a turbine's rotating blades; wind turbine power output is directly proportional to it, and it grows with the square of rotor diameter.

The percentage change in a PV module power, voltage or current per degree Celsius of cell temperature away from 25 °C.

A unit of energy equal to one billion kWh (10⁹ kWh) or 3.6 PJ; used for national and global electricity generation and consumption.

A unit of heat equal to 100,000 BTU (105.5 MJ, 29.307 kWh), used for billing natural gas to homes and businesses in the US and formerly the UK.

A localised area of a building envelope with significantly higher heat flow than its surroundings, caused by geometry or more conductive materials.

Storing energy as heat or cold, in sensible, latent (phase-change) or thermochemical form, for later use as heat or for power generation.

PV technologies that deposit absorber layers only a few micrometres thick onto glass, metal or plastic, mainly cadmium telluride, CIGS and amorphous silicon.

A nuclear fuel cycle that uses fertile thorium-232, which absorbs a neutron and decays to fissile uranium-233, as an alternative to uranium-plutonium fuel.

Electricity generated from the height difference between high and low tide, by impounding water behind a barrage or lagoon and releasing it through low-head turbines.

Electricity generated by underwater turbines placed in fast tidal currents, analogous to wind turbines but driven by water about 800 times denser than air.

The ratio of the linear speed of a turbine blade tip to the undisturbed wind speed; three-bladed utility turbines are designed for values of about 7 to 10.

A conventional energy unit equal to 41.868 GJ (10 Gcal, 11.63 MWh), the nominal heat content of one tonne of crude oil, used in energy balances.

Tunnel Oxide Passivated Contact: an n-type silicon cell whose rear uses an ultra-thin oxide and doped polysilicon layer to extract current with minimal recombination.

Electricity lost as heat and through non-technical causes between power plants and end users, usually expressed as a share of energy supplied.

The thermal transmittance of a building element: heat flow in watts per square metre per kelvin of temperature difference (W/m²K). Lower is better.

The process of increasing the share of fissile U-235 in uranium above its natural 0.711%, to make reactor fuel or, at very high levels, weapons material.

Renewable generation whose output varies with weather and time of day, mainly wind and solar PV, and cannot be fully dispatched.

Bidirectional charging that lets a parked electric vehicle export battery energy to the grid, providing storage and grid services.

A software platform that aggregates many distributed resources, such as batteries, EVs, heat pumps and rooftop solar, and operates them as a single dispatchable unit.

Weighted average cost of capital: the blended return required by debt and equity investors, used as the discount rate for energy projects.

The reduced wind speed and increased turbulence downstream of a wind turbine, which lowers the output and raises the fatigue loads of turbines located behind it.

The SI unit of power, equal to one joule per second; it measures the rate at which energy is produced, transferred or used.

Energy captured from the motion of ocean surface waves using devices such as point absorbers, oscillating water columns and attenuators; still at pre-commercial stage.

A two-parameter probability distribution used to describe how often each wind speed occurs at a site, defined by a scale factor c and a shape factor k.

Greenhouse gas emissions of a vehicle from producing and delivering its energy (well-to-tank) plus using it in the vehicle (tank-to-wheel), per unit distance.

The mean power available in the wind per square metre of area perpendicular to the flow, in W/m², the standard measure of wind resource quality.

The change in wind speed (and sometimes direction) with height above the surface, driven by surface roughness and atmospheric stability.

The motors, gears, bearing and brakes that rotate a wind turbine nacelle about the tower axis to keep the rotor facing the wind.