Glossary
Plain definitions of the terms used across the live board.
Electricity markets use a specialised vocabulary, and the same word can mean different things in different countries. This glossary defines the terms used on The World Electricity Clock in plain language. The definitions below are general and carry no live numbers; where a rule is specific to one market, the market’s own page says so.
If a number on the clock is unclear, read its badge first (Metered, Derived or Estimated) and then the matching entry here. The methodology page explains how each source is collected, and the page on negative prices explains why a price can fall below zero.
- MW and MWh
- A megawatt (MW) measures power, the rate of flow at one instant. A megawatt-hour (MWh) measures energy: one megawatt sustained for one hour. A plant rated 100 MW running flat out for two hours produces 200 MWh.
- Day-ahead market
- An auction held the day before delivery in which buyers and sellers agree prices and volumes for each interval of the next day. The result is a schedule, not a measurement.
- Real-time price
- A price that applies to the period actually being delivered, set shortly before or during it. In the Australian market it is the five-minute dispatch price.
- Imbalance price
- The price charged or paid for the gap between what a party contracted and what it actually produced or used. In Great Britain it is also called the system price.
- System price
- In Great Britain, the single imbalance price for a settlement period, reflecting the cost of the system operator's balancing actions.
- Settlement period
- The block of time for which energy is measured and paid. Great Britain uses 30 minutes, the Australian NEM five minutes, and the European day-ahead market 15 minutes.
- Market time unit
- The length of the intervals in which a market trades or clears. Europe's day-ahead auction moved from hourly to 15-minute units on 1 October 2025.
- Bidding zone
- An area in which one wholesale price applies. A country can have several; Denmark has two, DK1 and DK2.
- Interconnector
- A high-voltage line or cable that links two grids or markets and lets power flow between them, within a stated capacity.
- Capacity factor
- The share of a plant's maximum possible output that it actually delivers over a period. A wind farm with a capacity factor of 40 percent produces 40 percent of what it would at full power all the time.
- Installed capacity
- The maximum output a plant or fleet can deliver under rated conditions, in MW. It is not what it generates at a given moment.
- Curtailment
- Deliberately reducing the output of a power plant, usually wind or solar, because the grid cannot use or carry the power.
- Negative price
- A wholesale price below zero, meaning producers pay to inject power. See the page on why prices go negative.
- Price cap
- The highest price a market allows in an auction or dispatch, set by regulation. It limits price spikes.
- Price floor
- The lowest price a market allows. In the Australian NEM the floor is negative, so prices can fall below zero but not without limit.
- Pumped storage
- A hydropower scheme that pumps water uphill when power is cheap and releases it through turbines when power is needed. Also called pumped hydro.
- Embedded generation
- Generation connected to a local distribution network rather than the high-voltage transmission grid. Rooftop solar is the common example.
- Behind the meter
- Generation or consumption on the customer's side of the meter, such as rooftop solar, which reduces the demand seen by the grid but is not metered by the market operator.
- Transmission system operator
- The organisation responsible for running the high-voltage grid and keeping supply and demand in balance, for example Energinet in Denmark.
- Dispatch
- The instruction to a generator to produce a given amount of power. In the NEM, dispatch targets are recalculated every five minutes.
- Region
- The Australian NEM's term for a price area. The NEM has five: NSW1, QLD1, VIC1, SA1 and TAS1.
- Baseload
- Generation that runs steadily around the clock, typically large thermal or nuclear plants that are costly to switch off.
- Marginal price
- The price set by the last unit of supply needed to meet demand in an interval. All accepted bids in many markets receive that price.
- The part of generation that comes from renewable sources. On this site it is a derived figure based on the generation categories visible in each feed.
- Metered
- A badge meaning the value is taken directly from the operator's published measurement.
- Derived
- A badge meaning the value is calculated by us from metered values, such as a total, a share or a currency conversion.
- Estimated
- A badge meaning the value rests on published factors rather than a direct measurement. Carbon-intensity figures carry it.
- Paused
- A status shown when a source has gone silent or a metric has not updated within three times its interval. The page shows the last value and its time, not a guess.
- Carbon intensity
- The estimated greenhouse gas emissions per unit of electricity generated, usually in grams of carbon dioxide equivalent per kilowatt-hour.
- Lifecycle emissions
- Greenhouse gas emissions counted across a technology's whole life, including construction, fuel supply and operation, not only the stack or exhaust.
- Frequency
- The rate at which alternating current oscillates, 50 Hz in Europe and Australia. It drifts when supply and demand are out of balance.
- Spot price
- The price for immediate or near-term delivery, as opposed to a long-term contract. It is a wholesale price, not what consumers pay.
- Non-scheduled generation
- Smaller generating units that the market operator does not dispatch centrally. Their output may be estimated rather than metered in full.
- Ancillary services
- Services such as frequency control and reserve that keep the grid stable. They are bought separately from energy.