Glossary

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.

A

As of time

The time of the reading a figure is based on, shown in UTC next to each block. It tells you how old the number is, not when the page was loaded.

B

Badge

The label every figure carries: Metered, Derived or Estimated. It says whether the operator measured the value, we calculated it from measured values, or it was modelled.

Balancing authority

In the United States, the organisation responsible for keeping supply and demand in balance within a defined area. The EIA-930 data on our U.S. pages is reported by balancing authorities.

Bidding zone

An area within which the wholesale market sets one price. Denmark has two, DK1 in the west and DK2 in the east, so the two halves of the country can have different prices at the same moment.

C

Capacity market

A market that pays generators and other resources to be available in future, separately from payments for the energy they produce. PJM runs one; ERCOT does not have a centralized capacity market.

Clearing price

The price at which an auction matches the volume sellers offer with the volume buyers bid. Every accepted seller in that zone and period receives it.

CO₂ estimate

Our carbon figure: metered generation multiplied by published lifecycle emission factors per fuel. It is a partial estimate, not a measured emission intensity; the methodology names the factor table.

Curtailment

Reducing a plant’s output below what it could produce, usually wind or solar, because the grid cannot use or move the power. Curtailment is one way markets handle a surplus instead of ever lower prices.

D

Day-ahead market

An auction held the day before delivery that sets prices for each interval of the next day. Danish prices on our pages are day-ahead results, in 15-minute intervals since delivery day 1 October 2025.

Demand (load)

The amount of electricity being used at a moment, in megawatts. Each market defines it slightly differently; for example, behind-the-meter rooftop solar lowers the demand a grid sees.

Derived

A badge meaning we calculated the value from metered values: a total, a share, a sum or a currency conversion. Danish demand is Derived because Denmark publishes no near-real-time consumption figure.

Dispatch

The operator’s instruction to each plant on how much to produce in the next interval. In Australia’s NEM, dispatch runs every five minutes and sets the spot price.

E

EIA-930

The U.S. Energy Information Administration’s Hourly and Daily Balancing Authority Operations Report. It supplies hourly demand, net generation, interchange and fuel mix for our U.S. pages; the data is preliminary and delayed.

Estimated

A badge for values modelled beyond simple arithmetic. Estimated figures are labelled so they are never mistaken for measurements.

F

Frequency

The rate at which alternating current cycles, held close to a fixed nominal value of 50 Hz or 60 Hz depending on the country. Grid operators keep it stable by balancing supply and demand continuously.

FUELINST

Elexon’s dataset of five-minute generation by fuel type in Great Britain. It covers transmission-connected plant only, so most rooftop solar and small wind are missing.

G

Generation mix

The share of electricity produced by each source, such as wind, solar, gas, coal, nuclear and hydro, at a given time. It depends on what each operator reports and how it groups fuels.

I

Imbalance price

In Great Britain, the price charged or paid to parties whose actual position differs from what they contracted. Since the P305 modification in November 2015 there has been a single imbalance price for both directions.

Interconnector

A transmission link between two grids or bidding zones. When an interconnector is full, prices on its two sides can separate.

Interval

The period a price or reading covers: five minutes in the NEM, 15 minutes for Danish day-ahead prices, half an hour for British settlement, an hour for EIA-930. We store every interval by its start time in UTC.

M

Marginal plant

The last, most expensive plant needed to meet demand in a period. In many markets its offer sets the clearing price for everyone.

Market index price

In Great Britain, a half-hourly price derived from short-term trading. On our pages it comes from the APXMIDP provider and is shown next to the imbalance price.

Market price cap

The highest price a market allows. In the NEM it is set by the Australian Energy Market Commission; the cap for 2026-27 is A$23,200/MWh.

Metered

A badge meaning the value is taken directly from the operator’s published figure, without calculation by us.

MW and MWh

A megawatt (MW) is a rate of power at one moment; a megawatt-hour (MWh) is energy, one megawatt sustained for one hour. Wholesale prices are quoted per MWh.

N

Negative price

A wholesale price below zero, meaning producers pay to keep generating and buyers are paid to take power. It is a genuine market result, explained on the page on why prices go negative.

NEM

Australia’s National Electricity Market, operated by AEMO, covering Queensland, New South Wales, Victoria, South Australia and Tasmania. NEM market time is AEST, ten hours ahead of UTC.

Net generation

Electricity produced by plants minus what they use themselves. EIA-930 reports net generation for each balancing authority.

Net interchange

The balance of electricity flowing into and out of an area over its ties with neighbours. Positive and negative signs follow each source’s own convention, which the market page states.

P

Paused

The state of a figure whose source has gone quiet. A metric whose newest point is older than three times its interval shows its last value and time instead of a guess.

Pre-dispatch

AEMO’s published forecast of prices for coming periods. It appears only on the Australian cheapest-hours pages, labelled “forecast by AEMO”; it is not our forecast.

Provisional

Data that the operator may still revise. Nearly everything on the clock is provisional when first shown.

Pumped hydro

Storage that pumps water uphill when power is cheap and releases it through turbines when power is needed. Taiwan’s feed lists such units under storage; we show them as pumped hydro.

R

Record

The most extreme value of its kind in our own data window for one market, described as “record since” our first date, never as an all-time figure. Records start provisional and become confirmed by a second reading.

Renewable share

Renewable generation as a share of the generation the source reports. Where a feed misses small or rooftop plants, the share is of metered output, not of national demand.

S

Settlement period

The interval used to settle a market. Great Britain uses half-hours: 48 on a normal day, 50 on the long clock-change day in October.

Spot price

A price for electricity delivered in the current or a very near interval, as opposed to a contract for later. In the NEM it is the five-minute dispatch price.

Stale

A source whose data is late. If more than 20 percent of critical zones are stale, the page shows a stale band and keeps the previous release.

T

Transmission-connected

Plant connected to the high-voltage transmission network rather than to local distribution networks. Some datasets, such as FUELINST, count only this plant.

U

UTC

Coordinated Universal Time, the reference clock we use to store every timestamp. Pages show the “as of” time in UTC so markets in different time zones line up.

W

Wholesale price

The price of electricity traded between generators, suppliers and large buyers, before network charges, taxes and retail costs. It is not the price on a household bill.