Market Overview

The energy transition in Great Britain is well under way. Fossil fuels are on the out, and each year renewables make up a larger proportion of the power mix; in 2025, 44% of Great Britain's electricity was generated using renewables. Renewables are of course cheap and clean, but one consequence of a more renewables-focused power mix is that there is less system inertia, the physical flywheel effect that is used to resist sudden changes in frequency.

Grid frequency has to stay within a whisker of 50 Hz. Matching supply to demand, all else equal, is slightly harder when there are less heavy turbines to provide inertia on the supply-side of the grid. Less inertia means things move faster when something breaks, and requires faster responses to avert disaster. This problem is perfectly suited for batteries: they can go from idle to full output in under a second, in either direction. That capability is what the frequency response markets buy and why batteries' roles in the grids of the future will continue to grow.

The shift that created the market

The generation mix is the clearest way to see why these markets exist and why they have grown. Wind and solar rise; gas and coal fall; the system carries less and less inertia.

The relative mix matters because it shapes the underlying risk of frequency deviation and thus the opportunity for BESS sites to step in: high wind with low demand pushes frequency high, which the High-side services answer by charging, while low wind with high demand can cause dips, which the Low-side services answer by discharging. The High and Low in DCH, DCL and the rest name the frequency excursion being corrected, not the direction the battery moves power.

This plot shows a 28-day rolling mean, smoothing day-to-day noise while still capturing seasonal swings.

How batteries capitalise

A grid-scale battery in GB has three main routes to revenue: frequency response (ancillary services), wholesale arbitrage, and capacity markets. As capacity markets are longer-horizon auctions (one or four years out from delivery) with more site-specific physical limitations, this page covers the market for both the shorter term markets: frequency response and wholesale arbitrage.

Frequency response: contracted availability. NESO runs daily auctions for capacity that must react within seconds when frequency strays from 50 Hz. Win one and you are paid a £/MW/h availability fee for every hour you are committed, whether or not you are actually called. Predictable, contracted income, but each contract ties up part of the battery. A Low contract needs enough energy in store to discharge for its full delivery window, a High contract needs the same again as headroom to charge, and every MW sold in one direction counts against the battery's rating on that side, whichever service it is sold into.

There are three services, split by how fast and how long they must respond:

Service	Acts on	Full response within	Sustained for	Opposite-side reserve¹
DC — Dynamic Containment	Large deviations, ±0.2–0.5 Hz	1 second	15 min	10%
DM — Dynamic Moderation	Moderate deviations, ±0.1–0.2 Hz	1 second	30 min	20%
DR — Dynamic Regulation	Small, everyday deviations, ±0.015–0.2 Hz	10 seconds	60 min	40%

¹ Since 15 November 2024, each MW contracted also keeps a share of the battery's power free on the opposite side for recovering energy: a battery selling 10 MW of DR Low must keep 4 MW of charging capacity spare. The reserve shares come from the Response Services Procurement Rules (Schedule 1, “Reserved Capacity”); response times and delivery windows from NESO's Response Service Terms.

Each runs as two separate auctions: High, which responds to rising frequency by charging, and Low, which responds to falling frequency by discharging. Auctions clear per EFA block — six four-hour windows covering the day — so a battery's commitment can differ across a given day.

Wholesale arbitrage: opportunistic trading. Spot energy prices are dependant on many factors, which can lead to a instances of high peaks and low (and even negative) troughs for batteries to capitalise on. Simply, batteries can buy energy when it is cheap and sell when it is expensive, taking the spread minus round-trip losses & wear cost of cycling as profit.

The tension between these two is the subject of the Forecasting & Dispatch page: capacity committed to frequency response cannot be freely traded, so the operator must decide each day how to split it.

Frequency Response

Each service is bought through daily auctions, one for every product (the High and Low of each service) in every EFA block. The clearing price is the price of the marginal accepted offer for that product and block.

Two changes to the auctions divide the history in this data:

Legacy auctions, up to 1 November 2023. A unit could offer only one of DC, DM and DR in a block, and prices never went below zero.
Enduring Auction Capability (EAC), from 2 November 2023. A unit can split its capacity across all three services in the same block and offer several products in one order, and prices can go negative. The opposite-side reserve has applied since 15 November 2024.

EFA block timings

EFA Block 1 spans midnight (23:00 the previous calendar day to 03:00). All times are local GB time.

Clearing prices — 28-day rolling average by product

Individual auction results are averaged to a daily figure per product, then smoothed with a 28-day rolling window. Scroll through the story, or pick an auction era on the chart to explore it yourself.

2021–22

The early peak

DC Low averaged £17.51/MW/h across 2022, the highest annual average of any product in this data, with DR High not far behind at £11.53. Few batteries were yet competing for what NESO needed to buy.

2023

Then DC collapsed

New battery capacity arrived faster than NESO's requirement grew. DC Low averaged £2.70/MW/h in 2023, down 85% on 2022, and DR High fell to £1.10. The Low products of DM and DR held up far better, and have strengthened since.

Throughout

Low usually clears above High

In most blocks a service's Low product clears above its High product: % of DC blocks, % of DM and % of DR. Across the fleet, spare capacity to absorb power has generally been easier to find than spare capacity to inject it.

Since November 2023

High products go negative

EAC allowed prices below zero, and the High products went there almost immediately. Since go-live, DR High has cleared negative in % of blocks and DM High in %, while the Low products almost never have. A provider can offer DR High and DR Low in one order, accepted on its combined value. And DR High delivers free energy the battery can sell on, which is worth paying a little to hold.

28-day rolling average, £/MW/h

Summary statistics

Wholesale & settlement prices

Two prices matter for a battery trading energy. The market index (APXMIDP) is the half-hourly wholesale reference that the dispatch model on this site trades against. The imbalance price is what a party pays or receives for being out of balance in a settlement period.

Since Elexon's P305 took effect in November 2015, GB has settled imbalance at a single price: the System Sell Price and System Buy Price are the same number.

Market index and imbalance price

Lines are 28-day rolling averages of each day's mean price; the shaded bands run from the average daily low to the average daily high over the same window. The two prices move together on average, but the imbalance price swings further within a day: its daily range averages £/MWh against £ for the market index, and it is negative in % of half-hours against %.

Wholesale price spread

Daily peak-to-trough APXMIDP spread — the raw arbitrage opportunity available to a battery on any given day, before efficiency losses and cycling cost.

Thin line is the daily spread; heavy line is a 28-day rolling average.