GB transmission demand data is available in Ko: What it tells you and how to use it
GB transmission demand data is available in Ko: What it tells you and how to use it
Ko can now answer questions about GB electricity demand.
We've added ITSDO, Elexon's Initial Transmission System Demand Outturn, to the terminal. It measures GB Transmission System Demand in MW, updated every half hour, with data back to March 2016.
ITSDO includes station transformer load, pumped storage demand and interconnector exports. There's a second, related Elexon dataset, INDO (Initial National Demand Outturn), that excludes those additions. We prioritised ITSDO first because Elexon's own peak demand API is built directly on it.
ITSDO is measured at the transmission boundary, so it doesn't capture embedded, distribution-connected solar and wind generation. Their capacity has grown substantially since 2016, and that growth suppresses the demand figure ITSDO reports. A change in ITSDO over a long period reflects both underlying demand and that growth.
Before this update, Ko had no way to answer demand questions for GB. Now it can handle questions on peak demand, average demand and historic demand trends, at any timescale from a single day to the full data history.
Output: GB average transmission demand has fallen roughly 10% over the past decade, from 31.7 GW in 2016–2017 to 28.5 GW in 2025.
Notable points:
- 2020 sharp drop to ~28.4 GW: driven by the COVID-19 pandemic reducing industrial and commercial consumption.
- 2021–2022 partial rebound to ~29.6–29.8 GW as economic activity recovered.
- 2023 trough at ~28.0 GW, the lowest full-year average in the period. DESNZ data shows UK electricity consumption fell to a 1980s low in 2023, driven by continued high energy prices and the year was warmer than average, cutting heating demand.
- 2024–2025 slight uptick back to ~28.3–28.6 GW, though still well below pre-pandemic levels.
Follow up: How has solar buildout impacted average demand in GB?
Output: GB transmission demand peaked between 47.0 GW and 51.7 GW in every year from 2016 to 2025. The peak fell from 51.7 GW in 2017 to 47.0 GW in 2023, the lowest of the past ten years, before rising to 47.7 GW in 2025.
Despite a drop in peak demand generally over the past 10 years, in the past 3 years demand has increased. Is this showing an uptake in demand finally coming to the GB electricity system, especially when we consider embedded generation?
Follow up: What does the Modo forecast say about demand in Great Britain?
Output: GB transmission demand climbed through the heat wave in late June 2026, peaking at 36.3 GW on 25 June.
The increase wasn't just seen at the peak. The heat wave also kept demand elevated overnight. Between 23 and 26 June, average demand from midnight to 6am ran at 27 to 29 GW. This was 4-8 GW above the typical overnight range seen in June.
That points to sustained air conditioning load, a pattern that doesn't switch off after dark and is unusual for GB given historically mild summers.
The heatwave in late June saw prices in the intraday market hit £639/MWh on 23rd June, and on 24th June prices in the day-ahead market hit £420/MWh.
Follow up: What was the impact of the heatwave in June 2026 on the GB market?
More to ask Ko
