On TV Pickup, it's no longer such a thing as it was in the 1980s and 1990s; Tom Scott has a video mentioning it - Dinorwig pumped storage hydroelectric power plant in the UK. One of the reasons for building it was to provide power to the UK national grid at short notice, and the national grid organisation would study the TV schedules for the UK's 4-5 terrestrial channels, and put Dinorwig on standby for power generation. Now with so many different channels and streaming, there isn't the same effect of the country all having an ad break at the same time, and Dinorwig's main use now is in balancing calm moments of solar and wind power generation.

Source:

- https://youtu.be/6Jx_bJgIFhI?t=210