Last Monday, the Iberian grid suffered a disturbance in the south-west at 12:33. In 3.5 seconds this worsened and the interconnection to France disconnected. All renewable generation then went off-line, followed by disconnection of all rotating generation plant. The Iberian blackout was complete within a few seconds.

At the time the grid was producing 28.4 GW of power, of which 79 per cent was solar and wind. This was a problematic situation as solar and wind plants have another, not widely known, downside – one quite apart from their intermittency and expense.

This is the fact that they do not supply any inertia to the grid. Thermal powerplants – coal, gas, nuclear, for example – drive large spinning generators which are directly, synchronously connected to the grid. If there are changes which cause a difference between demand and supply, the generators will start to spin faster or slower: but their inertia resists this process, meaning that the frequency of the alternating current in the grid changes only slowly. There is time for the grid managers to act, matching supply to demand and keeping the grid frequency within limits.