The cause of last week's massive power outage in Spain and Portugal remains unclear but it has shone a spotlight on solar and wind energy, which critics accuse of straining electricity grids.

The rise of renewables presents a challenge for power grids, which must evolve to adapt as countries move away from fossil fuels.

Grid operators must ensure that electricity is constantly balanced between demand and supply.

A metric of this balance is the frequency of the electricity flowing through the grid, set at 50 hertz (Hz) in Europe and 60 Hz in the United States. If that number drifts too far off, it can jeopardise the grid.

Historically, the electricity system has relied on conventional power plants -- gas, coal, nuclear and hydroelectric -- that use spinning turbines to generate electricity.

These machines keep the frequency stable.

With their gigantic rotors spinning at high speed, they provide inertia to the system.

If a power plant fails or if electricity demand increases too quickly, they help stabilise the grid by releasing the kinetic energy stored in the rotors.

Instead of spinning machines, solar and wind farms use........

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