The solar eclipse could cause power cuts, Britain's National Energy System Operator has warned.
NESO has issued an Electricity Margin Notice (EMN) for Wednesday evening between 5pm and 8pm.
It requested a greater safety cushion due to a forecast 1,200-megawatt shortfall in system margins.
The operator emphasised that this is a precautionary, routine operational tool and that there is no risk to customer electricity supplies.
The grid is not expected to face an imminent blackout; instead, the market is being given an opportunity to make additional generation or flexibility available.
The warning comes as the moon will block up to 95 per cent of the sun.
This is expected to wipe out between 300MW and 1,100MW of solar power.
The timing will coincide with millions of people arriving home and switching on appliances.
The drop will happen much faster than a standard sunset, testing grid frequency stability.
Britain's heatwaves have also driven up electricity demand as households run air conditioners and fans in efforts to keep cool in the sweltering conditions.
Solar power has become an increasingly vital component of Britain's energy mix.
Last month it accounted for a record 14.4 per cent of the country's total electricity generation, meaning even a brief loss of sunshine creates a notable shortfall that must be compensated for.
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Without intervention, that gap would likely be plugged by firing up additional gas-fired power stations.
However, if enough households postpone energy-intensive tasks for a short period, that extra fossil fuel generation can be avoided.
Octopus Energy, the country's largest energy supplier, is also asking customers to reduce their electricity use between 6pm and 8pm today.
Customers who successfully lower their usage during the two-hour window will be rewarded with free electricity the following weekend through the company's Eclipse Power Down initiative.
Those taking part can also enter a photo competition for the chance to win solar panels fully installed at their home.
The winner can choose to receive a £12,000 cash alternative instead.
Massive queues have formed across Britain as people scramble to secure solar eclipse glasses before tonight's historic celestial event.
Tonight's solar eclipse is the most significant to be visible from the UK in 27 years.
It is Britain's deepest solar eclipse since 1999.
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