Britain will see its deepest solar eclipse in decades this evening, with as much as 96% of the sun blocked by the moon in parts of the country, the closest the UK has come to totality since the total eclipse over Cornwall in 1999. The dip lasts around two hours and will barely register on a typical home solar system's annual output, since the effect behaves much like a passing bank of thick cloud. It matters far more to the national grid than to your roof, which is why Octopus Energy is running a UK wide power down session between 6pm and 8pm tonight.
A solar eclipse reduces the light reaching a panel in the same way heavy cloud does. Dense cloud cover can cut a panel's output by up to 90%, and a near total eclipse behaves similarly for the period the sun is most obscured. The difference is timing, not mechanism. Cloud cover moves in unpredictably, while an eclipse follows a known schedule, in this case peaking at maximum coverage in the early evening and lasting roughly two hours from first contact to last.
Most UK systems will see output fall gradually as the moon moves across the sun, rather than drop suddenly, and recover just as gradually once it clears. If your system includes a battery, you are unlikely to notice anything at all. The battery and the grid connection fill the gap automatically, the same way they do on an overcast afternoon. Bypass diodes fitted to individual solar cells, and microinverters fitted beneath some panels, normally help limit losses when only part of an array is shaded unevenly. For an eclipse the whole array loses light at the same time, so this makes less practical difference than it does with a stray cloud or a shadow from a nearby tree.
The National Energy System Operator, which balances Britain's grid in real time, expects solar output to fall by roughly 700 MW during the eclipse, and says its own modelling shows a bigger drop of up to 1.3 GW cannot be ruled out. That coincides with the early evening period when millions of households return home and start using ovens, kettles and washing machines, so the grid has to source that power from elsewhere, typically gas, at short notice.
The next total solar eclipse visible from the UK will not arrive until 2090, and the Royal Observatory Greenwich has described this evening as the closest the British Isles will come to totality for quite some time. That rarity is part of why it is being treated as a live test of how the grid handles a sudden but entirely predictable dip in solar output, a scenario it will need to manage more often as more of Britain's electricity comes from the sun.
Octopus Energy, the UK's biggest supplier, wants its customers to trim their electricity use for the two hours either side of the eclipse's peak this evening, precisely when solar output falls furthest. Customers signed up to Octoplus and Saving Sessions who use less power than usual for at least half an hour of that window will not be charged for an hour of electricity the following Sunday, 16 August, in place of the usual Octopoints reward. There is also a separate prize draw, open to anyone who posts a public eclipse photo tagging Octopus Energy, offering a complete solar panel installation at no cost, or £12,000 in cash instead.
This is worth knowing even if you are not an Octopus customer, because it is a clear example of what the industry calls demand flexibility, shifting when electricity is used rather than how much is used overall. Octopus puts the UK's total household flexibility potential at around 13 GW, mostly sitting untapped in smart appliances, electric vehicles and home batteries that could be doing far more of this kind of work.
No. A properly designed home solar and battery system already behaves like a permanent, automatic version of tonight's power down session. When generation drops, whether from an eclipse, a passing cloud, or a grey January afternoon, the battery draws down first and the grid fills in behind it, without you touching a switch. Homeowners in southern England with a typical 4kWp system routinely see far bigger swings in daily output from ordinary weather across a year than they will see from two hours of eclipse tonight.
The event is still worth watching for what it says about where UK electricity is heading. As more homes generate and store their own power, evenings like this stop being a curiosity for astronomers and start being a live rehearsal for a grid that increasingly runs on variable, weather dependent generation. If you want to see the scale of variation a typical UK system already handles across a year, how rain affects solar panel output and a real household's solar generation data over several years both cover it in detail.
If you plan to watch the eclipse itself, use certified eclipse glasses rather than sunglasses. Looking directly at a heavily obscured sun without proper protection can still damage your eyes.