Anatomy of a cold snap: what actually happens to the grid
Why demand and price spike together, and why the grid operator starts paying almost anything to keep the lights on.
Demand doesn't move smoothly
Electricity demand already swings hard within a single day — low overnight, a morning ramp, a dip midday, and a sharp evening peak as homes switch on heating, cooking and lighting at once. A cold snap doesn't change that daily shape so much as it raises the whole thing: heating demand climbs across every hour, so the evening peak that was already the hardest part of the day to cover gets higher still, often several gigawatts higher than a mild day.
The grid has to be built and run to cover that worst case, not the average case — which is why a handful of genuinely cold days a year can matter more to how the market behaves than the other 360.
Why price rises faster than demand
Remember how merit order pricing works: generators run cheapest-first, and the last (most expensive) one needed sets the price for everyone. On an ordinary day, demand is usually met well within the cheap-to-mid part of the stack. On a cold-snap evening, demand can climb high enough that the grid has to call on its most expensive, least-often-used plants just to cover the last gigawatt or two — and because those plants are expensive, the clearing price for that half-hour jumps for every generator running, not just the marginal one.
This is why cold-snap prices don't rise in proportion to cold-snap demand — a 10% rise in demand can easily produce a much larger rise in price, because that last 10% has to be covered by the most expensive plants left in the stack.
The reserve margin
Grid operators don't plan to meet demand exactly — they keep a buffer of spare capacity above forecast peak demand, called the reserve margin, specifically so an unexpected plant breakdown or a colder-than-forecast evening doesn't tip straight into a shortfall. A cold snap eats into that margin from both directions: demand is higher than forecast, and cold weather is also when breakdowns are more likely (frozen equipment, strained transmission lines). A thin reserve margin going into a cold snap is one of the most reliable predictors of a genuinely tight, high-price evening.
When the margin runs out
If available supply still can't quite cover demand even after every plant that can run is running, the grid operator has a small number of last-resort tools — emergency imports, paying large consumers to reduce demand, and, only as an absolute last resort, planned rolling outages to protect the wider system from an uncontrolled failure. These are rare in Great Britain specifically because the market is designed to make the price signal painful enough, early enough, that the shortfall gets covered by the market first. A blackout is the system's failure mode, not its normal operating mode — and it's expensive in ways that go well beyond the clearing price.
Play it instead of reading about it
GridTradingDesk's cold-snap Challenge Day is modelled on exactly this pattern — demand climbing across the day, the stack getting squeezed, and a real decision about whether to hold a plant back, push it through a breakdown risk, or let the lights go out and take the Reliability hit. It's a fixed scenario, identical for every player, with its own leaderboard.