Weather and Energy Demand: Why the Forecast Moves Your Bill

Almost every large swing in British electricity demand starts with the weather. How cold it is decides how hard the country’s heating works. How hard the wind blows decides how much of the electricity comes from something with no fuel bill. How much sun reaches the panels decides the rest.

This page is the Met Office forecast for ten sites across England, Wales and Scotland, read as the drivers behind the numbers on our live electricity page. It is not a forecast to plan your weekend around — it is here to explain why demand and generation are about to do what they do.

Why weather drives demand

Britain heats its homes mostly with gas, and lights and runs everything else with electricity. Temperature moves both, and it moves them together.

The relationship is close to linear once you are below about 15°C. Every degree colder means every heated building in the country loses heat a little faster, and every thermostat responds the same way at roughly the same time. There is no smoothing effect from millions of independent decisions, because they are not independent — they are all reacting to the same weather.

That is why the coldest evenings of the year are also the moments the electricity system is under most strain, and why the market pays most for power at exactly those times.

The next two days

This is the current Met Office run for the ten sites. Note the spread — about 14°C between the coldest and warmest of them over the period, which is a reminder that "the weather in Britain" is never one thing.

Cold and still: the expensive combination

The single condition that stretches the electricity system is not extreme cold on its own. It is cold and still at the same time — high demand meeting low wind output.

Those two things often arrive together, which is the unhelpful part. The high-pressure systems that bring clear, freezing winter nights are the same systems that bring light winds. Meteorologists call the result a period of low wind and cold; the energy industry worries about it by name.

When it happens, gas plants have to fill the gap. Gas is usually the most expensive generation running, and in the way Britain’s wholesale market works the most expensive plant needed sets the price for everyone. So a still, cold day does not just cost more because the country is using more — it costs more per unit as well.

Milder or windier days work in reverse. Wind farms have effectively no fuel cost, so when they are producing they displace gas and drag the wholesale price down with them.

What the wind and sun figures do and do not tell you

Two honest caveats, because these numbers are easy to over-read.

The wind readings are ten metres above ground, at cities. Wind turbines are nothing like ten metres tall and most of Britain’s wind capacity is offshore, where the wind is stronger and far steadier. A city recording 15 mph does not mean the offshore fleet is becalmed. Use these as a signal about the weather system passing over, not as turbine input.

UV index is a proxy for sunshine, not a solar forecast. It tells you something real about how bright the day is, and therefore roughly how solar panels will be doing. It is not irradiance, it does not account for panel angle or shading, and it should not be read as a generation figure. If you want to know what solar is actually contributing right now, that is on the live electricity page.

Why ten sites and not a map

We publish ten named places rather than a national average or a colour-shaded map, and that is deliberate.

Ten points are a sample of populated places, not a grid. Averaging them would produce a “Great Britain temperature” that no one experiences and that would quietly weight cities over countryside. Interpolating between them would invent readings for places nobody measured. Either would look more authoritative and be less true.

So the page reports the warmest and coldest of these ten sites, which is a statement about a sample that happens to be accurate, rather than a statement about the country that would not be.

What this means for your bill

It depends entirely on the kind of tariff you are on, and the difference is bigger than most people expect.

• On a standard variable tariff, almost nothing happens in the short term. The price cap sets your unit rate for a whole quarter, so a cold snap changes how much energy you use but not what you pay per unit. Your bill rises because the meter turns faster, not because the price moved.
• On a fixed tariff, the unit rate is locked for the term. Cold weather still raises your bill through consumption, but the price is settled.
• On a tracker or agile tariff, wholesale movements reach you within a day or even half an hour. That is the trade: cheaper than the cap when wind is strong and demand is low, markedly more expensive on a cold, still evening. Our reviews of the Octopus Tracker and Octopus Agile tariffs go through how that works in practice.

If you heat with a heat pump, weather matters more than for anyone else: efficiency falls as the outside air gets colder, so the coldest days cost more per unit of heat as well as needing more of it. Our guide to air source heat pumps covers how much, and typical household energy use puts the numbers in context.