Carbon intensity: what it means and why it changes every 30 minutes

Carbon intensity is the number of grams of CO2 emitted for every kilowatt hour of electricity you consume. It is not a fixed number. It shifts every half hour as the UK grid adjusts its generation mix to match demand.

When wind turbines are spinning hard overnight and demand is low, the grid barely needs gas at all. Carbon intensity falls toward 50g CO2/kWh or below. When millions of households return from work and the kettle, oven, and TV all go on simultaneously, the grid calls on gas peaking plants to fill the gap. Carbon intensity can exceed 250g CO2/kWh at those moments.

The UK average grid carbon intensity has halved since 2012. In 2012 it sat around 500g CO2/kWh because coal was still generating a large share of UK electricity. By 2026 the average is closer to 150-200g CO2/kWh, as gas replaced coal and renewables replaced both. Wind alone now delivers around 30% of UK electricity. Renewables collectively exceeded 50.8% of generation in 2024, the first year they outpaced fossil fuels.

But the average hides the real story. The range is what matters. On the same grid, at different hours of the same day, carbon intensity can vary by a factor of five. Your choices about when to run appliances sit inside that range. You can use the green end of it, or the dirty end. The grid does not decide for you.

The daily pattern: when your electricity is greenest

The UK grid follows a predictable rhythm tied to human activity and the behaviour of wind and solar. Understanding this rhythm is the foundation of carbon-aware energy use.

10pm to 6am: the green window. Demand collapses as the country sleeps. Wind turbines, which care nothing for human schedules, often generate their full output through the night. Gas peaking plants sit idle. Carbon intensity drops to its lowest levels, typically 50-100g CO2/kWh, and on windy nights can fall below 40g. This is the window the grid wants you to use. On Octopus Agile, this window is also the cheapest.

6am to 9am: the morning ramp. Demand climbs steeply as households wake up. The grid calls on gas plant to supplement overnight wind generation. Carbon intensity rises toward 150-250g CO2/kWh depending on season and wind conditions.

12pm to 2pm: the midday lull. Demand moderates slightly after the morning peak. In summer, solar generation contributes meaningfully, suppressing carbon intensity to around 100-180g CO2/kWh. In winter, without meaningful solar, this period is less distinguishable from the broader daytime pattern.

5pm to 8pm: the peak. This is the dirtiest window of the day. Heating, cooking, and lighting demand hits its maximum. Gas peakers run at full output. Carbon intensity reaches 200-300g CO2/kWh on winter weekdays. On Agile, this period correlates closely with the highest prices of the day. The price signal and the carbon signal are pointing in exactly the same direction: avoid this window.

The seasonal pattern: winter highs, summer lows

Beyond the daily rhythm, the seasons shape UK carbon intensity in significant ways.

Winter (December to February) produces the highest carbon intensity peaks. Solar generation is near zero. Demand is at its annual maximum as homes heat up. Gas plant runs for longer and harder. Winter weekday evenings regularly push above 250g CO2/kWh. Overnight relief is still available, but the gap between peak and trough is at its largest. Shifting loads to the night in winter delivers the biggest carbon savings of the year.

Spring and autumn offer the lowest overall carbon intensity. Demand is moderate, wind is often strong, and solar contributes meaningfully without overwhelming the system. In spring, overnight periods can reach 30-40g CO2/kWh on particularly windy nights. These are some of the cleanest electricity hours available in the UK all year. They also coincide with some of the most dramatic Agile price drops, including negative pricing.

Summer produces the lowest peak intensity of any season. Solar generation is substantial through the middle of the day, often covering a significant share of daytime demand. Midday carbon intensity can fall to 80-120g CO2/kWh in July and August. Summer evenings are less extreme than winter because heating demand is absent, but the gas-peaker dynamic still applies at dinnertime.

The practical implication: if you are going to make one behavioural change, make it in winter. The carbon payback for shifting your appliance timing from peak to overnight is at its maximum in the cold months.

Regional differences: why Scotland's electricity is greener than London's

The UK carbon intensity figure published by National Grid ESO is a national average. It conceals dramatic regional differences that matter if you want to know the true impact of your electricity use.

Scotland has the lowest carbon intensity of any region in Great Britain. It combines substantial hydroelectric capacity with some of the largest offshore and onshore wind farms in Europe. At times of high wind and low demand, Scotland's regional carbon intensity can fall to 30-50g CO2/kWh. It regularly exports surplus clean power south.

The north of England benefits from proximity to large offshore wind installations in the North Sea. Carbon intensity here is typically below the national average and can be very low during strong wind periods.

The South East of England has the highest regional carbon intensity. It is furthest from major wind capacity, generates little renewable power locally, and depends heavily on interconnector imports and gas plant. On a calm winter evening, South East carbon intensity can be 150-200g CO2/kWh while Scotland sits at 40-60g CO2/kWh. Same hour. Same national grid. Five times the difference.

This has two practical consequences. First, the carbon benefit of timing your electricity use is present everywhere but most dramatic in the South East, where the gap between peak and off-peak carbon intensity is widest. Second, the regional variation reinforces why shifting load matters at a system level: consumers in low-carbon regions absorb surplus clean power and reduce the need for gas plant to fire up in higher-demand areas.

How to check live carbon intensity right now

Two tools give you real-time UK carbon intensity data, free of charge.

carbonintensity.org.uk is the official National Grid ESO API and its associated interface. It shows national and regional carbon intensity updated every 30 minutes, forecasts for the next 48 hours, and a breakdown of the generation mix behind the number. You can enter your postcode for a regional figure more relevant to your actual supply. The data is authoritative and free.

electricitymaps.com shows carbon intensity across European countries and regions in a visual format. It is useful for understanding where UK electricity sits in a continental context, particularly during periods when interconnectors are importing power from France's nuclear fleet or Norwegian hydro.

Both tools pair naturally with AgileAlert's live price dashboard. When Agile prices are lowest in your region, check the carbon intensity simultaneously. In most cases, the two numbers confirm the same story: the cheapest window is the greenest window.

Combining carbon intensity with Agile price: the double win

On Octopus Agile, electricity prices are set from the wholesale market, which responds to the same dynamics that drive carbon intensity. When wind generation is high and demand is low, wholesale prices fall because there is more electricity than the grid needs. When wind drops and gas peakers run to fill demand peaks, wholesale prices rise because that gas-fired generation is expensive.

The mechanism is distinct, but the outcome is almost identical. The windows when Agile is cheapest are the windows when the grid is greenest. The windows when Agile is most expensive are the windows when the grid is burning the most gas.

This is not a coincidence or a marketing claim. It is a structural feature of how electricity markets and generation physics interact. You cannot have surplus renewable electricity without both a price drop and a carbon drop. The two are inseparable.

In spring and autumn, the combination can be remarkable. On a windy night in March, you can find 3-5p/kWh on Agile and 40g CO2/kWh simultaneously. This is roughly one-tenth the carbon of peak-hour electricity and one-sixth the price of a standard variable tariff unit. Running your washing machine at that moment is both the smartest financial decision and the lowest-carbon version of that wash you could ever achieve.

Agile prices, checked on AgileAlert, are therefore a practical proxy for carbon intensity. You do not need to open two separate tools. The price signal is, in most conditions, a reliable carbon signal.

The maths: how much carbon does your timing actually save?

Abstract claims about carbon savings are easy to make. Here are the specific numbers behind appliance timing on Agile.

A typical Agile household shifts around 4kWh per day from peak hours (5-8pm) to overnight (10pm-6am). The carbon intensity difference between those windows averages roughly 180g CO2/kWh across the year, with the spread much larger in winter and smaller in summer.

The daily carbon saving from this shift: 4kWh x 180g = 720g CO2 per day.

Across a full year: 720g x 365 = 263kg CO2. That is roughly one-tenth of the average UK household's total electricity carbon footprint. From a timing change alone. No new equipment, no upfront cost, no change to the appliances you already own.

A single load of washing makes the difference vivid:

For EV owners, the numbers scale dramatically. A 60kWh EV charge at 5pm peak: 60 x 250g = 15,000g (15kg) CO2. The same charge overnight at 50g CO2/kWh: 60 x 50g = 3,000g (3kg) CO2. That is 12kg CO2 saved per charge. For a driver charging twice a week, that is over 1,200kg CO2 per year from timing alone.

Building a carbon-aware daily routine

The goal is not to obsess over carbon intensity every 30 minutes. The goal is to build a default routine that captures the overnight green window as a habit, not a daily calculation.

Most modern appliances help. Washing machines and dishwashers have delay-start functions that let you set a finish time rather than a start time. Programme your dishwasher to finish at 7am and it will run through the green window without you thinking about it. Do the same for the washing machine. If you have a smart plug, use it to schedule the kettle, dehumidifier, or air purifier for off-peak hours.

The windows to protect are simple: avoid 5pm to 8pm on weekdays for any flexible load. This is peak demand every day of the year. If you can move a task outside this window, you have eliminated your highest-carbon electricity use.

The windows to target are equally simple: anything between 11pm and 6am on a night when the wind is blowing. Check AgileAlert to confirm the pricing window, and the carbon benefit follows automatically.

Once the habit is set, you need not track carbon intensity directly. The Agile price signal does the work for you. The cheapest time to run your appliances is, in almost every meaningful circumstance, also the greenest.

Frequently asked questions

What is a good carbon intensity figure for UK electricity?
Below 100g CO2/kWh is excellent and indicates high renewable penetration. 100-200g is average for the current UK grid. Above 200g indicates significant gas generation, typically during demand peaks. The national average in 2026 sits around 150-200g CO2/kWh across all hours. Overnight and off-peak periods regularly achieve 50-80g.
Is Agile's cheapest time always the lowest carbon time?
In almost every case, yes. The mechanism is structural: wind surplus drives both low wholesale prices and low carbon intensity simultaneously. Rare exceptions can occur when gas prices are temporarily depressed by market factors unrelated to generation mix, but these are unusual. For practical purposes, treating the cheapest Agile window as the greenest window is a reliable rule of thumb that holds across the vast majority of hours.
Does my region affect how much carbon I save by timing appliances?
Yes. Households in the South East of England see the largest carbon differential between peak and overnight hours, because their regional grid is more gas-dependent. Households in Scotland or the north of England already benefit from lower average carbon intensity and may see a smaller proportional improvement from timing, though the overnight green window is still cleaner than the evening peak in all regions.
How does carbon intensity affect EV charging specifically?
EV charging is where carbon intensity has the largest absolute impact on any single household activity. Charging a 60kWh battery at peak intensity (250g CO2/kWh) produces 15kg CO2. The same charge overnight at 50g CO2/kWh produces 3kg CO2. For a driver who charges twice a week, shifting entirely to overnight charging saves over 1,200kg CO2 per year. That is a larger carbon saving than many EV drivers realise they can achieve purely through timing, before any other change.
Will carbon intensity keep improving as the UK grid adds more renewables?
Yes, the long-term trajectory is clear. The UK government's target is 95% clean power by 2030 and net zero electricity by 2035. As offshore wind, solar, and battery storage capacity grow, the carbon intensity of peak hours will fall alongside the overnight minimum. However, the difference between peak and off-peak will likely persist until long-duration storage can fully buffer the gap. Timing your electricity use will continue to deliver carbon benefits for years to come, even as the average improves.