What is carbon intensity?

Carbon intensity is the amount of carbon dioxide (CO₂) emitted per unit of electricity generated, measured in grams of CO₂ per kilowatt-hour (gCO₂/kWh). It tells you how clean the grid is at any moment. The UK average over the last 90 days was around 118 gCO₂/kWh; on the cleanest hours it drops below 90, on the highest-carbon hours it rises above 160.

What is the best time of day to use electricity in the UK?

Based on 90 days of National Grid ESO data, the cleanest hours of the UK week are typically Sunday between 2am and 4am, when overnight wind generation is high and demand is low. Mid-Saturday afternoons are also consistently low-carbon. The highest-carbon hours are weekday evenings between 5pm and 7pm, when household demand peaks and gas plants ramp up to meet it.

How is carbon intensity calculated?

Carbon intensity is calculated by dividing total grid CO₂ emissions by total generation in a given period. National Grid ESO publishes half-hourly figures using the actual generation mix - wind, solar, nuclear, gas, biomass, hydro, imports - multiplied by each fuel's emission factor. Our heatmap aggregates 4,000+ of these half-hour readings into 168 hour-of-day × day-of-week buckets.

Why is the UK grid cleaner at certain times?

Three things drive the pattern: wind output (highest overnight, especially in winter), solar (only during daylight, April–September), and demand (lowest overnight and on weekends). When low-carbon generation is high and demand is low - typically weekend afternoons and overnight - gas plants run less, so the grid is cleaner. When demand spikes on weekday evenings, gas plants fill the gap and intensity rises sharply.

How can construction projects reduce scope 2 carbon emissions?

Schedule grid-powered work - site huts, electric plant, off-site fabrication, electric arc steel, concrete batching with electric hoists - into the lowest-carbon hours of the week. Even shifting power-intensive operations from weekday evenings to weekend mornings can cut scope 2 emissions by 30–40% on identical workloads. The data also feeds tender narratives: most clients want evidence of a carbon-aware programme, not just a carbon target.

What is the average UK grid carbon intensity?

The 90-day UK rolling average is currently 118 gCO₂/kWh, down from around 200 gCO₂/kWh a decade ago. The annual average has fallen consistently as wind capacity has grown. UK grid intensity is now among the lowest in Europe, but the within-week variation is large - cleanest hours are roughly half the carbon of the highest-carbon hours.

What is embodied carbon in construction?

Embodied carbon is the total CO₂ emitted to manufacture, transport and install the materials in a building - separate from operational carbon (the energy the building uses once occupied). Concrete, steel and aluminium typically dominate. Lower-carbon alternatives like GGBS-blend cement, recycled steel, and engineered timber can cut embodied carbon by 30–60% on the same building. Our carbon calculator compares 100+ materials from the ICE database.

How can we reduce embodied carbon in construction projects?

Five high-impact moves: (1) specify GGBS or PFA blend cements in place of CEM I; (2) replace structural steel with engineered timber where feasible; (3) increase recycled content in aggregates, plasterboard and insulation; (4) reduce material quantities through better structural design, not finishes; (5) source locally to cut transport emissions. The biggest carbon wins almost always come from the structural frame, not the fitout.

Where does this data come from?

Grid carbon and generation mix data is published by National Grid ESO (the Electricity System Operator) via the open Carbon Intensity API at carbonintensity.org.uk. The API has half-hourly resolution back to 2018. Our 90-day rolling heatmap aggregates these readings by hour-of-day and day-of-week. Methodology and source attribution are listed below the heatmap.

Can I use this analysis in tender submissions and BREEAM reports?

Yes. The data is from National Grid ESO under the Open Government Licence and can be cited directly. For tenders, the heatmap supports a carbon-aware programme narrative; for BREEAM and similar frameworks, you can use the underlying half-hourly data to build project-specific scope 2 emissions evidence. Cite National Grid ESO Carbon Intensity API as the source, with the date of the data extract.

UK Grid Carbon Intensity by Hour

Name: UK Grid Carbon Intensity - Hour-of-day × Day-of-week 90-day heatmap
Creator: Fabrick
License: https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/

How to use UK grid carbon intensity in your project programme

Grid carbon intensity is the carbon dioxide (CO₂) emitted per kilowatt-hour of electricity generated. In the UK it varies by roughly a factor of two over the course of a typical week - cleanest in the early hours and on weekend afternoons, when renewables dominate; highest on weekday evenings, when gas plants ramp up to meet demand peaks. For construction projects running grid-powered plant, site cabins, electric arc steel or concrete batching, scheduling power-intensive work into the cleanest hours cuts scope 2 emissions on identical workloads.

The heatmap above aggregates 90 days of half-hourly readings from the National Grid ESO Carbon Intensity API into 168 hour-of-day × day-of-week buckets. Each cell shows the rolling average for that slot. The cleanest 10 windows and the highest-carbon 10 windows are listed below the heatmap so you can lift them straight into a programme document or carbon-management plan.

Methodology

Source. National Grid ESO Carbon Intensity API, half-hourly resolution, Open Government Licence v3.0.
Window. Rolling 90 days (configurable to 30, 180 or 365 days via the lookback selector).
Aggregation. Each half-hour reading is bucketed by the hour-of-day and day-of-week of its start timestamp (UTC). Bucket value = arithmetic mean of all readings in the lookback window. Actual values are used where published; the published forecast is used as a fallback for any gaps.
Best/highest-carbon windows. Sorted by bucket mean. "Cleanest" windows are the lowest 10; "highest-carbon" the highest 10. Percentages are vs the rolling overall mean.

What the data is good for

Carbon-aware programmes. Scheduling concrete pours, lift operations, off-site fabrication or other power-intensive work into the lowest-carbon hours.
Tender narratives. Demonstrating a programme that times grid-powered work to the cleanest hours, with cited evidence from National Grid ESO.
Scope 2 reporting. Project-specific scope 2 emissions can be calculated using the actual half-hourly intensity values during the project window rather than an annual average.
BREEAM and similar frameworks. Evidence for energy-management credits where the assessor accepts published National Grid ESO data.

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