When to Use Electricity
The cleanest and cheapest hours where you are
Great Britain’s electricity is not equally clean at every hour of the day, and it is not equally clean in every part of the country at the same hour. The regional model can therefore assign very different average gCO₂/kWh figures to the north of Scotland and the south east in the same half hour.
This page puts the two halves of that question together: the average carbon intensity forecast for electricity in your region in each half hour ahead, and the VAT-inclusive unit rate on the current Agile Octopus product shown here. It does not calculate an appliance bill or avoided emissions. Pick your region below. You do not need to enter a postcode.
London
Every carbon figure here is a forecast. Regional carbon intensity is modelled from national generation and estimated flows between regions — it is never measured at a region’s boundary, so there is no later outturn that confirms or corrects it.
Cleanest half hour
Thu 09:30
199 gCO₂/kWh
Lowest Agile unit rate
Thu 11:00
31.87p/kWh on Agile Octopus October 2024 v1
The cleanest half hour and the cheapest one are not the same. The cleanest costs 33.86p/kWh; the cheapest runs at 205 gCO₂/kWh.
Lowest two-hour averages
This compares four consecutive half hours and gives each one equal weight. It is useful for a roughly two-hour continuous load, but it is not the exact cost or carbon impact of a washing-machine programme: cycle lengths and electricity use within a cycle vary.
Lowest forecast average intensity
Thu 09:30 – 11:30
202 gCO₂/kWh on average, 32.58p/kWh
Lowest average Agile unit rate
Thu 10:00 – 12:00
202 gCO₂/kWh on average, 32.38p/kWh
The first 48 forecast half hours
- FromgCO₂p/kWh
- 09:30199g33.86p
- 10:00200g32.34p
- 10:30202g32.26p
- 11:00205g31.87p
- 11:30201g33.06p
- 12:00207g32.86p
- 12:30212g33.46p
- 13:00216g35.35p
- 13:30221g35.36p
- 14:00224g48.23p
- 14:30228g47.88p
- 15:00236g47.57p
- 15:30240g48.35p
- 16:00240g57.48p
- 16:30238g60.64p
- 17:00236g59.95p
- 17:30232g60.46p
- 18:00230g58.58p
- 18:30231g57.05p
- 19:00231g46.66p
- 19:30232g44.92p
- 20:00233g43.76p
- 20:30233g41.12p
- 21:00237g40.46p
- 21:30239g39.60p
- 22:00236g36.77p
- 22:30236g35.49p
- 23:00239g—
- 23:30239g—
- 00:00238g—
- 00:30240g—
- 01:00242g—
- 01:30241g—
- 02:00239g—
- 02:30237g—
- 03:00230g—
- 03:30228g—
- 04:00226g—
- 04:30223g—
- 05:00224g—
- 05:30228g—
- 06:00238g—
- 06:30246g—
- 07:00250g—
- 07:30251g—
- 08:00250g—
- 08:30244g—
- 09:00238g—
The carbon forecast runs 95 half hours ahead, and the table above shows its first 48 published slots. 68 later slots fall beyond the Agile rates published so far and show a dash. The held price series runs to Thu 23:00; Octopus says the following day’s rates are normally published between 4pm and 10pm.
Cleanest half hour ahead, by region
| Region | Cleanest | gCO₂/kWh | Dirtiest | Lowest Agile rate |
|---|---|---|---|---|
| Eastern England | Thu 11:30 | 161 | 239 | 33.63p at 11:00 |
| East Midlands | Thu 10:00 | 205 | 280 | 31.87p at 11:00 |
| London | Thu 09:30 | 199 | 279 | 31.87p at 11:00 |
| Merseyside and North Wales | Sat 08:30 | 47 | 103 | 35.39p at 11:00 |
| West Midlands | Fri 04:30 | 174 | 226 | 33.63p at 11:00 |
| North East England | Thu 09:30 | 1 | 1 | 33.63p at 11:00 |
| North West England | Sat 05:00 | 44 | 69 | 33.63p at 11:00 |
| Southern England | Thu 11:30 | 230 | 296 | 33.63p at 11:00 |
| South East England | Thu 09:30 | 234 | 325 | 35.39p at 11:00 |
| South Wales | Thu 11:00 | 353 | 391 | 35.39p at 11:00 |
| South West England | Fri 11:00 | 226 | 391 | 37.15p at 11:00 |
| Yorkshire | Thu 09:30 | 98 | 158 | 31.87p at 11:00 |
| Southern Scotland | Thu 09:30 | 0 | 0 | 33.63p at 11:00 |
| Northern Scotland | Thu 09:30 | 0 | 0 | 38.91p at 11:00 |
Southern Scotland, Northern Scotland are published at 0 gCO₂/kWh in every half hour of this window. That is the publisher’s own forecast rather than missing data — a region with nothing published shows a dash in these columns instead.
These are VAT-inclusive unit rates for the current Agile Octopus product held by this site. They do not include the daily standing charge. On a flat tariff the unit rate does not change by half hour, so the price comparison does not apply. Contains Carbon Intensity API data from National Energy System Operator, licensed under CC BY 4.0. Tariff data from the Octopus Energy public API. Energy Review is not affiliated with, or endorsed by, Octopus Energy.
The carbon figures describe the forecast average mix in each region. They do not measure the marginal emissions caused or avoided by one appliance, so use them to compare lower-intensity times rather than to calculate an emissions saving.
Past cheap windows
Is the cheap window always at the same time?
This is where the cheapest three hours have actually fallen on past days, not a prediction for tomorrow. The forecast for the day ahead is above.
Nearly always overnight, but not at a fixed hour. The cheapest three hours began between 10pm and 5am on 89.5% of days, and in the middle of the day on 8.5% — usually a sunny one, when solar pushes the midday price down.
- 12am–3am22.9%
- 1am–4am6.9%
- 2am–5am6.2%
- 3am–6am8.4%
- 4am–7am4.6%
- 5am–8am0.4%
- 6am–9am0.1%
- 7am–10am0.1%
- 8am–11am0.1%
- 9am–12pm0.4%
- 10am–1pm2%
- 11am–2pm3.5%
- 12pm–3pm2.5%
- 1pm–4pm0.1%
- 2pm–5pm—
- 3pm–6pm—
- 4pm–7pm—
- 5pm–8pm—
- 6pm–9pm—
- 7pm–10pm—
- 8pm–11pm0.4%
- 9pm–12am1.2%
- 10pm–1am—
- 11pm–2am40.1%
Only tariffs whose price actually changes through the day are counted. On a flat tariff every half hour costs the same, so there is no cheapest window to find - and flat tariffs are the majority of what this site stores, so counting them would invent a pattern. Based on 156,170 tariff-days over the last 365 days, with 348,250 flat-tariff days left out.
Why the carbon figure is always a forecast
National carbon intensity gets checked. A forecast is published ahead of time, the generation is metered, and an outturn figure replaces it — which is why the live electricity page can show what actually happened rather than what was expected to.
The regional figures work differently, and the difference matters. No meter records the carbon crossing the boundary between one distribution region and the next. What is published instead is a model: national generation, apportioned by what is connected where, adjusted by estimated flows between regions. There is no later measurement to validate it against, so it is not later replaced by a measured regional outturn. Treat these numbers as a guide to when the modelled average mix is lower, not as a measurement of the emissions caused or avoided by a household load.
If you are on a flat rate, only half of this applies
The price column is Agile Octopus, a tariff whose unit rate changes every thirty minutes with the wholesale market. Most households are not on one. On a standard variable rate — the tariff the price cap applies to — a kilowatt hour costs the same at three in the morning as at six in the evening, so moving a wash saves nothing at all on the bill.
Moving use to a lower forecast-intensity half hour aligns it with a cleaner modelled regional mix, but this average-intensity feed cannot quantify the marginal emissions consequence. It is also worth knowing the gap between the lowest and highest Agile unit rates before switching: whether a household saves depends on its actual half-hourly consumption, standing charge and the flat tariff used for comparison.
Why one region is cleaner than another
Wind farms, nuclear stations and gas plants are not distributed evenly across the country, and electricity does not travel freely between regions. When Scottish wind is abundant, the transmission network sometimes cannot carry all of it south — the constraint that shows up as the largest single line in what balancing the grid costs. The result is a region that is very clean while another, at the same moment, is burning gas.
Agile rates share a national wholesale input but also include regional and time-dependent factors. That is why their pattern can resemble the carbon forecast without either series determining the other, and why the lowest values need not occur in the same half hour.
Frequently asked questions
Is the carbon figure a measurement of what my region emitted?
No, and it never becomes one. National carbon intensity is published as a forecast and then confirmed by an outturn once the generation is metered. The regional figures have no outturn at all — they are modelled from national generation plus estimated flows between regions, because nobody meters the carbon crossing a distribution boundary. Everything on this page is a forecast, including the parts about the past.
I am not on a half-hourly tariff. Is this page any use to me?
The carbon half is still a guide to when the forecast regional grid mix has lower average intensity. The price half is Agile Octopus, which changes every thirty minutes; on a flat tariff the unit rate does not change by time, so the displayed price comparison does not apply. The regional intensity figure is not a marginal-emissions model, so it cannot tell you the emissions caused or avoided by moving one appliance.
Why do the cheapest and cleanest half hours often differ?
They are set by different things. Agile unit rates use wholesale costs plus regional and time-dependent pricing factors. Regional carbon intensity is a model of the average generation mix and estimated flows for that region. The two series can move together, but they are not measuring the same thing and need not identify the same half hour.
One region shows 0 gCO₂/kWh. Is that data missing?
No. Zero is a published forecast; a dash means no forecast was available. Regional figures are modelled and are not later replaced by a measured outturn.
Why do prices stop partway through the chart?
Agile prices are calculated from a formula related to wholesale costs. Octopus says the following day’s rates are usually published between 4pm and 10pm. Until those rates arrive, later price cells are shown as dashes rather than filled in. A separate warning appears if a rate is missing inside a price window that has otherwise been published.
