Energy data

When to Use Electricity

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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. A kilowatt hour used in the north of Scotland on a windy night carries a fraction of the carbon of the same kilowatt hour used in the south east while a gas plant is setting the price.

This page puts the two halves of that question together: what the electricity in your region is forecast to emit in each half hour ahead, and what a household on a half-hourly tariff would pay for it. Pick your region below — the postcode you might use to find it is never stored, and nothing here needs one.

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

Sat 14:00

68 gCO₂/kWh

Cheapest half hour

Fri 15:30

12.17p/kWh on Agile Octopus

The cleanest half hour and the cheapest one are not the same. The cleanest is far enough ahead that its price is not published yet, and the cheapest runs at 76 gCO₂/kWh.

The best two hours to run a wash

A cotton cycle runs about two hours, so this is the best window, averaged across it — not the best single half hour with a spike either side of it.

Lowest carbon

Sat 13:00 – 15:00

71 gCO₂/kWh on average

Lowest cost

Fri 14:00 – 16:00

72 gCO₂/kWh on average, 12.83p/kWh

The next 24 hours, half hour by half hour

FromgCO₂p/kWh
17:30130g34.92p
18:00137g35.27p
18:30147g37.23p
19:00149g28.60p
19:30158g28.38p
20:00168g28.41p
20:30179g28.62p
21:00182g28.02p
21:30188g28.15p
22:00194g27.44p
22:30181g26.31p
23:00182g23.79p
23:30178g23.79p
00:00172g22.74p
00:30175g22.18p
01:00178g20.71p
01:30175g20.16p
02:00172g19.80p
02:30171g19.38p
03:00187g19.80p
03:30185g19.38p
04:00173g20.64p
04:30178g20.22p
05:00181g22.89p
05:30177g22.76p
06:00174g23.02p
06:30165g24.30p
07:00153g21.69p
07:30144g23.03p
08:00140g19.59p
08:30130g17.92p
09:00106g17.49p
09:30101g13.79p
10:00103g14.84p
10:30104g13.71p
11:0096g13.76p
11:3083g12.95p
12:0086g13.14p
12:3086g13.36p
13:0085g13.29p
13:3082g12.55p
14:0069g13.29p
14:3072g13.19p
15:0071g12.65p
15:3076g12.17p
16:0084g28.59p
16:3093g29.36p
17:00107g33.76p

The carbon forecast runs 94 half hours ahead, about two days. 35 of them have no published price yet and show a dash — half-hourly prices are published only to Fri 23:00, because the following day’s rates are set in the late-afternoon day-ahead market. The table above shows the first 24 hours.

Cleanest half hour ahead, by region

Region	Cleanest	gCO₂/kWh	Dirtiest	Cheapest
Eastern England	Sat 14:00	54	225	12.96p at 15:30
East Midlands	Sat 12:00	27	263	12.17p at 15:30
London	Sat 14:00	68	194	12.17p at 15:30
Merseyside and North Wales	Fri 12:00	5	142	13.73p at 15:30
West Midlands	Fri 12:00	5	177	12.96p at 15:30
North East England	Sat 11:30	8	20	12.96p at 15:30
North West England	Fri 14:30	3	48	12.96p at 15:30
Southern England	Fri 12:00	54	211	12.96p at 15:30
South East England	Thu 17:30	64	217	13.73p at 15:30
South Wales	Fri 13:30	19	389	13.73p at 15:30
South West England	Fri 11:30	41	389	14.52p at 15:30
Yorkshire	Sat 09:30	22	169	12.17p at 15:30
Southern Scotland	Sat 03:00	0	3	12.96p at 15:30
Northern Scotland	Thu 17:30	0	0	15.31p at 15:30

Households on Agile Octopus, a half-hourly tariff. On a flat standard variable rate the price is the same in every half hour, so only the carbon figure varies. 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.

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 correct it against, so a regional figure that was wrong stays wrong and nobody finds out. Treat these numbers as a good guide to when, and a poor guide to exactly how much.

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.

It still moves the emissions, which is the honest reason to do it. And it is worth knowing the gap between the cheapest and dearest half hour in the table below before switching to a tariff that exposes you to it: a half-hourly rate is a genuine saving for a household that can shift load, and a genuine risk for one that cannot.

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.

Price does not vary that way. Wholesale electricity is traded for Great Britain as a whole, so a windy night is cheap in Cornwall and in Caithness alike. That is the structural reason the cleanest half hour and the cheapest one are so often near each other without being the same.

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. The price half is Agile Octopus, which changes every thirty minutes; on a standard variable rate you pay the same at 3am as at 6pm, so shifting a wash saves you nothing on the bill. It still shifts the emissions, and it still moves demand off the peak that the grid pays most to serve.

Why do the cheapest and cleanest half hours often differ?

They are set by different things. Price follows the wholesale market for the whole of Great Britain, so a windy night is cheap everywhere at once. Carbon follows what is generating near you and what is flowing in from neighbouring regions, which is why Scotland can be very clean at a moment when the price is set by a gas plant hundreds of miles south.

Why do prices stop partway through the chart?

Half-hourly prices are set from the day-ahead wholesale market, and the following day’s rates are published in the late afternoon. Until then the price column has nothing to say beyond 23:00 tonight, which is shown as a dash rather than filled in. The carbon forecast runs about two days ahead throughout.

Does running appliances overnight really help?

Usually, but not always, and that is the point of showing the figures rather than a rule. Overnight is often clean because wind keeps generating while demand falls away. A still, cold night is the opposite: demand stays high, gas fills the gap, and the small hours can be dirtier than the middle of a sunny afternoon.