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. 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
Wed 13:00
65 gCO₂/kWh
Cheapest half hour
Wed 11:00
16.46p/kWh on Agile Octopus
The cleanest half hour and the cheapest one are not the same. The cleanest costs 16.65p/kWh; the cheapest runs at 75 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
Wed 11:30 – 13:30
69 gCO₂/kWh on average, 16.89p/kWh
Lowest cost
Wed 12:30 – 14:30
70 gCO₂/kWh on average, 16.78p/kWh
The next 24 hours, half hour by half hour
- FromgCO₂p/kWh
- 19:00150g31.03p
- 19:30159g31.78p
- 20:00161g30.49p
- 20:30158g30.60p
- 21:00153g28.88p
- 21:30152g28.04p
- 22:00152g26.40p
- 22:30146g24.55p
- 23:00140g25.22p
- 23:30140g23.20p
- 00:00135g22.69p
- 00:30137g21.52p
- 01:00132g21.32p
- 01:30130g21.65p
- 02:00131g21.43p
- 02:30117g20.95p
- 03:00112g21.70p
- 03:30112g21.40p
- 04:00107g22.13p
- 04:30103g21.58p
- 05:0085g23.40p
- 05:3089g23.96p
- 06:0096g24.10p
- 06:3092g26.52p
- 07:00100g23.58p
- 07:3097g25.50p
- 08:0094g21.47p
- 08:3089g21.01p
- 09:0091g21.01p
- 09:3079g21.50p
- 10:0078g19.20p
- 10:3076g17.35p
- 11:0075g16.46p
- 11:3069g16.46p
- 12:0070g17.32p
- 12:3070g17.14p
- 13:0065g16.65p
- 13:3072g16.52p
- 14:0074g16.83p
- 14:3075g18.25p
- 15:0081g19.59p
- 15:3088g20.76p
- 16:0094g37.43p
- 16:30108g35.51p
- 17:00124g41.95p
- 17:30137g45.15p
- 18:00150g54.80p
- 18:30169g57.67p
The carbon forecast runs 96 half hours ahead, about two days. 40 of them have no published price yet and show a dash — half-hourly prices are published only to Wed 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 | Wed 13:00 | 22 | 164 | 17.46p at 11:00 |
| East Midlands | Wed 13:00 | 61 | 215 | 16.46p at 11:00 |
| London | Wed 13:00 | 65 | 201 | 16.46p at 11:00 |
| Merseyside and North Wales | Wed 08:00 | 31 | 224 | 18.46p at 11:00 |
| West Midlands | Wed 08:00 | 28 | 198 | 17.46p at 11:00 |
| North East England | Tue 20:30 | 12 | 17 | 17.46p at 11:00 |
| North West England | Wed 08:00 | 27 | 64 | 17.46p at 11:00 |
| Southern England | Wed 11:30 | 62 | 239 | 17.46p at 11:00 |
| South East England | Wed 13:00 | 103 | 221 | 18.46p at 11:00 |
| South Wales | Wed 15:00 | 41 | 384 | 18.46p at 11:00 |
| South West England | Thu 11:30 | 55 | 377 | 19.46p at 11:00 |
| Yorkshire | Wed 04:30 | 45 | 132 | 16.46p at 11:00 |
| Southern Scotland | Tue 19:00 | 1 | 2 | 17.46p at 11:00 |
| Northern Scotland | Tue 19:00 | 0 | 0 | 20.45p at 11:00 |
Northern Scotland ispublished 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.
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.
When households actually use it, measured
Everything above this point is about what the grid is doing. This is about what households are doing. SP Energy Networks and SSEN each release an aggregated half-hourly profile from their low-voltage networks, but their coverage and time conventions differ, so the two panels below stay separate rather than becoming one average.
Each is a limited window, and neither is a forecast or a model. They are what the meters recorded, within the scope stated on that panel.
SP Energy Networks feeder profile
Ayrshire and Clyde South and The Wirral only. Two parts of two distribution licence areas — Ayrshire and Clyde South inside SP Distribution, and the Wirral inside SP Manweb. Neither is a licence area in full. This is not Scotland, not England and not Great Britain, and no other network operator publishes an equivalent open measurement. Anywhere else is not covered, which is not the same as having no data. September 2025, published in January and February 2026 and not updated since. This is a measured month in the past, not a live feed.
Ayrshire and Clyde South
Part of SP Distribution. About 18,004 metered households across 1,001 street circuits, averaged over 23 days.
2.51× as much electricity in the busiest half hour of a weekday as in the quietest
Watt-hours per metered household per half hour. Weekday use peaks at 17:30 and is lowest at 05:00, as published — see the note on the clock below.
The Wirral
Part of SP Manweb. About 65,030 metered households across 1,774 street circuits, averaged over 28 days.
2.45× as much electricity in the busiest half hour of a weekday as in the quietest
Watt-hours per metered household per half hour. Weekday use peaks at 17:30 and is lowest at 05:00, as published — see the note on the clock below.
How many homes have a smart meter at all
The whole of SP Distribution and the whole of SP Manweb — a much wider area than the consumption sample above. Context for it, never a denominator for it.
- SP Distribution
- 60.6%1,187,350 active smart meters across 1,958,114 properties; 59% of them second-generation
- SP Manweb
- 63.7%942,682 active smart meters across 1,479,145 properties; 63% of them second-generation
About the clock. SP Energy Networks declares no timezone for these datasets and September is British Summer Time, so the clock times here are either UTC or local time published under a UTC offset — an hour apart, and the daily shape fits both. The ratio between the evening peak and the overnight minimum is the same figure under either reading, which is why it leads.
About the denominator. The denominator is smart meters, not homes. A feeder carries households without a smart meter whose use is not in these figures at all, so this is what the metered subset used and not what the street used. Feeders with fewer than this many reporting meters are excluded before anything is aggregated. SP Energy Networks suppresses below five; this is deliberately stricter, because a five-household group on a row that also carries a substation coordinate is not an anonymising crowd. It shifts the mean by about 3% in Ayrshire and Clyde South and under 1% in the Wirral, and the excluded feeders are disproportionately rural.
Contains data from SP Energy Networks, licensed under CC BY 4.0
SSEN measured feeder profile
Southern Electric Power Distribution and Scottish Hydro Electric Power Distribution only. SSEN's two distribution licence areas combined — southern England and north Scotland. Two of the fourteen GB licence areas. This is not Great Britain, not Scotland and not England, and the two areas cannot be shown apart: the published usage file carries no geography beyond the operator name, and the only thing that would separate them is a feeder-to-postcode lookup this programme deliberately never reads. A rolling window of recent days, refreshed daily. The publisher releases each day about three days later.
SSEN’s two licence areas combined
Southern Electric Power Distribution and Scottish Hydro Electric Power Distribution. About 2,063,796 metered households across 74,500 street circuits, averaged over 14 days.
1.85× as much electricity in the busiest half hour of a weekday as in the quietest
Watt-hours per metered household per half hour. Weekday use peaks at 18:30 and is lowest at 06:00, on this feed’s measured UK wall-clock basis.
About the clock. SSEN declares no timezone and publishes each stamp with a Z suffix, but the values are UK wall clock rather than UTC — settled on the two clock-change days, where a UTC series and a local one give different numbers of distinct labels and the data gives the local answer both times. The times here are therefore the household's own clock and need no conversion. The clock-change days themselves are excluded, because one label there means two different half hours.
About the denominator. The denominator is smart meters, not homes. A feeder carries households without a smart meter whose use is not in these figures at all, and it carries non-domestic connections too, so this is what the metered subset used and not what the street used. Feeders with fewer than this many reporting meters are excluded before anything is aggregated. SSEN suppresses below five and drops those rows entirely; this is deliberately stricter, because SSEN also publishes a feeder-to-postcode lookup that makes a five-household group locatable. It moves the mean by about -1.8% and excludes 21% of feeders but only 6.8% of metered households.
Contains data from Scottish and Southern Electricity Networks, licensed under CC BY 4.0
SPEN’s two published sub-areas show the shape particularly clearly: household electricity use roughly doubles and a half between the small hours and the evening peak, and both areas peak in the same half hour despite being three hundred kilometres and two licence areas apart. That is the demand the grid has to be built for, and it is why the difference between a clean half hour and a dirty one exists at all.
It is also the argument for shifting a wash even on a flat rate. The evening peak is when the most expensive and least clean plant is running, and a household moving a load out of it is moving it out of the half hour that costs the system most — which is the same thing the cost of balancing the grid measures from the other end.
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.
One region shows 0 gCO₂/kWh. Is that data missing?
No — it is the figure the publisher issued, and it arrives on the same feed at the same time as every other region. Where we hold no forecast at all the figure is a dash rather than a zero, and the table names any region published at zero right through the window so the two cannot be mistaken for each other. It is still a modelled regional figure with no outturn, so nothing later confirms or corrects a zero either.
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.
Related reading
Written by
Rob Gibbs
Hi, I'm Rob, and I run Energy-Review.co.uk. I initially started this project in 2018 when I was looking to switch energy suppliers and found there wasn't a website that provided simple, data-backed reviews on all the suppliers available. Since then, I have spent a lot of time (too much, some may say!) looking at all publicly available data about each supplier and writing reviews using this information. These reviews are updated as regularly as possible, and any data is backed up by a source where necessary. I have also started writing guides on various energy-related topics, which hopefully you will find useful. If you find any issues, please use our contact form to let us know.