The cleanest time to use electricity changes with the seasons

"Use electricity at night, when demand is low." It's common advice, and in winter it works. But we looked at a full year of carbon intensity data for Great Britain, from October 2025 to September 2026, and found that the cleanest time of day depends on the season. In summer, the night is actually one of the dirtiest times to switch things on.

What we found

The cleanest hour moves with the sun.

In spring and summer, solar panels make midday the cleanest time of day, cleaner than any winter hour. In autumn and winter, there is too little sunshine for that, so the small hours of the night stay cleanest.

The dirtiest hour is always in the evening, once solar output fades and gas power stations take over. It moves later as the evenings get lighter, from 17:00 in winter to 20:00 in summer.

The surprise: summer nights

Summer nights were the dirtiest nights of the year. Between midnight and 4am, carbon intensity averaged about 140–146 gCO₂/kWh in summer, against about 100–105 in winter. That's despite demand being low.

Possible reasons include lower wind in summer, so once the sun sets, gas has to cover more of the night. So the "use it at night" rule works in winter, but in summer it points you to some of the dirtiest hours of the day.

Summer also has the biggest gap between its cleanest and dirtiest hour: 85 gCO₂/kWh. That makes it the season when shifting your usage saves the most. Moving a 20 kWh electric car charge from 20:00 to 13:00 on an average summer day saves about 1.7 kg of CO₂, roughly what a mature tree absorbs in a month.

What you can do

• Spring and summer: run the dishwasher, washing machine or EV charger around midday, using a timer if you're out. Avoid overnight.
• Autumn and winter: overnight is still the cleanest time, especially between 1am and 4am.
• All year: avoid the early evening, from about 17:00 in winter to 20:00 in summer.
• Check before you switch on: these are averages, and a cloudy, still day can be much dirtier. The Energy Trace dashboard shows carbon intensity live.

How we did it

We used national half-hourly carbon intensity for Great Britain from the Carbon Intensity API, from 1 October 2025 to 30 September 2026, and averaged it by hour of day (UK time) for each meteorological season. The figures cover one year and the whole country; carbon intensity also varies by region, as we showed in Electricity isn't equally clean everywhere in the UK. The tree comparison uses the Arbor carbon equivalent calculator.