Find Your Greenest Hour
Enter your UK postcode to see the lowest-carbon half-hour to use electricity over the next 24 hours.
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How the Greenest Hour Calculator Works
Britain's electricity gets cleaner and dirtier through the day. When it's windy or sunny, more power comes from wind and solar and the grid is "greener"; when demand is high and the wind drops, gas fills the gap and it's "dirtier". This swings the carbon intensity of every unit of electricity — measured in grams of CO₂ per kilowatt-hour (gCO₂/kWh).
This calculator does three things in your browser:
- Looks up your electricity region from your postcode.
- Pulls the National Grid's live carbon-intensity forecast for that region over the next 24 hours.
- Finds the half-hour window with the lowest forecast carbon — your greenest hour.
Data sources: carbon figures from the National Grid Carbon Intensity API; postcode-to-region lookup from Postcodes.io (Open Government Licence). Figures are forecasts and update through the day, so check again closer to the time. This tool is for general guidance only.
Why Timing Your Electricity Use Matters
Running flexible appliances during the greenest hour means the electricity you use is generated with less gas and more wind, solar, hydro and nuclear. That lowers the carbon footprint of everyday chores — without changing what you do, just when you do it.
Will it save me money?
Only on a time-of-use tariff. If you're on a tariff where the price changes by time of day — such as Octopus Agile or an Economy 7 night rate — cheaper periods often line up with greener ones, so timing can cut both carbon and cost. On a standard flat-rate tariff, the price is the same whatever hour you run an appliance, so you'd save carbon but not money. We mention this plainly because it's an easy thing to get wrong.
What's worth shifting
Stick to flexible loads you can delay: the dishwasher, washing machine, tumble dryer, EV charging and an immersion heater. Things you need on demand — the kettle, oven, lights — aren't worth shifting.
Understanding the Carbon Intensity Index
Alongside the number, the National Grid rates each half-hour with an index: very low, low, moderate, high or very high. These bands are set by the Carbon Intensity API and shift with the seasons, so a "low" reading in summer may differ from one in winter. We show the live index next to the gCO₂/kWh figure so you can see at a glance how clean your greenest window really is.
| Index | What it means |
|---|---|
| Very low / Low | Lots of wind, solar and nuclear on the grid — the best time to run flexible appliances. |
| Moderate | A mixed grid — greener than the daily peak, but not the cleanest window available. |
| High / Very high | Gas is doing much of the work, often at peak demand — worth delaying flexible loads if you can. |
Note: exact gCO₂/kWh thresholds for each band are defined and updated by the National Grid Carbon Intensity API, not by HomeCalc.