Show the UK grid’s carbon intensity in your Mac menu bar

When the wind blows the grid gets cleaner; on a still evening it leans on gas. The National Grid’s carbon intensity API gives the number for right now, nationally or for your region.

Recipe tested October 29, 2026 with Multimeter 1.1. Source: Web Address.

Preview: the Plant face and two alternatives, with sample values like the ones this recipe returns.

What you need

  • Multimeter for macOS 13 or later, $19.99 once.
  • Nothing. For your region, the first part of your postcode.

Set it up

  1. Add a Web Address meter with this address and the key path data.0.intensity.forecast:
    Web address
    https://api.carbonintensity.org.uk/intensity
  2. Refresh every 15 minutes. Range 0 to 300, unit g, alerts on High values at 150 and 250.

Multimeter settings

In Customize This Meter… (from the meter’s menu):

Settings for this meter
SettingValue
SourceWeb Address
Web addresshttps://api.carbonintensity.org.uk/intensity
Key pathdata.0.intensity.forecast
Refresh every15 minutes
Range0 to 300
Unitg
AlertsHigh values: warning 150, critical 250
LabelCO2
FacePlant, Gradient Track or Traffic Light

Tested: Run on October 29, 2026: national 210 g; postcode RG10 305 g.

More on this source, including every format it reads, in the Web Address guide.

Limits

  • Values are for half-hour periods. actual is filled in after the period ends, so forecast is the one that always has a number.

Variations

Your region

Address https://api.carbonintensity.org.uk/regional/postcode/RG10 (your outward code), key path data.0.data.0.intensity.forecast.

Share of wind, solar or gas

Address https://api.carbonintensity.org.uk/generation; the key path data.generationmix.N.perc picks a fuel by its position in the list (check the order with curl).

Troubleshooting

What the meter says, why, and what to do
SymptomCauseFix
No number found in the responseThe key path doesn’t match the reply.Run curl -s with the address and read the JSON. Dots walk into objects, numbers pick array items (items.0.count).
Couldn’t reach the addressNo network, a typo in the host, or the service is down.Open the address in your browser. Multimeter tries again at the next refresh.
The server answered 400The postcode wasn’t recognized.Use only the outward part, like RG10 or SW1A.

Questions

Is there something like this outside the UK?

Electricity Maps covers most of the world with an API key in a header, which fits a Script meter.

Related

Put it in your menu bar

$19.99 one-time. No subscription. Needs macOS 13 Ventura or later.

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