GreenHour

The problem

Homes run big flexible loads — dishwashers, laundry, EV charging — at whatever time is convenient. But the carbon intensity of the grid swings 3–5× across a day: the same wash can emit far more CO2 at 6pm than at 2am. Almost nobody knows when their electricity is cleanest, so avoidable emissions happen every single day.

What GreenHour does

GreenHour is a one-page web app that reads the free, government-published National Grid ESO Carbon Intensity API and tells a UK household the greenest hour in the next 48 hours to run a flexible load — then keeps a verifiable ledger of the grams of CO2 avoided. Every number on screen links back to the exact government data point it came from, so a judge can re-derive every claim.

Why it's honest (and defensible)

• Savings are measured against a named, editable baseline — your usual run time — not an inflated 48h average.
• An always-visible line discloses that the API reports average, not marginal grid intensity, so the figure is framed as an estimate, not an over-claim.
• On an API outage it reuses the last-good real data behind a "stale data" banner and never fabricates a greenest hour.

Impact

Shifting a single load is small, but the ledger scales it honestly ("at this pace ≈ X kg CO2/year"), and the method ports to any grid that publishes half-hourly intensity — a measurable, replicable path to real reductions.

How we built it

Vanilla HTML/CSS/JS, zero dependencies, works offline. The 48-hour chart is hand-drawn inline SVG; the ledger lives in localStorage. The pure calculation core is covered by a dependency-free test suite (node greenhour.test.js, 19 checks) — including a test asserting the honest baseline claims less than an inflated one.

Built with amini-code's multi-agent build-and-judge loop; an adversarial judge panel caught and fixed an inflated-baseline honesty bug before submission.

Built With

• carbon-intensity-api
• css
• html
• javascript
• national-grid-eso
• svg