What this page shows
The frequency of the Great Britain electricity grid, live. Britain runs at a nominal 50 Hz, and the actual figure moves constantly: it rises when generation exceeds demand and falls when demand exceeds generation. Frequency is therefore the grid’s heartbeat — the one number that tells you, in real time, whether supply and demand are in balance across England, Scotland and Wales.
The reading above is measured independently at a probe in London sampling the mains waveform and publishing once a second. A finer grained 10 Hz trace is retained for more granular analysis of frequency disturbance events. Timestamps will soon be GPS synchronised to permit phase angle differences to be determined for different locations around the country.
What the numbers mean
Not every wobble is news. Three bands are worth knowing.
49.985 – 50.015 Hz is the deadband. Inside it, frequency response services are not obliged to act; this is the grid simply breathing.
49.8 – 50.2 Hz is what NESO calls the normal operating range. The figure above turns amber when it leaves this band, which happens from time to time and is routine rather than alarming.
49.5 – 50.5 Hz are the statutory limits, set by the Electricity Supply Regulations. Excursions beyond them are rare and serious — the August 2019 blackout saw frequency reach 48.8 Hz. The figure turns red here.
Why it matters
Because electricity cannot be stored in the wires, generation must match demand instant by instant, and frequency is the error signal that says whether it does. When a large generator or interconnector trips, frequency falls within seconds, and batteries, pumped storage and part-loaded plant respond automatically to arrest it. Watching this number is watching that control system work. As Britain’s grid takes on more wind and solar and retires the spinning mass of coal and gas, holding frequency steady has become harder, more expensive, and a good deal more interesting.
Use the data — it’s free
Two endpoints. No key, no sign-up, CORS open so you can call them straight from a browser:
GET /api/freq — the latest reading.
GET /api/series — the last 10 minutes at 1 Hz.
Both return JSON as [[unix_seconds, hz], …], newest first. Use it
for dashboards, research, teaching, art, a lamp that changes colour — whatever you
like. A credit and a link back are appreciated but not required. Please keep polling to
once a second or slower.