How the UK electricity grid balances supply and demand in real time
The National Grid operates at exactly 50Hz. Not approximately. Not most of the time. Exactly. Every second, generation must match consumption. When the two fall out of balance, frequency drifts, and the system faces serious consequences.
This balancing act happens continuously. At 7am, millions of kettles switch on simultaneously. At 10pm, a popular television programme ends and millions of people head to bed. Factories start shifts. Industrial processes ramp up. Cloud passes over solar farms. Wind speed drops over Scotland. Each event shifts the supply-demand equation in real time.
To meet this challenge, National Grid dispatches generation assets in sequence: cheapest first, most expensive last. Baseload nuclear runs continuously. Gas peakers fire up only when needed. Interconnectors pull power from France or Norway when domestic supply is tight. The price at any moment reflects the cost of the marginal unit of electricity required to balance the system.
That marginal cost changes constantly. On a still winter evening with high demand and low wind output, it is expensive. On a breezy Sunday morning in spring with plentiful solar and subdued demand, it can fall to near zero. The 30-minute settlement period is the mechanism that translates this physical reality into a price signal.
Why half-hourly pricing reflects reality better than flat rates
Standard variable tariffs charge you the same rate at 3am as they do at 6pm. This is convenient, but it is a fiction. The electricity flowing through your meter at 3am cost almost nothing to generate. The electricity at 6pm on a cold February evening cost a great deal.
With a flat tariff, the supplier averages these costs out and adds a margin. You pay a blended rate that neither rewards you for consuming at cheap times nor signals to you that consuming at expensive times is genuinely costly for the system.
Half-hourly pricing removes that fiction. When generation is abundant and cheap, the price falls. When generation is scarce and expensive, the price rises. This is not a tariff design choice. It is an accurate reflection of what electricity actually costs to produce in that 30-minute window.
Octopus derives Agile prices from the EPEX Spot wholesale market, where electricity is traded in half-hourly blocks. The prices published each afternoon for the following day are the real market clearing prices for each settlement period, passed through to customers with a modest markup. Check the AgileAlert live dashboard to see exactly what each half-hour slot costs today.
The times when prices move most dramatically (and why)
Two windows define the daily price landscape on Agile. Understanding them changes how you think about electricity.
The evening peak: 4pm to 8pm. Demand surges as people return home, cook dinner, run appliances, and turn on heating. Renewable generation often falls as calm weather settles in the evening. Gas peakers fire up to fill the gap. Prices regularly reach 25 to 50p per kWh during this window. On cold, still evenings in winter, they can approach the 100p cap. This is the most expensive electricity of the day by a wide margin.
The overnight trough: 11pm to 6am. Demand drops sharply as households and most businesses go dark. Wind turbines continue generating regardless of the hour. Solar has ceased, but wind often strengthens overnight. The gap between supply and demand widens. Prices compress to 2 to 8p per kWh. On high-wind nights, they fall further still, sometimes reaching zero or going negative.
Between these extremes, the middle of the day offers moderate prices, typically 8 to 16p, shaped by solar generation and commercial demand patterns. The weekend profile differs from weekdays. Industrial shutdowns reduce demand on Saturday and Sunday mornings, pushing prices lower than on equivalent weekday slots.
These patterns are predictable at a broad level, but the specific prices within them are not. A weather forecast revision overnight can shift tomorrow's prices significantly. This is why prices are only confirmed for the following day at around 4pm each afternoon.
How this creates the savings opportunity
The average Agile customer saves approximately £440 per year compared to the Ofgem price cap rate, according to Octopus Energy. That figure comes entirely from one source: shifting consumption away from expensive periods and into cheap ones.
You do not need to use less electricity. You use the same electricity at different times. A dishwasher on a timer set for 2am instead of 7pm costs the same water and the same energy but a fraction of the price. An EV charged overnight at 3p per kWh instead of during the evening peak at 35p uses the same electrons but costs ten times less.
This is load shifting, and the half-hourly price structure is what makes it possible and measurable. Standard tariff customers have no financial incentive to run their dishwasher at 2am versus 7pm. Agile customers have a direct, quantifiable reward for doing so. The live price dashboard makes it easy to identify exactly when that reward is largest.
Households with EVs amplify these savings significantly. A typical EV with a 60kWh battery, charged entirely overnight at 4p average, costs £2.40 per full charge. The same charge at peak Agile rates of 35p would cost £21. The difference is £18.60 per charge cycle. For a household charging three times per week, that is over £2,900 per year in avoided costs.
What smart meter technology made possible
Half-hourly pricing for households was technically impossible before smart meters. Traditional accumulation meters record total consumption but provide no information about when that consumption occurred. Your supplier received one reading per quarter and had no way to distinguish cheap-time units from expensive-time units.
Smart meters transmit half-hourly consumption data automatically. Your supplier can now see exactly how many kilowatt hours you used in every 30-minute window and apply the correct price to each. The meter communicates through the Smart Metering System, a national infrastructure that underpins all time-of-use tariffs in the UK.
As of Q4 2025, 71% of UK households have smart meters installed, according to the Department for Energy Security and Net Zero. This is the infrastructure base that makes tariffs like Agile viable at scale. Without it, the price signal would exist in the wholesale market but have no way to reach individual households.
The remaining 29% without smart meters cannot access Agile. This is the single technical prerequisite for switching. If you have a smart meter, the mechanism that reads your half-hourly consumption is already in place. The price signal is already reaching your home. Whether you benefit from it depends on which tariff you choose.
Time-of-use tariffs currently reach only 9% of UK households, according to Nesta research from 2025. That gap between smart meter penetration and TOU adoption represents millions of households paying flat rates for electricity that the grid prices dynamically. The technology is installed. The savings are available. The awareness is the missing link.