How Electricity Is Actually Traded Day Ahead

Electricity is bought and sold before it is produced, in a process that runs every day on a fixed clock. The two-stage structure explains why prices appear in two forms and why they sometimes diverge sharply.

The day-ahead auction schedules the system

Generators submit offers describing what they will produce at what price, and buyers submit expected consumption. The operator solves for the least-cost combination that meets demand in every hour while respecting transmission limits.

The result is a schedule and a price for each hour of the following day, and participants are financially committed to it.

This gives plants that need hours to start the notice they require, which is a physical necessity rather than a market convenience.

Real time settles the difference

Actual demand never matches the forecast, and plants trip offline. A second market operates in short intervals through the delivery day to balance the difference.

Participants that deviate from their day-ahead position buy or sell the shortfall at real-time prices, which can be far higher or lower than the day-ahead price.

Real-time prices are therefore volatile by design, while day-ahead prices are smoother because they reflect expectations rather than events.

Prices differ by location

Because transmission has limited capacity, the cheapest generator cannot always serve a given area. The price at each point reflects the cost of serving that specific location.

When a line binds, prices on either side separate, sometimes dramatically, and that separation is the signal that transmission is scarce there.

Negative prices appear where generation cannot easily be turned down and there is nowhere for the power to go, which is increasingly common in areas with heavy renewable output.

Offers describe physics as well as price

A generator's offer includes the cost of starting up, minimum output once running, and how quickly it can change output. The optimization must respect all of it.

This is why a plant is sometimes scheduled to run at a loss in one hour: committing it is cheaper overall than starting a different unit later that evening.

Operators make additional payments to cover those situations, and these uplift charges are a recurring subject of market redesign.

Why the two-market structure persists

Scheduling ahead lets slow physical assets be committed sensibly; settling in real time forces participants to face the consequences of being wrong.

Storage and flexible demand sit naturally in the gap between them, earning from the spread, which is why market rules governing that boundary now attract so much commercial attention.