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US power markets price the same hour of electricity twice: once in a day-ahead auction, and again in real time as it's delivered. The difference between the two prices is called the DART spread, and it's one of the most watched numbers in power trading.
Grid operators run a day-ahead market every afternoon: an auction that sets an hourly price for each hour of tomorrow. Most electricity is bought and sold here, because everyone wants to plan. Then, on the actual day, reality drifts from the plan. Demand comes in higher, a plant trips offline, clouds roll over a solar farm. So the operator re-runs the auction every five minutes and sets a real-time price that reflects what's actually happening.
DART = real-time minus day-ahead. It's the difference between what the market expected and what happened.
Real-time above day-ahead means something tightened: a heatwave, an outage, a wind forecast that didn't show up. Real-time below day-ahead means oversupply, often a sunnier or windier day than predicted. A spread near zero means the forecast was good.
Because you can act on it. A battery that expects real-time prices to spike above the day-ahead forecast holds its charge and sells into the spike. Traders go further: markets allow purely financial bets, called virtual bids, that pay off when day-ahead and real-time diverge in the direction you predicted. Whole desks do nothing else.
The DART spread also works as a scorecard of a grid's predictability. Texas runs famously wild spreads. California's midday spreads lean negative when solar overdelivers. A node's DART history is one of the most direct measures of local grid stress available.
One afternoon reading from California's SP15 hub: day-ahead cleared at $20.12, real-time printed $13.69. DART spread: −$6.43. Translation: the day-ahead market over-forecast the price, likely extra solar. A battery charging in that hour paid a third less than the plan said it would.
Our API serves the DART spread per node as a first-class endpoint, matched hour by hour, and the live map shows the real-time side as it prints.
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