Ancillary Services Market Structure

The energy market pays generators for megawatt-hours delivered. The ancillary services market pays them for capabilities held in reserve. These are separate markets, clearing at separate prices, and the interaction between them determines whether the grid can actually use the energy it buys.

RTOs run several ancillary services markets, each corresponding to a different timescale of grid response. Regulation reserves (seconds to minutes) handle continuous small imbalances. Spinning reserves (minutes) cover sudden generator trips. Non-spinning reserves (10-30 minutes) provide slower backup. Each product clears at its own price, set by the marginal cost of the resource providing it. A battery might earn $15/MW-hour in the regulation market while a gas turbine earns $5/MW-hour in spinning reserves.

PJM's regulation market, 2023. PJM pays regulation providers based on both capacity (being available) and performance (responding accurately). A battery that follows the regulation signal with 95% accuracy earns roughly 2x the payment of a gas unit at 70% accuracy, because PJM's "mileage" multiplier rewards precision.

Why not just procure all ancillary services from the cheapest source?

Because different services require different physical capabilities. Regulation needs fast, precise ramping. Spinning reserves need sustained output for 30+ minutes. A battery excels at regulation but may lack the energy duration for spinning reserves. A gas turbine is mediocre at regulation but can sustain output for hours. The market structure matches capabilities to needs through separate clearing prices.

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