Ancillary services
The standby products — reserves and fast response — that grid operators buy to keep the system stable when something goes wrong.
Keeping the lights on takes more than matching supply and demand on paper. The grid needs resources standing by to react in seconds or minutes when a power plant trips offline, wind dies down faster than forecast, or demand jumps. Ancillary services are the market products through which the grid operator buys that readiness — paying resources to hold capacity in reserve rather than sell it as energy.
ERCOT buys several distinct products. Regulation Up and Regulation Down (RegUp / RegDown) are the fine-tuning: resources that automatically nudge output up or down every few seconds to hold system frequency steady. Responsive Reserve Service (RRS) is the emergency brake — capacity that responds within seconds to minutes when frequency drops after a big plant failure. ERCOT Contingency Reserve Service (ECRS) restores the system's cushion in the minutes after an event, and Non-Spinning Reserve (Non-Spin) is slower backup that can be brought online within about half an hour.
For batteries, ancillary services have historically been the most attractive first market: a battery can promise fast response with almost no fuel cost, earning capacity payments simply for being available. As more batteries have entered, these small markets have become crowded, pushing storage operators increasingly toward energy arbitrage.
For the technical reader
Each service is defined by response speed, duration, and qualification requirements, and is procured as capacity (MW held back from energy dispatch) with clearing prices set by co-optimization against energy in the day-ahead market. Holding a resource out of energy to provide reserves has an opportunity cost, and co-optimization internalizes it: in equilibrium, an ancillary clearing price reflects the foregone energy margin of the marginal provider.
ERCOT's stack, roughly by response time: RegUp/RegDown (seconds, AGC-following), RRS (frequency-responsive, including fast-frequency-response variants well suited to inverter-based storage), ECRS (10-minute deployment), and Non-Spin (30-minute, can be offline). Under the current design, day-ahead awards create an obligation across the operating day; RTC+B will move ancillary procurement into real-time co-optimization every five minutes.