How it works
The grid must match supply and demand second by second, or its frequency drifts from the nominal 50 or 60 Hz. Ancillary services are how the operator keeps it there. Frequency response reacts within seconds, often automatically through the plant's own controller. Reserves follow in stages over minutes, replacing the fast response and restoring frequency. Other services include voltage support through reactive power, and black start, the ability to restart the grid after a blackout.
System operators buy these services separately from energy, through auctions or contracts. A provider is paid for holding capacity ready, in $/MW per hour, and sometimes also for energy delivered when it is activated. It must prove it can respond at the required speed, pass tests and keep headroom spare while committed. Batteries suit the fast services, because they respond in milliseconds and can move both up and down. As more batteries have entered, prices for these services have fallen in several markets.
Why it matters
For a battery developer, ancillary services can be a large share of early revenue, and the outlook for them drives the financing case. These markets are small in MW, so they saturate as batteries arrive. A model that holds today's prices flat for fifteen years will overstate value. IPPs with flexible plant decide each period whether to hold capacity for reserves or sell it as energy. Buyers pay for ancillary services indirectly, through grid charges.
A worked example
A 50 MW battery is contracted for a frequency service at $12/MW per hour, for 20 hours a day. Holding that capacity earns 50 × $12 × 20 = $12,000 a day, or $12,000 × 365 = $4,380,000 a year. If the price halves to $6/MW per hour as more batteries compete, the same commitment earns $2,190,000. The four uncommitted hours each day are left for energy trading.
Illustrative figures.
Across markets
US grid operators co-optimise reserves with energy in a single clearing and price both together. Europe procures balancing reserves country by country under common rules, with cross-border platforms for some products, and Great Britain runs its own fast frequency products. India introduced market-based ancillary services more recently, with secondary reserves dispatched by Grid-India and tertiary reserves procured through the power exchanges.