Ancillary Services
Definition
Ancillary services are services that keep the grid reliable beyond the delivery of energy itself. EIA defines them as 'services that ensure reliability and support the transmission of electricity from generation sites to customer loads,' including 'load regulation, spinning reserve, non-spinning reserve, replacement reserve, and voltage support.' Grid operators buy them in markets or under tariffs, usually paying for capacity held ready and sometimes for energy delivered when called.
Why it matters
Ancillary services were the first large market for grid batteries, and many projects still depend on them. The markets are small relative to energy markets, so prices can fall as more batteries enter, and rule changes such as co-optimization alter how they are priced. Investors should check what share of a project's revenue comes from these services, in which market, and what the projection assumes about future prices and competition.
Common misconceptions
- •Ancillary service revenue is not contracted; it is merchant unless sold forward or under a toll.
- •Recent ancillary prices are not a long-term forecast; small markets can saturate.
- •Holding capacity for a reserve service uses part of the battery's power and energy that cannot then be sold elsewhere.
- •The label is only a starting point. Investor outcomes usually depend on the governing documents, cash-flow priority, valuation policy, disclosure quality, and manager discretion surrounding the term, not just the shorthand definition used in marketing materials.
Technical details
Main service types
Regulation: continuous adjustment up and down to hold frequency.
Spinning or responsive reserve: capacity online and ready within minutes.
Non-spinning or contingency reserve: capacity that can start or respond within a longer window.
Voltage support and black start: usually procured under specific arrangements.
Questions to ask
Which services and which market rules apply?
What price is assumed, and how does it compare with recent clearing prices?
How much of the battery's capacity is committed to each service?
What happens to revenue if the market saturates or the rules change?
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- Why do batteries provide ancillary services?
- Because they respond in milliseconds to seconds, faster than most generators. NREL describes ancillary services as providing power 'to maintain generation-load balance and prevent frequency fluctuations,' services that need fast response more than long duration. Ormat says its storage segment earns its revenues 'mainly from the sale of ancillary services in the merchant market and /or tolling agreements and RA contracts.'
- What ancillary services does ERCOT buy?
- Vistra's 2025 10-K lists 'regulation service, responsive reserve service, ERCOT contingency reserve service, and non-spinning reserve service,' provided by 'generators, energy storage, and qualified loads.' Under real-time co-optimization, as energy prices rise, ERCOT 'will go short on ancillary services,' converting that reserve capacity to energy.
- How are ancillary services paid?
- Usually by a clearing price per MW of capability held for each interval, set market by market. Some products also pay for the energy actually delivered or for performance. Prices are set by the grid operator's rules and can change when the rules or the number of providers change.