Ancillary Services Overview

The electricity you pay for on your bill is only part of what keeps the lights on. A second, invisible market runs in parallel, selling services most customers never hear about.

Ancillary services are grid support products that system operators procure separately from bulk energy to maintain frequency, voltage, and the ability to restart after a blackout. Five categories: frequency regulation (seconds-scale output adjustments to hold 60 Hz), spinning reserves (already-running generators that can ramp within 10 minutes), non-spinning reserves (offline units startable in 10-30 minutes), voltage support (reactive power injection), and black start (restarting generation without grid power after a total blackout).

In PJM, frequency regulation alone is worth hundreds of millions of dollars per year. A generator providing regulation gets paid for that capability, not just for the energy it produces.

Tracing what happens after a generator trips offline. A 500 MW coal plant unexpectedly trips. Grid frequency begins dropping from 60 Hz.

What responds first: spinning reserves, or the system operator making a phone call?

Automatic response. Spinning reserves respond without human action. Governors on already-running turbines sense the frequency drop and open their throttles within seconds. Only after automatic response stabilizes frequency does the operator assess whether non-spinning reserves need dispatch. Variable renewables (solar, wind) are optimized for cheap energy production but are not naturally suited to provide spinning reserves or voltage support, which is why integrating large renewable shares requires rethinking who provides these second-by-second services.

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