The invisible infrastructure keeping your lights on — explained through direction (up, down, symmetrical) and the critical distinction between capacity and energy.
Every ancillary service product has a direction. This determines whether a provider must be ready to increase output, decrease it, or both simultaneously — and fundamentally shapes who can qualify.
Upward reserve
The provider increases generation or decreases consumption. Activated when frequency falls — demand exceeds supply. A thermal plant ramping up, a battery discharging, or an aluminium smelter halting production are all upward actions.
Freq drops → UP fires
Downward reserve
The provider decreases generation or increases consumption. Activated when frequency rises — surplus supply. Solar curtailment, a battery charging, pumped hydro pumping. Increasingly critical on high-renewable grids with midday surpluses.
Freq rises → DOWN fires
Symmetrical reserve
The provider holds a midpoint and must ramp both directions simultaneously on command. This is the most demanding qualification: a battery must stay near 50% SoC, a plant at part-load. FCR is always symmetrical. Commands a premium in auctions.
Equal headroom both ways
Ancillary service markets pay for two distinct things. Conflating them is the most common mistake — especially for storage assets trying to stack revenues.
Payment for being available: having technical capability and reserved headroom, regardless of whether you are ever called. This is the reservation fee — you are paid to stand ready.
Capacity payments are the foundation of ancillary service revenue for assets that are rarely activated (e.g. black start, replacement reserves). They also reward fast-response assets like batteries that provide high option value at low activation frequency.
Payment for actual energy delivered when activated. This only flows on activation. For upward activation, providers receive activation price × MWh injected. Downward activation may earn a saving or a separate payment.
Energy payments can dwarf capacity payments during scarcity events when activation prices spike. But for storage, every MWh delivered depletes state of charge — so unlimited energy revenue isn't possible. This is the fundamental constraint.
Adjust the parameters to see how capacity and energy payments interact — and how activation depletes state of charge.
Click each service. Inside, you'll find direction badges (▲ ▼ ↕) and payment structure (capacity / energy) for every product.
Choose whether generation is lost (▲ UP reserves activate) or excess generation spikes frequency (▼ DOWN reserves activate).
Services activate in tiers from milliseconds to hours. Symmetrical products (FCR, FFR) are always on. Directional products are called on demand.
Note the asymmetry: downward capability (▼) is often more constrained for thermal plants — and more valuable on grids with high renewable penetration.
✦ Excellent · Good · Capable* (with compliance) · Limited · ✗ Not possible