State of charge (SOC)
How full a battery is right now, usually expressed as a percentage of its usable energy capacity.
State of charge is the fuel gauge of a battery: how much energy it holds right now relative to how much it can hold. A 200 MWh battery at 50% SOC has 100 MWh available to discharge. It is the single most important operational fact about a storage asset, because it defines what the battery can actually do next — a battery at 5% can promise very little discharge, no matter how large its nameplate rating.
For grid batteries, SOC is strategy made visible. An operator who expects an evening price spike will spend the afternoon charging toward full; one who expects cheap midday power will run the morning down toward empty. Watching a fleet's state of charge is therefore a way of reading the market's collective forecast: when every battery in Texas tops up at 4 p.m., the fleet is bracing for an expensive evening.
SOC also carries obligations. A battery that has sold reserves to the grid operator must hold enough charge to deliver if called, so its usable range for arbitrage shrinks. Managing that budget — energy for trading versus energy promised as insurance — is the core discipline of storage operations.
For the technical reader
SOC enters dispatch optimization as the state variable linking time steps: SOC_{t+1} = SOC_t + η_c·charge_t − discharge_t/η_d, bounded by usable capacity limits that operators often keep inside the cells' physical 0–100% to manage degradation. It is what makes storage dispatch a genuinely dynamic (path-dependent) problem rather than a series of independent interval decisions — the marginal value of stored energy at any moment is an opportunity cost over future price distributions, not a fuel cost.
Ancillary awards impose SOC feasibility constraints (enough headroom to absorb for RegDown, enough energy to sustain RRS/ECRS deployments for their required durations), and ERCOT's RTC+B design brings SOC accounting directly into the market clearing rather than leaving it to each operator's telemetry. Measurement itself is nontrivial: BMS estimates drift and are recalibrated at rest, so reported SOC is an estimate with error, not a gauge reading.