An availability guarantee is a contractual clause that ensures an energy system - such as a BESS or power plant - remains online, functional, and available to deliver services during a predefined portion of a reporting period. It is typically expressed as a percentage of the time the system is operational relative to the total time it is expected to be available. The availability guarantee differs from the performance guarantee in that it measures system readiness rather than actual output or efficiency. For example, a battery that is fully operational but not dispatched may still meet availability requirements, even if no energy is delivered. This guarantee is often linked to service payments and can include penalties for excessive downtime or scheduled maintenance overruns.