Loss of load
Loss of load in an electrical grid is a term used to describe the situation when the available generation capacity is less than the system load. Multiple probabilistic reliability indices for the generation systems are using loss of load in their definitions, with the more popular being Loss of Load Probability (LOLP) that characterizes a probability of a loss of load occurring within a year. Loss of load events are calculated before the mitigating actions (purchasing electricity from other systems, load shedding) are taken, so a loss of load does not necessarily cause a blackout.
The concept of probabilistic assessment of power resource adequacy dates back to the 1930s. A foundational paper was published by Calabrese in 1947, which introduced a method to calculate the expected number of days when peak daily electricity demand would exceed the available generating capacity. This paper also started the tradition of describing the reliability metrics with multiple different, and loose, phrases like “loss of load duration” and “expected total number of days of loss of load".