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.^[1] Multiple probabilistic reliability indices for the generation systems are using loss of load in their definitions, with the more popular^[2] being Loss of Load Probability (LOLP) that characterizes a probability of a loss of load occurring within a year.^[1] 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.

Multiple reliability indices for the electrical generation are based on the loss of load being observed/calculated over a long interval (one or multiple years) in relatively small increments (an hour or a day). The total number of increments inside the long interval is designated as ${\displaystyle N}$ (e.g., for a yearlong interval ${\displaystyle N=365}$ if the increment is a day, ${\displaystyle N=8760}$ if the increment is an hour):^[3]

• Loss of load probability (LOLP) is a probability of an occurrence of an increment with a loss of load condition. LOLP can also be considered as a probability of involuntary load shedding;^[4]
• Loss of load expectation (LOLE) is the total duration of increments when the loss of load is expected to occur, ${\displaystyle {LOLE}={LOLP}\cdot N}$. Frequently LOLE is specified in days, if the increment is an hour, not a day, a term loss of load hours (LOLH) is sometimes used.^[5] Since LOLE uses the daily peak value for the whole day, LOLH (that uses different peak values for each hour) cannot be obtained by simply multiplying LOLE by 24;^[6] although in practice the relationship is close to linear, the coefficients vary from network to network;^[7]
• Loss of load events (LOLEV) a.k.a. loss of load frequency (LOLF) is the number of loss of load events within the interval (an event can occupy several contiguous increments);^[8]
• Loss of load duration (LOLD) characterizes the average duration of a loss of load event:^[9] ${\displaystyle {LOLD}={\frac {LOLE}{LOLF}}}$

A typically accepted design goal for ${\displaystyle LOLE}$ is 0.1 day per year^[10] ("one-day-in-ten-years criterion"^[10] a.k.a. "1 in 10"^[11]), corresponding to ${\displaystyle {LOLP}={\frac {1}{10\cdot 365}}\approx 0.000274}$. In the US, the threshold is set by the regional entities, like Northeast Power Coordinating Council:^[11]

resources will be planned in such a manner that ... the probability of disconnecting non-interruptible customers will be no more than once in ten years

— NPCC criteria on generation adequacy

• Value of lost load