Modern economies and infrastructure sectors rely upon secure electricity supplies. Due to sectoral interdependencies, major interruptions cause cascading effects in the economy. This paper investigates the economic effects of major power supply disruptions taking such interdependencies into account. We apply a dynamic in-operability input-output model (DIIM) to 101 sectors, including households, of the Scottish economy in 2009 to explore the direct, indirect, and induced effects of supply interruptions. We estimate the societal cost of energy not supplied (SCENS) due to an interruption. The results show that the most economically affected industries, following an outage, are different from the most inoperable ones. The results also indicate that SCENS varies with the duration of a power cut, ranging from £4,300/MWh for a one-minute outage to £8,100/MWh for a three-hour (and higher) interruption. The results can be used to design policies for contingencies and preventive investments in the power sector.