Abstract Generation adequacy is a key ingredient to security of electricity supply. We develop a stochastic model of demand and supply (from different technologies) for measuring it from a physical or technical point of view. We adopt several metrics of supply shortfalls. Next we demonstrate the model by example. Because of limited interconnections with neighboring countries, Spain can be considered an electric island. We get numerical estimates of the parameters underlying the model. We then resort to Monte Carlo simulation to derive the risk profile of the adequacy metrics in the coming decades. We consider up to ten scenarios, with different demand paths and generation parks. The proposed deployment of renewable technologies and the envisaged closure of coal-fired and nuclear stations result in greater risk of shortages. For one, assuming that demand grows at 1.36% per year, from 2020 to 2030 the expected energy not served increases more than 400-fold as coal and nuclear capacities are reduced; the factor runs into the tens of thousands by 2040 and 2050 when those technologies cease to operate. These results are potentially important for policy makers, system operators, and power companies involved in the construction of the European internal electricity market.