From an economic point of view it would be entirely correct to internalise grid-level system costs into the plant-level levelised cost of electricity production in order to derive the true economic cost to society of different technologies and to enable policy-makers to make least-cost choices maximising social well-being. While estimates of the capacity credit of variable renewables, their ability to fully substitute for dispatchable capacity, vary widely and depend heavily on local circumstances, they rarely exceed 10% of total capacity and decline with rising shares of variable renewables in electricity production. Estimating the costs of such back-up capacity is less straightforward than it seems.