Power system balancing will become increasingly important to secure a reliable European energy supply, as the share of intermittent supply increases (e.g. variable generation from wind and solar PV). This paper shows, in a quantitative way, the limitations of relying exclusively on flexibility in generation as the future shares of intermittent supply increase. Literature and data on intermittent supply and existing scenarios are reviewed. Costs related to increasing shares of intermittent supply are assessed. Quantifiable indicators relevant for electricity systems with a high share of intermittent supply are developed, namely (a) flexible supply (generation units that can quickly change output); (b) balancing need (which measures the needed flexibility of the power system as the difference between peak and off-peak residual demand (net of intermittent supply)). There is an externality of increasing the share of intermittent supply by increasing the power system balancing costs. If the cost of integrating intermittent generation was to be limited to about 25 billion EUR per year, which is an addition of 24% per unit intermittent generation cost, the intermittent supply share cannot reach more than 40% in the European power market. The final choice of an acceptable cost increase will be a political choice. (c) 2013 Elsevier Ltd. All rights reserved.