Carbon capture and storage (CCS) is being widely discussed as a possible mitigation option for limiting the emissions of carbon dioxide (CO2) from fossil fuel burning power plants. The implementation of CCS requires the resolution of a number of difficult policy, engineering and economic issues. Here we address the efficacy of CCS from the perspective of climate science. Implementation of CCS makes power stations less efficient, in the sense that they produce more CO2 for a given output of electricity, a feature which is characterised by the so-called energy penalty. The captured CO2 is then stored in, for example, geological storage reservoirs, from which some small fraction is expected to leak back into the atmosphere each year. We use a set of relatively simple models of carbon capture, the atmospheric carbon cycle and climate, to quantify, for a range of CCS engineering and implementation parameters, the amount of leakage from these reservoirs that can be tolerated to ensure that CCS leads to less, rather than more, climate change. We demonstrate that up to the year 2100, for almost all the parameters that we consider, application of CCS is beneficial. However, in some cases the benefit might be small. We also consider a much longer time horizon (out to the year 2500). We find that while many parameter combinations still lead to a benefit, there are some cases for which application of CCS leads to greater warming than had it not been applied at all. The largest single controlling factor is seen to be the storage reservoir retention time. Many previous studies focused on the use of those storage reservoirs with very long retention times, but we demonstrate that the use of less resilient reservoirs might also provide a climate benefit during the 100 to 500 year time horizon. The largest absolute benefits of CCS to global temperature are found for high future emission scenarios. These absolute benefits also increase as the climate sensitivity of the model is increased.