In many power systems large thermal generating units, which were primarily designed to resist creep damage caused by base load operation throughout an effective service life of more than 40 years, are being operated cyclically as a result of market liberalization and the rapid expansion of intermittent renewable energy sources. This type of off-design operation results in accelerated rates of life consumption due to the initiation of fatigue-related damage mechanisms which these units were not designed to withstand. This issue is of particular concern to the owners and operators of thermal generators in the Irish all-island system because of the significantly increased levels of cycling duty that their units will be required to perform as a result of plans to integrate very high levels of wind power by 2020. The impacts of cyclic operation on unit operating costs, scheduling and availability has largely been overlooked in renewable energy integration studies. The authors draw on the results of recent studies in Ireland and elsewhere to relate fatigue-life consumption (measured in total lifetime starts) and damage accumulation (measured in annual maintenance costs) to create a model which can be used to forecast lifetime hot, warm and cold per-start costs for a typical base load unit in a range of market and wind-penetration scenarios.