The electricity sector in OECD countries is on the brink of a large shift towards low-carbon electricity generation. Power systems after 2030 may consist largely of two low-carbon generator types: Intermittent Renewable Energy Sources (IRES) such as wind and solar PV and thermal generators such as power plants with carbon capture. Combining these two types could lead to conflicts, because IRES require more flexibility from the power system, whereas thermal generators may be relatively inflexible. In this study, we quantify the impacts of large-scale IRES on the power system and its thermal generators, and we discuss how to accurately model IRES impacts on a low-carbon power system. Wind integration studies show that the impacts of wind power on present-day power systems are sizable at penetration rates of around 20% of annual power generation: the combined reserve size increases by 8.6% (6.3-10.8%) of installed wind capacity, and wind power provides 16% (5-27%) of its capacity as firm capacity. Thermal generators are affected by a reduction in their efficiency of 4% (0-9%), and displacement of (mainly natural gas-fired) generators with the highest marginal costs. Of these impacts, only the increase in reserves incurs direct costs of 1-6€/MWhwind. These results are also indicative of the impacts of solar PV and wave power. A comprehensive power system model will be required to model the impacts of IRES in a low-carbon power system, which accounts for: a time step of <1 h, detailed IRES production patterns, flexibility constraints of thermal generators and interconnection capacity. Ideally, an efficient reserve sizing methodology and novel flexibility technologies (i.e., high capacity interconnectors and electricity storage and DSM) will also be included.