We investigate the annual cycle of the radiation at the top of the atmosphere (RTOA) and of the surface energy budget on the basis of a general circulation model with intermediate complexity. The model is based on a standard spectral dynamical core and includes an idealized radiation scheme with continuous computation of energy fluxes. The surface energy budget is taken into account by means of a slab ocean with prescribed lateral oceanic heat flux convergence. The moisture budget is based on a new transport scheme and simple parameterizations of condensation and convection. Subgrid-scale parameterizations include gravity waves and turbulent diffusion. Each parameterized process is formulated in an energy conserving fashion such that the resulting numerical error of the mean RTOA is about 0.1 W/m/m. On the seasonal timescale, the annual variation of the RTOA is synchronous with an equally strong imbalance of the surface energy budget. We run the model either with or without orbital eccentricity. Either case exhibits a pronounced annual cycle of the outgoing long-wave radiation (OLR) of a few W/m/m, with the minimum occurring in the northern hemispheric winter. This variation results from the hemispheric differences in the distribution of land and ocean surfaces, which are characterized by different heat capacities and albedos. It is synchronous with an annual variation of the global-mean surface temperature of a few K. The implication of this finding is that long-term changes of the RTOA due to internal variability of the climate system are well possible. In particular, if the ocean absorbs heat while the global-mean surface temperature decreases, the RTOA is positive.