We investigate the accuracy of a MEMS-based nanocalorimeter for determining specific heat in the temperature range from 300 K to 800 K. By comparing DC differential scanning measurements of materials of known specific heat capacities with detailed finite element studies, the dominant sources for heat losses and inaccuracies have been identified. The main source of error comes from radiative losses at elevated temperatures, but even near room temperature thermal gradients and thermal conductivity in the specimen lead to moderate discrepancies. Furthermore, studies on de-wetted Au films and Bi particles show that specimen emissivities differ strongly from literature values, presumably due to effects associated with the specimen dimensions being smaller than the thermal wavelength. By including the emissivity as a fitting parameter in the simulations, specimen heat capacities can be obtained to an accuracy of better than 7% over the entire temperature range.