Heat produced at the surface of Mercury by the highly time-variable precipitation of charged particles need not be radiated away immediately but can also be conducted into the interior of the planet. For a given precipitated energy flux density, the rise of surface temperature can be computed taking both heat conduction and radiation into account. When the energy input varies on time scales shorter than a characteristic period, estimated to be one (terrestrial) day for conditions typical of Mercury's dark side, heat conduction dominates over radiation and the predicted surface temperature rise becomes negligibly small. Possible observable calorimetric effects are therefore confined to long time scales, and their magnitude is constrained by the average rate of energy input from the solar wind together with limits on the observable area of precipitation. ¿ American Geophysical Union 1987