A researcher investigates the thermal storage properties of basalt rock for use in a concentrated solar power plant. They use an electrical cartridge heater to heat a 2.5 kg cylindrical basalt core sample to determine its specific heat capacity.

The researcher calculates the expected temperature increase of the basalt rock sample based on the electrical energy input. However, the experimentally measured temperature increase is lower than this calculated value.
Explain why the actual temperature increase of the basalt rock sample is smaller than calculated.