Using the model of a cylinder-type heat source, the power loss owing to heat conduction in laser cutting and welding of metals is calculated analytically. The case of laser cutting is described by taking into account the influence of the generated cutting kerf using numerical calculations. Both the analytical and the numerical solution for the power loss deposited into the material are well described by approximative formulae. The theoretically predicted power loss into the cut workpiece is confirmed by measurements of the temperature rise within the metal sheet in laser cutting experiments.