This energy-over-time lesson turns a steady power budget into transferred energy by multiplying watts by elapsed seconds. The 24 W for 5 s gives 120 J exactly only for the stated constant-power interval. Real circuits can drift with heating, changing load resistance, source sag, switching transients, timer calibration, finite precision, and measurement uncertainty.
A power value over a time interval gives an exact energy transfer. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.