To determine the heat energy absorbed by water in an insulated cup, one can use the power or power density of the microwave or sunlight source rather than the indirect method involving Planck's constant. The discussion emphasizes calculating the time required for water to boil from 23 degrees Celsius to 100 degrees Celsius, assuming no heat loss. Key factors include the surface area exposed to sunlight and the heat capacity of the water. On a bright day, sunlight provides approximately 1 kW/m² of power, though some energy may be reflected. Accurate calculations depend on these variables to estimate the heating time effectively.
