A Watt is a unit of power, defined as the rate of energy transferred [per second]. The energy is usually defined as Joules; therefore, one Joule per second is one Watt.
When measuring heating effects, a unit of thermal energy known as a calorie is used. One calorie is 4.184 Joules.

Materials have a property called thermal capacity or specific heat. This is a measure of how many calories are needed to raise 1.0 gram of the material one degree Centigrade. The thermal capacity of water at 15 deg. C is 1.0 calorie. That is 4.184 Joules or 4.184 Watt-seconds. The entire energy could be transferred to the water in one millisecond at a rate of 4184 watts to produce the same temperature rise of one deg. C.

The thermal capacity of materials changes slightly with temperature primarily due to changes in density, and very dramatically at phase transitions, such as ice melting and water boiling.

The specific heats of a few selected elements at 25 deg. C are: