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Although the calorie is a very practical unit because it permits the measurement of heat by its thermal effect on water and is easy to measure, this unit of measure has the defect of being different from the ones we use in the rest of Physics to measure any increase in energy. For example in the toaster electrical energy is converted into heat. How can we make the electrical energy correspond to the heat generated.
We all remember that the SI unit of energy is the joule. So, generalizing on the previous example: How many joules are equivalent to a calorie?
In the following visual we answer this type of question. An electric heater with a known power in watts (remember that a watt is one joule per second), will supply electrical energy to a container of water. Let's suppose that the system is appropriately insulated so that all the energy of the heater is used to heat the water, considering that used in the air and in the container itself as negligible. | 
