Unit of rate of heat transfer is Watt.

Explanation:
Heat transfer is the process of energy transfer from one body or system to another due to a temperature difference between them. It occurs through three main modes: conduction, convection, and radiation. The rate at which heat is transferred is an important parameter in many engineering applications.

The rate of heat transfer is defined as the amount of heat transferred per unit time. It is commonly expressed in units of watts (W), which is a measure of power. Power is the rate at which work is done or energy is transferred. In the context of heat transfer, power refers to the rate at which heat energy is transferred.

The unit of watt is derived from the basic SI units of length, mass, and time. It is defined as one joule per second (1 W = 1 J/s). A joule is the unit of energy, and when heat energy is transferred at the rate of 1 joule per second, it is equivalent to 1 watt.

In other words, if a system or body is transferring heat energy at a rate of 1 watt, it means that 1 joule of heat energy is being transferred every second. This rate can be higher or lower depending on the specific application. For example, a small electronic device may transfer heat at a rate of a few watts, while a large industrial process may involve heat transfer in the range of hundreds or thousands of watts.

In conclusion, the unit of rate of heat transfer is the watt, which represents the rate at which heat energy is transferred.