The figure above shows the input and output parameters for a transmission. Torque conversion requires a torque which supports the differential moment between output and input torque. This is a characteristic of transmissions.
Similar to speed converters, in the case of torque converters, the overall power input which goes into the transmission is not available at the output. Hence the transmission efficiency can be defined as the ratio of the power delivered by the transmission to the power supplied to it.
The power loss can then be calculated over the difference between the power input and the power output.