In the solar-assisted series hear pump system, coefficient of performance (COP) can be increased by transferring solar radiation heat to the heat pump evaporator. In the present study a method of calculating the ideal design limit for the collector area based on the second-law efficiency is given by using a set of experimental data on a certain design of a solar-assisted series heat pump system for domestic heating during the heating season. A comparative study is made between the actual and ideal conditions, and a minimum required collector area is presented.