With ngspice you may not only simulate electrical circuits, but also “thermal circuits” and the interaction of both.
So for example there is an amplifier with power transistors. They will dissipate electrical power and heat up. The “thermal circuit” now consists of the heat generated in the device, the heat flow through the package to the heat sink and finally into the ambient. This flow is somewhat limited, and therfore the device temperature increases. Fortunately “thermal” circuits may be translated into electrical circuits and then be simulated with ngspice.
I have set up a tutorial at
with circuits entered by Eeschema, using electro-thermal device models from semiconductor makers (Power MOS and GaN devices) and some new KiCad symbols to follow these models.