Here's a tricky question about effect of heat, specific to the Prius:

The Prius ICE runs in very rich mode for the forst two minutes after a cold start to warmp up rapidly. AFAIKleaving this rich mode is in part dependent on mesured enine temperature. FE is usually abysmal during this time, which is why many short trips in the Prius tank the mileage. A block heater would help.

The Prius also uses an electric heating element to help heat the cabin, if the ICE is not yet warmed up sufficiently. Thus, if you use heat during the first 2 minutes, you will be using the electric heater element, which should suck up energy via the electric generator/motor.

However, this same process (electric heating) should increase the load for the ICE during startup, and may shorten the warmup period of rich running at low FE, saving fuel. The question is: how does this balance out.
Any thoughts?